Building a flat roof

Flat roofs are common on a large number of small buildings such as house extensions, garages and porches. Their popularity lies in the fact that they are easily constructed—the same lengths of timber serving as both rafters and ceiling joists—and are relatively inexpensive to build since fewer materials are required.

They are equally suited to buildings which stand on their own or to those, such as house extensions, which are attached to an existing building. And unlike pitched roofs they can be finished in a number of relatively inexpensive materials. Felt, for instance, is more easily laid than the more traditional slates or tiles but provides a resilient and waterproof roof covering.

Here the erection of the wall plates and rafters is covered. The next section of this project, deals with the actual roof covering.

Pitching a flat roof

Despite their name, flat roofs are never completely flat—they are built with a slight pitch or slope allowing rainwater to run off quickly. The exact amount of pitch varies greatly. The minimum pitch is 1:60, and may be as much as 1:6 or more before the roof technically becomes a lean-to.

However, the best pitch to aim for on most types of flat roof is around 1:36. This will allow rain to run away freely and at the same time provide a roof which is pleasing to look at.

Parts of a flat roof

Before attempting to build a flat roof, it is essential to understand the component parts:

Wall plates: These are lengths of timber 100mm wide and 50mm deep which are bedded on top of the side walls to form a level surface on which to support and secure the rafters. With masonry walls of the cavity type, the plates should be laid on top of the inner leaf and the brickwork on the outer wall built up level with the top of the timber plate to provide adequate support. In a timber-framed house, the wall plate is simply another length of timber nailed to the top plate. Joist hangers: With many projects, your fiat roof will be built against an existing wall of the house.

In the UK, the modern method of attaching the ends of the roof rafters to the side of this wall is by the use of special steel joist hangers which are built into the existing wall. These are available in a number of sizes, from large builders’ merchants or timber yards, to accommodate any size of rafter.

A second type of joist hanger is used to join rafters running at right-angles to one another. These are utilized to form a frame around a projection on the existing wall, such as a chimney or an external corner.

Where the existing building is timber-framed, there are other methods of attaching the roof joists to the side of the wall. If the building is brick-clad, with full-size bricks, then joist hangers may be used if these are available. An alternative method might be to remove the covering from the wall and nail noggins–short horizontal timbers-between the studs of the wall. Then rest the ends of the roof rafters on the noggins and nail them securely in place.

Rafters/joists: On a flat roof, the same timbers usually act both as roof rafters and ceiling joists. They are normally set at 400mm centres; use only sound, well-seasoned timber.

Most rafters are 50mm thick, but their depth has to be carefully calculated according to the span of the roof and the load they are intended to bear. The panel above gives sizes for the UK; in Canada, joist sizes depend on the snow loads in your area, and on the quality and species of timber you use they could be as much as twice those shown in the panel. Consult your local building code to find out the right size.

Another consideration is space for insulation. This is usually placed within the thickness of the rafters, and there must be a clear space between the top of the rafters and the bottom of the roof covering for ventilation to prevent condensation forming and leading to rot. In the UK. 75mm to 100mm of insulation plus a 50mm air gap is usually sufficient, and all but the smallest rafters are deep enough to accommodate this.

In Canada, though, the air gap should be at least 150mm, and insulation may be 200mm or more in thickness. Rather than use a solid rafter.

Firring pieces: These are tapered lengths of timber the same width as the rafter to which they are fixed. They give the required pitch to the roof and mean that while the outside of the roof slopes to carry away rainwater, the internal ceiling actually remains level.

Where the pitch of the roof runs at right-angles to the rafters, you must decide which way you want the roof jj to slope and nail the firring pieces across the timbers accordingly. -a Firring pieces can be cut on site a using a circular power saw but this is $ often a tedious business and it is difficult to judge such fine angles accurately. It is far better to carefully work out the pitch of the roof beforehand and then ask the timber yard to cut a number of firring pieci-s for you while you are ordering the rest of the timber. Many timber merchants supply ready-cut firring pieces in a number of standard pitches and these must simply be cut to length. Sarking: Decking or sarking is the name given to the timbers which cover the rafters and are used as a base for the final covering. Rough sawn timber of a suitable length, measuring roughly 150mm x 25mm is usually employed, though tongued-and-grooved boarding gives better results. Sheets of 18mm chipboard coated with bitumen can be used as an alternative.

Fascias: These are wide boards fixed around the edge of the roof and nailed firmly to the ends of the rafters. As well as having a purely decorative function, they protect the roofing timbers from the intrusion of damp and provide a secure fixing for the guttering.

Soffits: These are timber coverings fixed to the underside of the rafters where they protrude beyond the wall of the building. They protect the roof timber from damp as well as stopping birds from nesting under the eaves and between the rafters. Restraint straps: In a high wind, even the most solidly built roof is likely to suffer damage. Steel restraint straps—available from most builders’ merchants—can be fixed across the tops of the rafters immediately above the line of the wall plates to help keep the roof intact under such conditions.

Strutting: Even wood which has been well seasoned will tend to warp when it is used as a rafter- to prevent this. wood or metal struts are used to make the whole structure more rigid. Special steel struts or pieces of 50mm x 50mm timber can be used, fixed in an ‘X’ shape. Solid timber blocking fixed squarely between the joists can also be used- but the timber must be less deep than the joists so as to maintain the right size of air gap. Fix blocking so that its lower edge is flush with the lower edge of the joists.

The straps are about 25mm wide and are made in a variety of sizes so that the roof can easily be spanned using a number of them.

Planning the work

Time spent in calculating the type and amount of materials you need—as well as planning the type of roof you require—prevents expensive mistakes later on.

Start by working through the list of components and jot down the materials you require. Th^n calculate the pitch of the roof using a ratio of 1:36 and relating this to the span which needs to be covered. From this you can work out the angle of the fining pieces.

When calculating the number and type of rafters that you need, remember always to order one size larger if you are in any doubt about the load they might have to bear. All of the rafters should be at least 150mm longer than the span of the roof so that they can protrude over the edge and help stop damp and rain intruding into the upper brickwork.

Once you have compiled your list of necessary materials, try to select sound and well-seasoned wood from the timber yard—especially that for the rafters. Timber with large knots should be avoided and if the wood has not been tanalised or treated against rot and woodworm—leaving it a tell-tale green colour—it should be coated before fixing with a suitable wood preservative. Care needs to be taken to treat any wood which is cut during construction in the the same way. Pay special attention to end grains which are particularly prone to dry and wet ro+

Setting the wall plate

Start the building work by putting the wall plates into position on top of the inner leaf of the supporting walls. Two plates are needed when dealing with a detached building and only one plate where the roof joins an existing wall.

It is best to prepare for the wall plates by leaving the inner leaf one course of bricks lower than the outer skin. Once a thick bed of mortar is laid on top of the inner leaf, the wall plate should be exactly level with the outside wall when it is in place.

Mix up a 1:3 mortar and trowel it thickly across the top of the inner leaf, taking care not to allow any mixture to drop into the cavity. While the mortar is still wet, lift the plate into position and place it squarely on top of the inner leaf. By tapping it gently with the heel of the trowel handle it should be possible to bed it correctly.

Use a spirit level to ensure that it is perfectly aligned with the outer leaf and then scrape away excess mortar with the blade of the trowel. If the plate is found to be too low, remove it and add more mortar before continuing work.

Fixing joist hangers

While the mortar sets beneath the wall plate, you can fix joist hangers along the existing wall, if this is the method of attachment you are using. First, measure the distance from the floor to the top of the wall plate on the opposite wall. If the floor is level this measurement should be transferred across to the other wall and used as a guide for positioning the joist hangers.


Start from the centre of the wall and work towards both ends, marking where the holes are to be cut out. The centre of each hanger should be 400mm apart and the bottom of each hanger exactly level with the top of the wall plate on the opposite wall.

Once you have marked and checked each of the hanger positions and are confident that they are aligned exactly, start to cut out the slots using a plugging chisel and a hammer. Be sure to wear safety glasses when doing this to protect your eyes from dust and j flying masonry. a The holes should be about 75mm gwide, the depth of a hanger bracket m and run about 65mm into the brickwork. Try to widen them towards the back so that mortar can more easily be inserted once the hangers are positioned.

If any holes prove difficult to cut due to the hardness of the brick, use a power drill fitted with a masonry bit to make a number of holes along the line of each slot. Then use a plugging chisel and a club hammer to cut away the brickwork joining the holes to leave a neat, well-tailored slot to accommodate the bracket.

Once all of the holes have been cut, and the hanger tried for size, make sure that each cavity is free of dirt and loose particles. Any dust that is difficult to reach be removed by using an old plastic detergent bottle as a bellows to blow the hole clean. Then fill the container with water and use it to douse all of the brickwork around the inside of the hole. This will Wash away any traces of loose dust and help the mortar to adhere once it is pushed into the slots.

To bed the hangers into position, prepare a 1:3 mix of mortar and use a trowel to feed the material into each hole until it is about 10mm from the opening. While the mixture is still pliable, drive each of the hangers in turn into place and adjust their final position by inserting pieces of slate coated with mortar around the edges of the aperture.

Once you are certain that the hangers are aligned correctly, point around the outside with mortar and leave both the wall plates and hangers to set for at least 24 hours before fitting the rafters.

Erecting the rafters

Once both the wall plate and the joist hangers are firmly in position, each of the rafters in turn should be slotted into place, fixed into the hangers, and rested on top of the plate. Leave the rafters overhanging the eaves by at least 150mm so that they can later be cut to length.

Check that the rafters are correctly aligned and exactly parallel to one another by measuring the distance between them at top and bottom. Mark their positions with chalk on the top of the wall plate in case they are accidentally displaced later while you are working.

The rafters should be secured with pairs of struts nailed between the facing sides. Position the struts in a line mid-way across the span and fix them with galvanized nails, hammered through into the rafters themselves.

The structure should be further strengthened at this stage by fixing restraining straps across the top of the rafters directly above the line of the wall plate. The straps should be indented into the woodwork so that a level surface is maintained for the decking. Do this by marking where each of the straps crosses the rafters and then chiselling a channel to accommodate them. Secure the straps to the rafter and the wall using sheradized screws and wall plugs.

Finally, to prepare the roof for almost any eventuality—such as a high wind—strengthen it further by attaching the foot of the rafters to the wall plate. Although this is often done by skew nailing through the lower end of each rafter into the wall plate below, a much neater method is to use right-angle steel brackets. Place these in position and use- galvanized nails or screws to fix them both into the side of each rafter and to the top of the plate.

Trimming the rafters

If the rafters have been erected correctly, they should protrude well beyond the eaves and your first job is to cut them to length. The exact distance depends on a number of factors including the height of the building, the area and pitch of the roof, and also how the building is to look once it is finished. If you are building an extension, the figure should be specified in the plans. But as a general guide, the roof should run about 100mm beyond the eaves. This protects the top of the wall from penetrating damp. In Canada, the overhang is typically 450mm to 600mm.

It is important to mark and cut the rafters accurately so that the roof line is left level and square. To do this, first measure and mark the two end rafters to the correct length using a try square and plumb line as a guide. Then stretch a chalk line along the top of the roof between these two marks.

If the line is pulled tight and then plucked, it will leave a mark across the top of the rafters so that each can be squared off accurately. This mark can then be transferred onto the vertical face of each rafter—again using a plumb line—so that the timber can be sawn accurately.

Where your building plans specify that the roof projects on more than one side of the building, this can be achieved by skew nailing stub noggins the same size as the rafters at right-angles to the main roof. These are spaced at 400mm centres and laid on a wall plate bedded on the inner leaf of the side wall. A new rafter is then nailed across the ends of the noggins, which are cut to length so that the new roof protrudes over the eaves by the right amount.

Fascias and soffits

To provide a clean cut finish to the edge of the roof and to help protect it from the rain, fascia boards and soffits should be fixed to each overhang before the timbers are covered. You can use fioorboarding for these, providing they are at least 25mm deep to give them protection against the , weather. Further waterproofing should be provided by applying a clear preservative and an undercoat of paint to the timber before it is fixed in place —a topcoat can be added once the roof is finished.

Attach the boarding with galvanized nails across the bottom of the roof and under the eaves, but leave a small 10mm gap between the back of the soffit board and the wall to help ventilate the roof. Finally, fix a batten of wood 25mm deep and 50mm wide along the top edge of the eaves. This will be used later to secure the felt tightly around the front edge of the roof.

Firrings and sarking

Once all the main roof timbers are in position, fix the fining pieces with galvanized nails. These can run at right-angles to the main timbers or be placed directly on top of the rafters, according to how you want the roof to slope. But if you nail them across the rafters, spacing should not be more than 300mm because of the increased loadbearing factor.

With the firring pieces positioned, nail sarking on top to form a firm base for the felt covering. The sarking can consist of boarding or impregnated roofing chipboard or, in Canada, sheathing-grade plywood.

When using flat or tongued-and-grooved boarding, start from the bottom of the roof and work upwards, driving two nails into each board where it crosses a rafter. If you hammer each nail in at an angle, the boards will be held more securely and the head will sink into the surface. Tongued-and-grooved boarding should be cramped during fixing to stop the joints from spreading.

Flat boards have a tendency to ride up and warp even if they are nailed securely, but this can be avoided by nailing them heart side downwards.

If you are using chipboard or plywood to cover the roof timbers, this is usually supplied in standard sheet sizes and must be tailored to fit. The sheets should meet half way across a rafter and be fixed at regular intervals with galvanized nails in a similar way to the boarding.

Tying in the roof

If the roof butts against a masonry wall, you should, at this stage, cut a channel—or chase—into which you can insert flashing once the roofing felt has been laid. Doing this before you apply the final covering stops dirt and dust getting mixed up with the felt and bitumen.

The chase must run through a bed joint in the brickwork of the adjoining wall, approximately 150mm above the sarking to prevent rain splashing up from the roof during a heavy storm and intruding into the wall below.

Cut out the chase with a hammer and a plugging chisel or bolster, wearing safety glasses to protect your eyes. Dig out the mortar and chop away the brickwork with a bolster above the joint to form a channel 25mm wide and about 40mm deep. Once you have cut a neat groove all along the wall, and made sure that it is brushed free of any dirt and loose material, you can proceed to lay the roofing felt.

Applying the felt

Although sheet metal is often used to cover a flat roof, felt is far more popular since it is inexpensive, easy to lay and yet provides a resilient waterproof covering. It is usually laid in at least three courses: an underlay fixed with nails, followed by two layers bonded with hot bitumen. The felt will be more manageable and contain less creases if it is unrolled and left for 24 hours before fixing. To save space, cut it roughly to length as you unroll it then stack the lengths, one on top of the other, in some suitable outdoor space.

Do the cutting with either a sharp handyman’s knife or the blade of a plasterer’s trowel. By running either of these along a metal straightedge, the felt can be scored and then picked up and cleanly broken.

In order to make the roof completely watertight, a number of flashings are required; these can either be bought ready-made in a number of standard sizes or formed from bend-able rolls of galvanized zinc. For a typical roof adjoining another building, you need three flashings—two 150mm wide and another 250mm wide —long enough to stretch the width of the roof. These should be prepared before you lay the felt so that they are ready to fix in position.

Since a large quantity of bitumen must be heated to fix the flashings and top layers of felt, hire a specially-made gas burner, pot and galvanized bucket for the purpose; attempting to fix up some type of home-made equipment is dangerous and makes the whole job more difficult.

When mixing and handling the bitumen—which comes in the form of large hard lumps before it is heated— take special precautions to prevent any of it accidentally spilling and coming into contact with bare skin. Wear a sturdy pair of boots to protect your feet and heavy-duty rubber gloves with sleeves rolled down to cover your hands and arms. Use the galvanized bucket to carefully trans- fer the material once it is heated.

How you fix the felt at the edge of the roof depends on how your roof is designed. If the sides are capped by parapet walls, try to keep all three layers of felt as close to the base of the parapet as possible so that a flashing can be inserted once the roof has been covered. Where the roof protrudes at the sides or is detached, lap each layer of felt over the edge by 75mm and nail it to the fascia.

Lay the first layer of felt directly on top of the sarking. Start from the edge of the roof and unroll it in the opposite direction to the fall, random nailing as you go with galvanized nails spaced at 150mm centres.

Lengths should overlap each other by at least 75mm, each overlap being nailed firmly to the sarking below. Once you reach the top of the roof, cut each length to size and allow it to ride up the adjoining wall by 25mm.

The next stage is to fix the two smaller flashings in position at right-angles, one along the top of the eaves, the other where the roof butts against the adjoining wall. Pour a small quantity of the hot bitumen onto the flashings, then carefully lift them into place and push hard against the roof surface to bed the material securely.

Now lay the second layer of felt, starting from the top of the flashing you have just fixed at the eaves. Pour a thin coating of bitumen on top of the roof just in front of where you are working and unroll the felt slowly on to it, adding more bitumen as you go. Take special care to overlap adjoining lengths neatly so that you do not leave unsightly lines which might show through once the roof is finished.

When you reach the top of the roof, continue over the flashing and cut each length of felt so that it can be tucked into the bottom of the chase.

The third and final layer is now laid, starting at the bottom edge of the roof and ending at the foot of the wall at the top. Again, this should be bonded to the surface below with hot bitumen and each length overlapped by at least 75mm.

The final step is to position the large cover flashing across the junction between the roof and the adjoining wall. This should be tailored to fit so that it tucks into the top of the chase above the second layer of felt.

To finish off the chase and make it completely waterproof you must point the gap left above the cover flashing with 1:3 mortar. But to hold the flashing temporarily in position while you do this, first cut a number of wooden wedges and push them in to the chase.

Next wet the opening thoroughly and push the mortar into the gap using a small pointing trowel. Leave this to harden for at least 24 hours then, when you are confident that the flashing is held securely, remove the wedges and finish off the chase with fresh mortar.

Against timber-clad walls, it is usual to take all the layers of felt at least 150mm up the wall behind both the siding and the wall-sheathing paper. The building paper and siding are then replaced, leaving a gap of about 50mm between the bottom of the siding and the top of the quadrant fillet.

Watertight finishes

To prevent surface water blowing across the roof and penetrating the brickwork, a DPC must be inserted into any parapet walls which exist. This must be at least 150mm above roof level, and the base bedded on top of the roof with hot bitumen.

The outside of the roof is now finished except for the installation of guttering along its lower end. But if you want to use the area as a roof garden or seating space, special asbestos tiles must be applied over the felt to help spread the load. Remember that if the roof is to be used regularly for this purpose, safety rails must be constructed around the outside of the area and stairs or a first floor door built to provide safe access. All of this will require building permission. layers of feltdecking firring piece

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