Roofing

ROOFS IN GENERAL

The roof is a very important part of the building structure. It performs several essential functions:

• It gives shelter to people.
• It provides shade.
• It isolates the building from cold and heat.
• It keeps out dust and dirt.
• It protects the interior of the building.
• It sheds rainwater.

ROOF TYPES

There are many different types of roofs. in Rural Building we deal only with the following four types:

The lean-to roof (Fig. 1)

The pent roof (Fig. 2)

The gable roof (Fig. 3)

The hipped roof (Fig. 4).

These roofs will be described in detail in the following sections of the book. in the next few sections we will describe some factors which are important in the construction of all roofs. These must be understood well before we can go on to describe the construction details of the particular roof types.

SIZE OF THE ROOF

The cost of roofing sheets will be a significant part of the cost of the whole building. Therefore it is the size of the roofing sheets which will determine the size, and especially the width, of the whole building.

For this reason, we make an outline design of the roof before we determine the other measurements of the building. We cannot design the building first and later fit a roof on it. The outline design tells us the width that our building should have so that we can fit a roof on it without unnecessary and wasteful cutting and trimming of the sheets.

To make the outline design of the roof, we need to know:

• the pitch cf the roof
• the effective length of the sheets
• the distance the roof will project past the outside walls of the building.

ROOF PITCH

The angle of the slope of the roof is called the pitch. Most roof types have a standard pitch. If corrugated sheet materials are used, the pitch angle should be between 15 and 20 degrees (Fig. 1).

EFFECTIVE LENGTH OF THE SHEETS

The effective length of roofing sheets is the length of the sheet (x) minus the overlap (y) between the sheets (Fig. 2),

The minimum overlap in the length for corrugated sheeting materials is 15 cm. The most common length for roofing sheets is 244 cm.

In order to use the sheet materials as economically as possible, we use either 1, 1-1/2, 2, 2-1/2, or 3 (and so on) sheets to cover the distance from the highest point of the roof to the lower edge. Thus the effective length will be:

for 1 sheet 241 cm

for 1-1/2 sheets 244 cm + 122 cm - 15 cm = 3ul cm

for 2 sheets 244 cm + 244 cm - 15 cm = 473 cm

for 3 sheets (244 cm x 3) - (15 cm x 2) = 702 cm

and so on.

OUTLINE DESIGN

The outline design should be made in as large a scale as possible, since it is used to find some measurements for the future building design.

a. Draw a horizontal line (line 1).

b. Draw line 2. The angle between line 1 and line 2 should be the angle of the pitch of the roof.

c. Extend line 2 to show the projection of the roof beyond the wall of the building. Mark point x at the end of line 2.

d. Measure the effective length of the sheets from point x, remembering to subtract the overlap. Mark point y on line 2.

e. Make a line perpendicular to line 1 and passing through point y. This is line 3. Mark point z on line 1.

f If a lean-to or pent roof is planned, the length of line 1 up to point z gives the width of the building. Now indicate the supporting walls and the sheets with the ove\-lap. in a lean-to roof, remember that the end of the sheet at the top is enclosed in the wall; and for the pent roof, the sheets project past the wall on die top side.

g. If a gable roof is planned, draw the other half of the roof in the same manner. The extended line 1 will give the width of the building.

h. indicate the supporting walls, sheets and the overlap.

TERMS:

The point where the sloping lines meet, point y, is called the "ridge".

Line 3 is the "rise" of the roof.

If the horizontal line (line 1) projects beyond the supporting walls, the projecting part is called an "overhang".

If the sloping part projects beyond the wall, the projecting part is called an "overhanging eave".

The lowest part of a sloping roof is called the "eave" (point x).

LOADS

In order to build a good strong roof, it is necessary to take into account certain forces that will affect it. We call these forces 'loads".

Some of the loads come from the weight of the roof itself and the ceiling. These are called "dead loads". They consist of: (see Fig. 1)

• the weight of the roof covering
• the weight of the roof construction
• the weight of the ceiling
• the weight of anything attached to the ceiling such as ceiling fans or light fixtures.

Other loads are caused by external (outside) forces such as rain or wind. These are called "external loads". External loads are: (see Fig. 2)

• windloads
• rainloads

All loads, external and dead loads, must be taken into account in the planning and construction of a roof.

The most dangerous load in Ghana is the windload. Strong winds or storms can cause a lot of damage to a roof if it is not well made and securely fixed to the building.

One particular danger is the suction on one side of the roof caused by the wind, which creates pressure on one side and suction on the other side of the roof (Fig. 2). This suction on roofs can be demonstrated by an experiment. Fold a sheet of paper in half and blow over the top edge. The other half of the paper will be lifted up by suction.

SHAPE OF THE ROOF WITH RESPECT TO WINDLOAD:

Roof constructions can be severely damaged by storms. Apart from the precaution of anchoring the roof well, which will be discussed later, there are some ways to shape a roof so that it is less vulnerable to the force of the wind:

Consider carefully the pitch of the roof. Roofs with a pitch of less than 10 degrees are much more prone to high suction forces.

15 to 20 degrees is a better roof pitch, because it causes less suction.

Consider making a hip roof instead of a gable roof. This will decrease the suction even more.

COMPARE TRADITIONAL ROOF CONSTRUCTION AND MODERN LIGHT-WEIGHT STRUCTURES

LIGHT-WEIGHT STRUCTURES

As the span (width) of the building increases, the strength of the roof construction must also be increased, Formerly this was done by using thicker and wider timbers (Fig. 1).

This method resulted in very heavy and expensive roof constructions. Large timbers are normally much more expensive than smaller timbers.

This kind of roof construction is now uneconomical, as well as difficult because of the complicated joints needed for large timbers.

In modern light-weight structures, the strength of the roof is increased for larger spans by building up brace structures in the shape of triangles (Fig. 2), instead of using larger timbers. This construction uses small timbers, which are cheaper and lighter in weight; therefore we get a light-weight structure.

Light-weight structures are less costly, and result in a smaller dead load on the roof. Another advantage is that we can use simple joints instead of complicated roofing joints.

Since the construction is built up from small members, it is sometimes called a "built-up structure". This is the only type of roof construction we will learn in Rural Building.

- TERMS:

The sloping members are called "rafters" (Fig. 2, a).

The horizontal member is called the "tie beam" (b).

The members strengthening the construction are called "braces" (c).

BASIC SHAPE OF THE ROOF CONSTRUCTION

It is essential that the roof construction remains rigid. To ensure this, we need to find a shape for it which cannot be distorted.

- NOTE: The only construction shape that cannot be distorted is a triangle.

The strength and efficiency of the triangular construction can be proven by experiment:

For example, a square shaped construction (Fig. 1) can easily be distorted. The only resistance to the distortion is in the corner joints. This construction is called a ,1non-perfect" or imperfect structure.

A triangle shaped structure cannot be distorted (Fig. 2). When force is applied to any part of it, the frame remains rigid. This construction is called a "perfect" structure.

As soon as a diagonal brace is introduced to a square shaped frame (Fig. 3), it becomes rigid. The brace changes the square shape into two triangles, making it a perfect structure.

If the brace were to be installed vertically or horizontally instead of across the diagonal, the structure could still be distorted because it would still be non-triangular, and thus an imperfect structure.

There are many examples of triangular constructions in our surroundings. Look at a bicycle frame, scaffold braces or braces for a door frame, for example.

In Fig. 4 are shown some perfect structures.

In Fig. 5 are shown some imperfect structures.

In roof construction, it is essential to make only perfect structures.

TECHNICAL TERMS

TIE BEAM: This is the horizontal member of the roof structure which ties together the feet of the rafters (Fig. 1, below).

RAFTERS: These are the sloping members which give support to the purlins (Fig. 1, below).

BRACES: These are the members which strengthen the construction.

ROOF TRUSS: This is the structure made up of the rafters, tie beam and braces, which forms the main load carrying unit in some kinds of roofs.

PURLINS: These members lie across the rafters and support the roofing sheets.

RIDGE: This is the highest point of a roof construction.

RISE OF THE TRUSS: This is the vertical height of the truss (Fig. 1, a), measured between the highest point of the truss and the soffit of the tie beam.

SPAN OF THE TRUSS: This is the clear horizontal distance between the internal faces of the rafters (b) at the point where they meet the soffit of the tie beam.

SPAN OF THE BUILDING: The building span is the clear horizontal distance between the inside faces of the walls which support the roof (c).

OVERHANG: When the tie beam projects beyond the supporting wall, the projecting part is callod the overhang (d). The overhang is measured square to the wall.

OVERHANGING EAVE: When the rafters project beyond the supporting wall, the projecting part is called an overhanging eave (e). The width of the overhanging eave is measured square to the wall.

EAVE: This is the lowest part of the overhang or overhanging eave (point x).

LENGTHENING JOINTS FOR LIGHT-WEIGHT STRUCTURES.

Always select the timbers before starting work on the roof construction. Keep the long straight pieces for the rafters, tie beams, and purlins; and the short pieces for braces.

Sometimes pieces of the right length may not be available. Then it may be necessary to join the members in order to make them longer.

All joints used for lengthening the members of the roof construction will be fish joints with one or two fish plates. These will be discussed in detail in the next section.

Keep in mind that each lengthening joint in the roof construction will weaken the roof. Therofore, try to select the tlmbers so that you don't need to use many lengthening joints.

WHERE TO PLACE A LENGTHENING JOINT

It is often difficult to judge where to place a lengthening joint. in general, one can say that it should be as close as possible to a support such as a wall or pillar, but we must also keep in mind that the joint is weak, so it cannot be in a place where it is under strain.

For example- A 6 m long tie beam is needed. The only long piece of wood you have is 5. 70 m long. If you join that to a 30 cm piece to make 6 m, then the joint will indeed be near a load-carrying wall - but in relation to the entire structure the joint will be in the wrong place, because most of the weight of the truss is concentrated in that area, whore wo now have a weak joint.

In this case, a bettor solution is to join two shorter pieces to got the required length.

Lengthening joints should alt/o not be placed where the braces moot the member. NOTES;

FISH JOINTS

We have said that a fish joint will be sufficiently strong for most parts of the roof construction. However, when we assemble a fish joint, we have to follow certain guidelines to end up with a strong joint.

A fish joint will usually be assembled with nails. The strength of the nailed fish joint depends first on the number of nails used; and second on whether one or two fish plates are used. The length of the fish plates should be at least 5 times the width of the joined members.

SHEAR STRESS IN A JOINT WITH ONE FISH PLATE:

If one nailed fish plate (Fig. 1) is used, the nails tend to shear off (break) at the joint line between the members and the fish plate because of the force exerted as the members try to move apart (Fig. 3). The joint is said to be in single shear (Fig. 2).

In order to prevent the members from moving apart and shearing the nails, it is necessary to clinch the nails, so that they are anchored firmly (Fig. 4).

SHEAR STRESS IN A JOINT WITH TWO FISH PLATES:

When two nailed fish plates are used (Fig. 5) to join the members together, the nails tend to shear at two points. The joint is said to be in double shear (Fig. 6).

In this joint there is less tendency for the members to pull apart, since there are two fish plates (Fig. 7). The nails must go through both fish plates.

NOTE: The strength of a joint in double shear (with two fish plates) is approximately twice that of a similiar joint in single shear (with one fish plate).

NAILING THE FISH JOINT:

We have seen that the strength of a nailed fish joint depends on the number of nails used in it and on the number of fish plates. It is important that the nails are fixed correctly, or the joint will be weakened.

• The nails should be evenly distributed over the entire fish plate.
• Nails should always be staggered.
• Nails should always be blunted.

If thick nails are used, they have a tendency to split the wood. Often staggering and blunting cannot prevent this splitting. in this case it is necessary to drill holes for the nails. The holes should have a slightly smaller diameter than the nails.

There are certain rules for spacing the nails on the fish plate. These are:

In the direction of the grain, the distance between the nails should be 10 times the thickness of the nail (Fig. 1, below).

The distance to the margin of the fish plate should be: in the direction of the grain, 10 times the thickness of the nail; and across the grain it should be 5 times the thickness of the nail (Fig. 1).

OTHER LENGTHENING JOINTS

Other joints besides the butt joint with fish plates are sometimes used to lengthen the members of a roof construction. These joints require more work to make and they are not necessarily better than the ordinary fish joint. They are used only if extra strength is required.

Those joints are:

Joints 1-6 are normally used for joining rafters, tie beams or purlins in the length. They can be used with one or ti/o fish plates.

Joints 7-9 are used mostly for joining wall plates in the length. The corner joint for a wall plate can also be a halving joint.

CONSTRUCTION DETAILS

LEAN-TO ROOF

A lean-to roof is a sloping roof attached to the wall of another building. It is 'leaning" against the building. It is usually used for a small store or similiar building which is attached to an existing building. The main members are: (Fig. 1)

• the wall plate (a): 5 x 10 to 15 cm
• the rafters (b): 5 x 7,5 to 10 cm
• the purlins (c): 5 x 7,5 cm
• the fascia board (d): 2,5 x 20 to 30 cm
• the sheet material (e)

The above measurements can be used as a guide in selecting timbers for this kind of roof.

Below are some rules for the construction.

A good method for fixing rafters onto the main wall is to chisel holes in the wall into which the rafters can be fitted. The holes should be deep enough to ensure a safe rest for the rafters (Fig. 1).

The lower part of the rafter should rest on a wall plate (a) or a concrete belt. The rafter and wall plate should be firmly anchored through at least 3 courses of blocks.

The rafters should each be fitted with a "bird's mouth" near the end (f). The depth of the bird's mouth should be 1/5th of the width of the rafter. This is so that the rafters rest securely on the horizontal support.

The purlins (c) should be well secured to the rafters.

To prevent water from entering between the wall and the sheeting material, the part of the sheet which attaches to the main wall should fit into a recess in this wall, so that the ends of the sheets can be plastered over later (Fig. 1).

In an open fronted structure, the roof is supported by pillars on the open side, instead of by a wall. The plate which rests on the pillars and supports the rafters is called the "eave plate" (Fig. 2, g). The size of the eave plate should be about 5 x 7,5 to 10 cm.

Remember that in an open construction the pillars must not only support the dead load of the roof construction, but also must be strongly anchored in the ground so they cannot be pulled out by a strong wind catching the roof from underneath.

Note that in an open-fronted construction side walls are to be avoided. If they are required, there must be openings in them to permit wind to escape. A building with an open front should never face in the direction of the prevailing wind (the direction from which the wind usually comes).

PENT ROOFS

A pent roof is a roof which slopes to one side. It differs from a lean-to roof in that it is not attached to the wall of another building, but is supported by its own walls.

In Rural Building, we deal with two types of pent roof: the ordinary pent roof and the enclosed or parapetted pent roof.

ORDINARY PENT ROOF:

In this roof, the rafters and purlins project beyond the outside walls and a fascia board is fixed all around the building. The pitch of this roof will usually be about 15 degrees. The main members are: (Fig. 1)

• the wall plate (a): 5 x 10 to 15 cm
• the rafters (b): 5 x 15 cm
• the purlins (c): 5x7,5 cm
• the fascia (d): 2,5x 20 to 30 cm
• the sheet materials (e)

The above timber measurements can be used as a guide in selecting the timbers for an ordinary pent roof; that is, a pent roof with a span of less than 3,5 m. The distance between rafters should be 1 to 2 m, and the distance between the purlins depends on the size of the sheet material.

There are certain rules for constructing this type of roof:

The rafters should always be fitted with a bird's mouth so that they rest securely on the wall plate (Fig. 2).

Wall plates, rafters, and purlins should be well anchored (see the section on anchorage which comes later in this book).

The pent roof is often used because it is cheaper to construct than other roofs, since only rafters and purlins are used, and no tie beam, braces, etc. are needed.

ENCLOSED OR PARAPETTED PENT ROOF:

in this roof, the higher wall and the two sloping walls enclose and protect three sides of the roof. The parts of the walls which project above roof level are called parapets. Parapets help to reduce suction on the roof and to keep the sheets in place. The pitch of this roof will be about 15 degrees (Fig. 1).

The parts of the roof are:

• the wall plate
• or concrete belt (a)
• the rafters (b) 5 x 7,5 cm
• the tie beam (c) 5x7,5 cm
• the braces (d) 2,5x7,5 cm
• the purlins (e) 5x7,5 cm
• the fascia (f) 2,5 x 20 to 30 cm
• the sheet material (g)
• the parapet (h)

The measurements above can be used as a guide for constructing this kind of roof. The distance between the rafters should be 1 to 2 m, and the distance between the purlins will depend on the size of the sheet material.

Keep in mind the following construction pointers:

A built-up structure is used for this roof (a half truss). We will learn about how to make the truss in a later lesson.

Before the half truss is erected, all the walls should be built to the level of the tie beam. The truss is then erected and the walling continued,

There should be an expansion gap at the end of each rafter and tie beam (Fig. 1, x) where they fit into the wail; and at the ends of the purlins (Fig. 2, x). The gap prevents the wall from being cracked when the wooden member expands.

The sheet material should be fixed so that about l/3rd of a corrugation on each side of the roof is enclosed in the wall (Fig. 2). To keep water out, this edge should be [Minting upwards (Fig. 2a).

Leave about 10 cm space between the last truss and the wall so that there is enough space for plastering (Fig. 2, y).

GABLE ROOF

This is a roof which slopes down en the two sides of the ridge and has a gable on one or two end walls. The gable is the triangular shaped part of the end wall where it comes up to the sloping edges of the roof.

The advantage of the gable roof over the pent roof is that it can be constructed to permit cross ventilation. It can be used for large or small spans.

In Rural Building, the kind of gable roof we make is constructed with built-up trusses. The main parts and members of the gable roof are (Fig. 1):

• the wall plate or concrete belt (a)
• the rafters (b) 5 x 7,5 cm
• the tie beam (c) 5 x 7,5 cm
• the braces (d) 2,5 x 7,5 cm
• the purlins (e) 5x7,5 cm
• the fascia (f) 2, 5 x 20 to 30 cm
• the sheet materials (g)

The above measurements should be used as a guide when you make this type of roof. The distance between the trusses will be about 200 cm. The last truss should be close to the wall, except if parapets are used. in that opis leave about 10 cm space for mortaring.

The gable ends can be constructed so that the purlins project beyond the gable and the fascia boards are nailed onto them. The end sheets should be nailed so that the last corrugation folds down over the top of the fascia.

Another way of constructing the gable is to put a parapet on top of the gable in the same way as for a parapetted pent roof. Again, the sheets should be enclosed in the parapet wall by about 1/3rd of a corrugation, with the edge pointed up.

The parapet can be on top of the gable wall only (Fig. 2), or the wall can project vith the parapet beyond the ends of the sheets (Fig. 3).

DESIGNING BUILT-UP TRUSSES

Built-up trusses are used in gable roofs to support the purlins and the roof covering.

In constructing these trusses, we must remember the principle of "perfect structures" (see Light-Weight Structures), otherwise the roof construction may get distorted.

The following points will show why roof trusses are built up in the way they are.

1. Figure 1 shows that a roof construction of only rafters tends to spread the walls. This is because the roof doesn't have a triangular shape, so it is not a perfect structure. This construction could cause damage to the walls, the roof, find the whole building.

2. As we see in Fig. 2, as soon as a tie beam is introduced we get a triangular shaped, perfect roof structure.

3. As the span of the roof increases the tie beam tends to sag.

4. To prevent the sagging of the tie beam, braces are introduced. Because there are 4 members (2 rafters and 2 braces) that meet at the ridge, there; ?y a danger of over-nailing the joint, which would weaker- the truss. Therefore we need to find another way of fixing the braces while keeping a perfect structure.

5. This is a possible solution to prevent overnailing.

6. As the span increases still further the rafters will also be longer and they v/ill tend to bend.

7. To prevent the rafters from bending, more braces are introduced,

JOINTS FOR THE TRUSS

At the ridge, two rafters mutrt be joined together. This is done with a halving joint {Fig. 1).

The halving joint is assembled with a few long nails. They should be long enough that they can be clenched on the opposite side. Take care not to overnail the joint.

The rafter - tie beam joint can be a fish joint with two nailed fish plates (Fig. 2). After the fish plates are fixed, they must be cut so that they don't project past the edge of the rafter.

If large trusses are made, short braces can be used to strengthen the joint instead of fishplates (Fig. 3).

The braces are added according to the required pattern of the truss. At the place where the two braces meet, they should be cut ao that the areas where they are nailed to the truss (areas a and b) are as large as possible and of equal size on both braces (Fig. 4).

This is done by placing them on top of each other, marking and cutting during the assembly of the truss.

All of these reinforcements can be added on either one or both sides of the truss, depending on the strength that is needed for the construction.

MARKING OUT BUILT-UP TRUSSES

Before you start to mark out the trusses, measure the span of the building at several points and find out the span of the truss. Make sure that the truss is long enough to cross even the widest span of the building.

Do not forget the difference between the span of the building and the span of the truss. You must take into account any verandahs or overhangs in the span of the truss. The outline design will help you to find out the different spans for the building and the truss.

The marking out is done in a cleared, flat area of the building site. After the area is cleared, the shape of the trusses is laid out with iron pins knocked in the ground and with the mason line.

SEQUENCE OF OPERATIONS:

a. Place pegs 1 and 2 at a distance equal to the span of the truss, and stretch a line between them.

b. Divide the distance from point 1 to point 2 in half, and mark point 3.

c. From point 3, erect a line perpendicular to line 1-2 (use a large square or the 3-4-5 method).

d. Place peg 4 along the line from point 3, at the distance of the required rise of the truss.

e. Check that the distances 1-4 and 2-4 are equal.

f. Check that the roofing sheets will fit as planned, including a sufficient overlap. Check this by measuring from peg 1 to peg 4, then add the overhang and the projection of the fascia board.

ASSEMBLY OF BUILT-UP TRUSSES

Before you assemble the truss select ths timbers, measure them against the setting out, and if necessary join pieces to get members with the required lengths (see Lengthening Joints.

- SEQUENCE OF OPERATIONS:

a. Lay the tie beam along the line from peg 1 to peg 2.

b. Place the two rafters between pegs 1 and 4; and between pegs 2 and 4. Note the positions of pegs 1, 2 and 4 with respect to the boards. This is very important. Rafters should be slightly longer than needed. This allowance is cut off after the trusses are erected.

c. Mark the halving joint on the rafters at the ridge and cut it.

d. Mark the cutting lines on the tie beam and cut the ends.

e. Assemble the ridge joint with a few nails; and the rafter to tie beam joint with fish plates.

f. Mark the positions of the braces according to the plan, on the rafters and tie beam.

g. Lay the braces one over another on top of the tie beam and rafters. Use a single nail to fix them temporarily.

h. Cut the braces to the correct shape, keeping in mind that the size of the area where each is nailed to the truss must be the same for all of them (arrows).

i. Fix the braces with nails.

j. After the braces are fixed, If necessary a second fish plate can be fir^d on the other side ol the rafter-tie beam joint. Turn the truss over and fix Jiis plate. Sometimes it is also necessary to fix braces on both sides.

k. Mark the positions of the purlins on the rafters. This will make work easier later on.

1. The other trusses can be assembled on top of the finished first truss.

- NOTE: Always use dovetail nailing, throughout the truss.

ERECTING A ROOF STRUCTURE FOR A GABLE ROOF

As soon as the trusses are finished, the roof can be erected. If wood preservatives are used they should be applied before the truss is erected. Creosctu is often used as a preservative for trusses. If the roof will not be erected Immediately, keep the trusses dry and in a shady place.

SEQUENCE OF OPERATIONS:

a. Hang a truss upside-down across the span of the building, at each gable end (Fig- 1).

b. Turn the trusses over and plumb them (Fig. 2).

c. Secure the trusses at the bottom with anchor rods and brace their temporarily from the rafter to the wall plate.

d. Fix a line from one ridge to the other (Fig. 2).

e. Hang the other trusses, one after the other, upside-down between the walls. Turn each one over and plumb It according to the line before hanging the next one. Brace them temporarily.

f. Nail the purlins according to the measurements of the roof covering (the sheets) and secure them (a section on Anchorage is included later in this book). If the rafters overhang to cover a verandah, make sure that the eave plate is also well secured.

g. Fix a line at the bottom end of the rafters, ou the gable ends of the roof, at the level of the required overhang. Cut off the ends of the rafters according to the line.

h. Cut off the purlins to the correct length.

i. Build up the gables.

j. Nail the fascia boards on the purlins and rafter ends.

Once the roof structure is erected, cover it as soon as possible to keep out the sun and rain; otherwise the wood may start to twist and the joints will loosen. If the trusses rest on a concrete belt, put roofing felt between the concrete belt and the tie beam to help prevent dampness.

PURLINS

The function of the purlins is to support the roofing sheets and to stiffen the whole roof structure. They are supported by the trusses and sometimes the gable ends. The lengthening joints for purlins can be fish joints.

The size of the purlins should be 5 by 7, 5 cm.

The lowest purlin of the construction is called the eave purlin (Fig. 1). The highest one is called the ridge purlin.

To make a wider area for nailing the sheets, lay the purlins flat on the rafters. If larger distances must be bridged, lay the purlins on edga. This reduces the nailing area, but it prevents the purlins from sagging.

The distance between the purlins depends on the size and type of sheets which are used for the roof covering. If thick sheets with the normal 244 cm length are used, then one purlin at each end of the sheet plup one in the middle will be sufficient. For very thin sheets one extra purlin should be added for each sheet.

If the building will require more than 2 sheets to cover the length of the rafter, be careful that the purlins are set at the correct distance apart.

It makes work much easier if you mark the positions of the purlins on the rafters as you assemble the trusses,

FASCIA BOARDS

Fascia boards are often added to the roof construction. They are wide boards which are fixed on the rafter ends and the purlin ands at the gables of the roof construction.

Fascia boards are about 20 to 30 cm wide and 2,5 cm thick. A small rebate can be made into the lower edge of the board to give it a better appearance (Fig. 1).

If the rafter projects below the fascia it can be cut off as shown in Fig. 2.

The eave purlin should be at the correct height so that it does not interfere with the fascia. The fascia boards are fixed before the sheets are laid.

A butt joint is sufficiently strong to use for lengthening the fascia boards, if a fish plate is nailed behind it for strength (Fig. 3). The corner joints should all be mitre joints so that the end grain of the boards is not exposed (Fig. 4).

Before the boards are fixed they should be painted, since it will be difficult to reach some parts later, especially the top edge. Pay special attention to the places where they are joined together. The end grain should be soaked with paint to prevent water from entering there.

VERANDAHS

When a building with a gable or pent roof has a verandah, there are two possible methods for constructing a roof to cover it;

By extending the tie beams to cover the verandah (Fig. 1)

By extending the rafters to cover the verandah (Fig. 2).

REMEMBER: Open verandahs should not face the direction of the strongest winds.

EXTENDING THE TIE BEAMS:

The part of the tie beam that covers the verandah is called the overhang (Fig. 1). For a small overhang, supporting pillars are not necessary, but it should be braced very well. If the overhang is wide, then pillars are needed to support it (Fig. 1).

If the building has a gable roof, an ordinary truss construction can be used, with the span of the truss including the width of the verandah.

Since the parapetted pent roof has a half truss construction with a tie beam, this construction may also be used with it.

EXTENDING THE RAFTERS:

The overhanging rafters cover the verandah. Note that the building must be high enough so that the lower edge of the verandah is not inconveniently low (Fig. 2). This construction can be used with either an ordinary pent roof or a parapetted pent roof as well as with a gable roof.

The feet of the rafters are supported by an eave plate or a concrete eave beam which rests on pillars. The joint between the rafters and the eave plate should be a bird's mouth to ensure proper support for the rafters. The depth of the bird's mouth should be about l/5th of the width of the rafter.

The eave plate is normally set edgewise on the pillars. in most cases 5 x 10 Od-um is large enough for the eave plate. Take care that it is well anchored on the pillars.

There are certain lengthening joints which are often used for joining eave plates. These are described in the chapter on "Lengthening Joints".

The pillars themselves have to be well anchored in the ground, since they not only have to carry the weight of the structure but also have to hold the roof down in heavy storms (see Lean-to Roofs).

At times the builder may choose to construct the verandah roof using a combination of projecting rafters and projecting tie beams.

ANCHORAGE

Whatever the form of the roof construction, it must be tied securely to its supporting walls or pillars. If this is neglected, the roof can easily be damaged or even torn off in strong winds.

The following are some points which are important in anchoring a roof.

ANCHORAGE RODS:

The roof is secured to the walls by these iron rods. They can be fixed in the wall during the wall construction if the positions of the trusses have been decided, or else they can be fixed in the finished wall.

In the first method, the rod is inserted in the bed joint as the third course below the top of the wall is laid. The rod is bent at a right angle so it can fit into the cross joint (Fig. 1); and it is positioned just inside the face of the wall.

When the next course is laid, the block above is chiselled out to make a shallow groove (approximately 1 cm deep) for the rod (Fig. 1).

The rod fits into the next cross joint and is bent gradually inwards toward the top centre of the wall, coming out where it can pass later through the wallplate or concrete belt (Figs. 1 & 2).

When the truss is erected, the rod is bent around the rafter and tie beam both, and secured with nails (Fig. 2).

With the second method, the rod is inserted through a hole chiselled in the bed joint of the third course from the top, after the wall is complete. The ends of the rod are bent up and over the tie beam and rafter, overlap each other, and are secured on both sides with nails (Fig. 3).

The second method is chiefly used when the roof of an existing building has to be secured. If you are constructing a whole new building, it is best to plan ahead a bit and use the first method, which is both easier and stronger.

ANCHOR BEAM:

Once the wall is completed, the top of the wall may be covered with a strip of reinforced concrete all around the outside walls of the building. This is the anchor beam, also known as the ring beam or concrete belt. The roof anchorage passes through this belt. The anchor beam performs three functions:

It keeps the outside walls of the house together, thus strengthening them.

It provides a firm base for the trusses and distributes their weight over the softer landcrete blocks below.

It anchors the trusses and adds weight to help keep the roof construction from being lifted up in heavy winds.

WALL PLATE:

For smaller spans the Rural Builder can install a wooden wall plate instead of an anchor beam. The roof anchorage passes through the wall plate and through at least three courses of blocks.

There are several joints that may be used to connect pieces lengthwise to make the wall plate. The wall plate should be about 5 cm thick by 15 cm wide.

TRUSSES:

The trusses should be evenly spaced across the roof, with one at each gable end and the rest about 2 m apart from each other. Every truss should be anchored to both the walls that it rests upon. The anchorage rod is bent around both the tie beam and the rafter, and secured with nails (Fig. 2).

PURLINS:

Tie all the purlins to every rafter, preferably with iron rods. These are shaped as shown in Fig. 1, by hammering the rod flat on both ends and drilling holes in the ends, then bending the rod to a U-shape.

If these fixings can't be made, it is possible to make wooden fixings (Fig. 2). Two wooden pieces are used, one on either side of the purlin.

Barbed wire can also be used as an alternative to the iron rods. Wrap it firmly around and nail it securely.

Whatever fixing you use, take care to fix the nails so they are not pulled out by the force of the wind on the roof. Fig. 3 shows two badly nailed fixings.

ROOF COVERING

See the Reference Book, pages 207, 230 and 231 for descriptions of the sheet materials and roofing accessories; also see pages 239 and 240, Tables of Figures.

ALIGNMENT OF THE SHEETS

If possible, always start laying the sheets from one end of the roof so that the free ends of the sheets face away from the direction of the wind. This reduces the danger of the sheets being blown away as they are being installed.

Start laying from one end of the building to the other. As each new sheet is laid, lift the edge of the previous one so that it overlaps the new sheet by 2 corrugations. Each sheet u. thus held in position by the one previously fixed, so they are more easily aligned in the correct position.

Exact sidalaps (2 corrugations) and endlaps (15 cm) are essential to make the roof waterproof.

NAILING:

When you nail corrugated roofing sheets to purlins, always nail through the top of the corrugation. This is so that rain will tend to run away from the nail (Fig. 1).

Each nail should be driven in until the roofing felt just touches the sheet. If It is driven further, the nail will flatten the corrugation and distort the sheet. This can cause the roof to leak.

The sheets should be nailed to all the purlins. Nail every second corrugation in the sheets along the eave purlin and along the ridge purlin; and also on the end sheets at the gables. Over the rest of the roof, nail at every third corrugation over the purlins (Fig. 2).

HOW TO LAY THE SHEETS:

Before the sheets can be laid, the gables must be built up to the correct height. Make sure that all the nail heads of the wooden construction are punched well below the surface.

Fix a line on the eave to indicate the desired projection of the sheets. Note that the sheets should project about 2 cm over the fascia board in order to provide a drip overhang.

It helps to station a helper at eave level to observe if the general line of the sheets is straight and that they project uniformly. Another helper can check that the sheets are straight along the corrugations from the eave to the ridge.

The helpers can also check from both sides of the roof to see If the corrugations match at the ridge line (Fig. 1).

For nailing the sheets it is best to have one man at each purlin.

SEQUENCE OF OPERATIONS:

In order to join the overlaps correctly, follow this sequence, starting from the end of the roof which is in the direction where the strongest winds are coining from (here it is the east) (Fig. 1).

a. Place sheets 1 and 2 loosely along the gable end, with sheet 2 overlapping sheet 1 by 15 cm in the length (Fig. 2).

b. Align them wJ.th the help of the two observers on the ground.

c. Secure the sheets with one nail in the middle. One man should do the securing, or the sheets might shift because of the shocks of the hammer blows.

d. Have the observers cteck that the sheets are still straight in both directions.

e. Nail the sheets home with all of the required nails except the nails at the edges where the next sheet will be fixed under.

f. Place sheet 3, lifting up the edge of sheet 1 until it overlaps sheet 3 by two corrugations (Fig. 2).

g. Place sheet 4, lifting sheet 2 until it overlaps sheet 4 by two corrugations. Sheet 4 should overlap sheet 3 in the length by 15 cm (Fig. 2).

h. Align sheets 3 and 4 with the help of the two observers on the ground.

i. Secure the sheets with one nail each at the centre.

j. Recheck the straighthess in two directions.

k. Nail the sheets home with all the required nails except the ones at the edge where the next sheet fits.

1. Continue with this sequence until the whole roof is covered.

m. Proceed to build the parapets if they are required.

If no parapets are made, the last corrugation at the gable ends can be lapped over the fascia board, turned down and nailed onto the face of the fascia. Placo the nails at intervals of about 10 cm.

SIDELAPS

Since corrugations often get distorted or flattened, it can sometimes be difficult to get an exact and watertight sidelap between the sheets.

To straighten the sheet and get a good sidelap, fix all the required nails in the middle corrugation after first securing the sheet with one nail and checking the stralghthess. Then press the sheet to the correct position and nail the rest of the sheet (Fig. 1).

Do not try to get a sheet in the correct position by nailing the corners where the 4 sheets meet. This will distort the sheet even more.

RIDGE CAPS

If no ready-made ridge caps are available, they can be made on the site with aluminium or galvanized iron sheets. With the help of two straight-edged boards, the sheets are bent to the required angle on the work bench (Fig. 2).

It is better to make the ridge cap so that the bend goes across the corrugations, rather than along them. This ensures a tighter fit, but the corrugations on both sides of the roof have to match exactly (see Reference Book, pages 231 & 239).

HELPFUL HINTS FOR ROOF COVERING

Put all nails into the washers with the roofing felt before you start laying sheets, to prevent delays.

The roofing felt can be cut with an iron pipe that has been filed to a cutting edge at one end (Fig. 3).

Use soap to make driving nails easier in hard wood.

There should be people on the ground ready to hand up the sheets.

Before you put a sheet in place, make a mark on the previous sheet to show where the purlin is for nailing.

Always use a line at the eave, with supports if necessary to keep it straight.

Have a long, slim and pointed punch ready. It can be made locally from an iron rod. This punch is used to pierce holes through in places where the nail must pass through 4 sheets. Without piercing the hole for the nail first, there is a danger of flattening the corrugation and distorting the overlap while nailing it.

If a nail begins to bend and should be pulled out, use a round piece of timber or metal as shown in Fig. 4.

Be careful in handling the sheets. They have sharp edges which can cut you deeply.

If a corrugation has been badly damaged through pulling out a nail, try to restore the area around the nail hole to as near as possible the original shape. A round piece of metal can be put under the corrugation to help in this.

If the sheets have to be cut, do not leave the cut-off pieces lying around. They can still be used for other work.

Do not leave sheets lying where people might step on them and flatten the corrugations.

Be sure, especially in seasons where there is a danger of storms, that no sheets are left loosely nailed to the roof, or left on the ground without being weighted down or otherwise secured. A sheet blown around by the wind can easily kill someone.

HEAT PENETRATION - INSULATION

One of the main goals of building in the tropics is to reduce heat penetration into the building. There are a number of building constructions which help to make the building cooler.

Build in an east-west orientation, so the sun hits directly on only the end walls of the building.

Construct a verandah to shade the wall.

Construct overhanging eaves or overhangs to shade the walls.

Use an open roof construction (no ceilings) on the verandahs, overhangs and overhanging eaves; to permit cross ventilation between the roof and ceilings in the rooms.

Make ceilings in the rooms so that the sun cannot heat the room directly through the roofing sheets and so that there is cross ventilation between the sheets and the ceiling.

Whsn possible, use a combination of any or all of the above constructions.

LIGHTENING

To prevent lightening frcsn striking a building, you can install a lightening conductor. This is a pointed copper rod, which is fixed above the highest point of the roof and connected to a copper rod driven into the ground. Long buildings need more than one lightening ,. jd. Follow the manufacturer's instructions to install the conductor.

HIP ROOFS

A hip roof is often chosen as an alternative to a gable roof. It has a nicer appearance, and it is less vulnerable to suction from heavy winds. However it takes more work to construct it, and requires more timber than a gable roof. Also there is a certain amount of waste involved in cutting the sheets at the corners. The hip roof is more apt to leak unless it is constructed exactly right.

The main part of the construction is the same as for the gable roof. in addition, a half truss and two hip trusses are needed for each hip (Fig. 2).

The two hip trusses run diagonally from the ridge of the last full truss to the corners of the building. The hip rafter has to sit higher than the other rafters, so that the purlins butt into it at the side instead of lying on top of It, This is so that the sheets will be able to fit smoothly around the hip (Fig. 1).

The half truss is set out in the same way as the full truss, with only half the span. If the tie beams of the other trusses will project beyond the walls, the half truss tie beam has to project by the same amount (Fig. 3).

SETTING OUT AND CONSTRUCTING THE HALF TRUSS

Before the half truss is fixed, all the other trusses have to be in place.

To assemble the half truss, use the same setting out which was made for the full trusses. Simply construct half of a full truss. The last brace near the ridge should be set back about 10 cm from the end, to leave space for fixing the trusses together (Fig. 1, arrow).

When the half truss is ready, fix it temporarily into place against the full truss. Remember that if the other tie beams project past the walls, the half truss tie beam must also project, by the same distance.

POSITION AND MEASUREMENTS OF THE HIP TRUSS

Follow the sequence below to find the measurements for the hip truss, as well as its correct position.

a. Lay two purlins flatwise near the eave and ridge, across the last 3 full trusses of the roof. Make sure that they are aligned parallel to the wall, and nail them temporarily (Fig. 2).

b. Fix a line along the top surface of each purlin. The lines should extend past the ends of the purlins to the end wall of the building, where they can be held taut by a temporarily fixed pole. The lines must be taut and parallel to the side wall. Each line should touch the whole length of the top surface of the purlin (Fig. 2)

c. Have two men put the hip rafter in position, holding it so that one end is at the ridge and the other end is above the corner of the building. The top surface of the hip rafter should just touch both lines. Thus, the top of the hip rafter is on the same level as the top of the purlins.

d. Now another man can measure the length of the hip truss tie beam, which will be the horizontal distance from the centre of the full truss tie beam (Fig. 2, point a) to the soffit of the hip rafter outside the wall (point b). This distance is the span of the tie beam for the hip truss (see Fig. 3).

e. To obtain the rise of the hip truss, measure the distance from the soffit of

the full truss tie beam to the soffit of the hip truss rafter (Figs. 2 & 3, point a to point c).

Now that you have the measurements for the hip truss, you can go ahead to construct it. Measure the other hip truss for the opposite side of the roof in the same manner.

CONSTRUCTING THE HIP TRUSS

Look at the hip roof design in the Drawing Book, page 108.

SETTING OUT:

a. Drive pegs 1 and 2 in the ground at a distance equal to the span of the hip truss (the span is measured as described on the previous page). Fix a line between the pegs (Fig. 1).

b. Fix a line perpendicular to line 1-2, starting from peg 1 (Fig. 1).

c. Drive peg 3 at a distance equal to the rise of the hip truss (Fig. 1).

ASSEMBLING THE HIP TRUSS:

d. Lay the hip tie beam against pegs 1 and 2 (Fig. 2)

e. Lay the hip rafter against pegs 2 and 3 (Fig. 2). Make sure that the tie beam and rafter are correctly positioned with respect to the pegs.

f. Cut the rafter-to-tie beam joint. Mark the inside end of the tie beam (a) and the inside end of the rafter (b). Make sure that the rafter has a long projection on the eave end (Fig. 3). This projection is trimmed only just before the fascia board is fixed (c).

g. Nail the braces and the fish plates.The last vertical brace on the ridge end is set back (Fig. 3, x) so that there is space to fit the trusses together later. Nail a short vertical brace (Fig. 3, d) where the tie beam meets the supporting wall.

h. Cut the ends of the rafter and tie beam (ends a. and b) at a 45 degree angle, as shown in Fig. 4.

i. Assemble the second hip truss on top of the first one.

ERECTING THE HIP OF THE ROOF

When the half truss and hip trusses are ready, the hip structure of the roof can be erected. The connection between the tie beams of the half truss, hip trusses, and full truss can be made as shown in Fig, 1. The rafter connection is made as shown in Fig. 1 at the beginning of the Hip Roof section.

FIXING THE PURLINS:

The purlins which butt against the hip rafter must be cut at exactly the correct angle so that they will fit snugly against the rafter.

Mark the positions of the purlins on top of the hip rafters, and insert a nail at the mark to support the purlin during marking (Fig. 2).

Place the purlin against the nails and mark the cutting lines. Use a short piece of wood as a guide (Fig. 2). Make the marks on both sides of the purlin at once, then mark both sides at the other end.

Cut the butt joints and fix the purlin,

Fix the rest of the purlins in the same manner. The layout for the purlins on a roof with a single sheet length on each side is shown below (Fig. 3).

COVERING A HIP ROOF

Nail two purlins parallel to the hip rafters, as shown in Figs. 1 and 2. The extra purlins provide more area for nailing the hip ridge cap.

Lay the first sheet at the hip rafter in position and mark with i straight edge where it should be cut. Cut the sheet and nail it in position before you mark and cut the next sheet. Continue in this way until the hip is covered.

A hip cap, similar to the ridge cap, must be used to cover the hip line, where the sheets meet. If no ready-made caps are available, one can be made as explained on page 205.

HIP AND VALLEY ROOF

When a roof is constructed for an L- or U-shaped building, a hip and valley roof is normally used.

The main problem of this construction is to make a truss which provides sufficient nailing area for the valley and also for the hip. This is solved by assembling two trusses, with spacers in between to connect them (Fig. 1). The spacers provide the required distance between the trusses. This distance is important, because the valley should be wide and deep enough so that rain can run down it easily and so it does not get blocked by leaves (Fig. 1).

As in the hip roof, the hip rafters and the valley rafters must be set higher so that the purlins meet them at the sides, and don't lie on top as they do with the other trusses.

If a large span must be covered with a hip and valley truss, it may be necessary to add some half trusses onto the hip and valley truss to provide enough support for the purlins (Fig. 2).

POSITION AND MEASUREMENTS OF THE HIP AND VALLEY ROOF

Start by erecting poles in the positions shown in Fig. 1, and fix lines at the ridge.

Erect the ordinary trusses according to the lines in their correct places (Fig. 2). Refer to the section on erecting a gable roof.

Lay purlins temporarily on top of the rafters, as was done with the hip roof.

Fix lines parallel to the walls along the tops of these purlins, at eave height on both sides of the roof (Fig. 2).

Set the ridge lines higher, by the amount of the purlin thickness (Fig. 2).

Now you will have three points where the lines cross. These are: point A at the ridge; point B at eave height, and point C on the other side at eave height (Fig. 2). These points mark the outer surfaces of the hip and valley truss,

Next construct a lightweight, temporary model truss. Hold it in position on top of the roof (Fig. 3). Adjust it until points A, B, and C touch the outer surface of the truss.

Now the model truss will have the correct dimensions and can be used as a model for constructing the two trusses of the hip and valley truss.

CONSTRUCTION OF A HIP AND VALLEY TRUSS

Take the model truss down and set it on the ground in a cleared and level spot.

Place an iron peg at each side of the model where the rafters meet the tie beam (Fig. 1, pegs 1 and 2) and one peg at the ridge (peg 3). Remove the model truss.

Now lay the actual tie beam on the ground against the pegs.

Lay the two rafters on the ground in the correct positions. Be sure that the members are in the correct positions with respect to the pegs, as in Fig. 1.

Cut the rafter-to-tie beam joint, and the ridge joint (Fig. 2).

Nail the braces. Take care that the braces are set back from the top of the rafters on the valley side, so there is room to construct the valley later (Fig. 3).

Assemble the second truss on top of the first one. Be careful to nail the braces so that later when the two trusses are combined, the braces will all be on the inside (Fig. 4).

When both trusses are ready, they can be set in place on top of the wall. Then the spacers can be nailed to keep the trusses at the correct distance apart. Remember that the distance between the trusses should be wide enough (approximately 15 cm) to provide a wide valley.

COVERING A HIP AND VALLEY ROOF

First fix the fascia boards and the purlins. Fix a line at eave level and cut the rafters according to this line. Then nail the eave purlin and the fascia boards.

At the place where the fascia boards meet at the valley, cuts have to be made in the fascia boards according to the shape of the gutter (Fig. 1).

Now fix the gutter in the valley. Lay a strip of sheeting metal lengthwise in the valley in the form of the required gutter. Fix it there temporarily with nails. The final fixing will be done as the other sheets are laid, since they have to be laid on top of the gutter. The overlap for the gutter sheets should be about 30 cm in the length.

Now lay the first sheet into position at the valley, mark and cut it. Nail this sheet and place the next sheet in position. Continue in this way until the hip and valley are covered.

Install the hip cap as explained for the hip roof.

The ridge cap on the main ridge is always covered last, since it has to overlap all the rest.