How to build your own flat roof

While the most common type of house roof is pitched and covered with tiles or slates, there are other kinds of covering and you may have one on your property. The main roof can be flat rather than pitched or there may be a flat roof on a room extension, garage or other small-scale construction; some roofs are corrugated and you may have a shed with a roof covered in roofing felt. Whatever the type of roof, it is worth checking for signs of deterioration – where repairs are needed, it may be possible for you to carry THEM OUT yourself.


A faulty roof may be indicated by a damp patch on a ceiling. Tracing the source ofa leak in a flat roof can be difficult since water may have travelled some distance before it shows as a damp patch. Therefore looking at a roof directly above a damp patch will not necessarily show where the problem lies and you should examine the entire roof for damage.

If the roof looks in reasonable condition, it should be sufficient to apply two coats of heavy duty liquid bitumen coaling to waterproof it. Alternatively you can apply an overall plastic membrane – this will be covered later in the Course. Both treatments can be carried out on roofing felt, asphalt, zinc, lead or concrete.

BITUMEN PROOFING Before applying this waterproofing liquid, remove dirt and dust with a wire brush and a knife or wallpaper scraper and sweep the roof to remove debris. If there is any moss and lichen, scrape it oil’and treat the roof surface with a proprietary fungicide solution, allowing this to dry before carrying out the waterproofing treatment. Bitumen proofing can be applied by brush to damp as well as dry surfaces; you will find dipping the brush in water from time to time makes it easier to spread the bitumen. Make sure the first coat is thoroughly dry before applying the second.

Where there is more serious damage to the roof covering, such as severe cracks, loose areas and bad wrinkles, the roof should be stripped and the covering replaced. In the case of an asphalt roof. this work should be left to a professional. Where a concrete roof has cracked, trowel on a mortar mix to fill the cracks before treating with bitumen proofing. Where the roof has a timber base, check the timber is sound and remove and replace any unsound boards before carrying out further repairs. Roofing felt Flat roofs covered with bituminous roofing felt are quite easy to strip and replace. On house extension or garage roofs there are usually three layers of felt fixed down over a roof deck of timber boards or chipboard sheets. Tear off the old felt, using a sharp knife to cut round the nails. It is best to wear tough gloves when doing this job since the felt is rough and may cut or scratch your hands. Remove the old fixing nails with a claw hammer or hammer them as flush with the surface as possible. Sweep the surface clean and check it is smooth high points can be taken off with an abrasive disc fitted to an electric drill and depressions levelled with an exterior grade filler. Nail a 50 x 38mm batten along the outside edge or verge of the roof to serve as a drip rail over which felt will later be fixed to form an apron; this weathcrproofs the edge of the roof and throws water clear of the walls of the building or into a gutter, if one is fitted. If the fascia board projects upwards at the verge, you should also nail a 75mm wide angled timber fillet behind the fascia. Before you buy fell, consult your local building inspector to check that the type you have in mind complies with the current Building Regulations. Felt manufacturers make recommendations regarding the kind of felt to be fitted to suit the type of roof and will supply comprehensive fixing instructions. Where possible, cut the felt roughly to length and leave it flat for at least 24 hours before fixing to minimize the effects of subsequent curling and stretching. Usually, you will have to fix the first layer of felt to the roof boards with galvanized clout nails at 150mm intervals. Fix from the centre of the sheet outwards to prevent wrinkles and bubbles forming and secure subsequent layers of felt by brushing on roofing felt adhesive. Also apply roofing felt adhesive over the surface of the top layer of felt and sprinkle on small stone chippings to protect the felt and give a non-slip surface – or you can spread on a special chipping compound. Protect the gutter from the chippings by blocking it with newspaper or rags while you are carrying out this operation. To form the weatherproofing apron at the eaves, the middle layer of felt should be cut a little shorter than the other layers; cut a separate strip, nail it to the drip rail with galvanized clout nails and double the edge over to lie flush with, and butting against, the second layer. At the verge, a similar piece of felt is nailed to the drip rail and doubled over to form the apron and then taken over the roof to overlap the top layer of felt.

Zinc roofing Normally a zinc roof is constructed in tiers called drips and each drip is subdivided into sections or zinc trays: the trays are separated by timber battens or rolls which are also covered with zinc. While extensive repairs are probably best left to a roofing specialist, you can replace a damaged tray yourself. Where the tray meets a retaining wall there will be a flashing overlapping the tray. Lever this up and use a claw hammer or pincers to remove the nails from the zinc capping covering the rolls. Open the welted joint at the bottom edge of the tray, release the retaining clips which hold the tray in place and slide the tray free. Use the old tray as a pattern to cut the new one to size with tin snips. Repeat for the cappings, remembering to make sure on both occasions you fold out the welted joint to cut the new zinc to the right size.

Using a batten of suitable length as a former, bend up the sides and end of the new zinc to form the upstands of the tray, clean all round with wire wool, and solder the corners. Fold the capping in the same way as the old capping. Place the new tray in position, lap the flashing over it and butt the upturned sides hard up against the rolls, folding the retaining clips over the upstands. Secure the new capping in place with galvanized nails and solder the edges of the capping at the roll end where they butt against the zinc tray underneath.

Alternatively you can strip the zinc covering and replace it with roofing felt. You can replace the zinc with metal-faced glass fibre bitumen sheeting. but this type of work is best left to a specialist. LEAD ROOFING Normally a lead roof should last for a long time; but if there are small damaged patches in a roof, you can solder on small pieces of lead to cover the damage. Buying sheets of lead for more extensive repair work is expensive and it is cheaper and simpler to strip off the lead and apply three layers of bituminous felt.


A single sheet of mineral-surfaced roofing felt is normally used to waterproof a shed roof, but sometimes it may be covered with bitumen strip slates which simulate a tiled roof. In both cases, you can patch small tears and cracks with new pieces of roofing felt stuck down with roofing felt adhesive. If the covering is in very bad condition, it should be removed and replaced. Before carrying out replacement, remove any protruding nails with a claw hammer or drive them in flush with the roof. FELT COVERING Lay new bituminous roofing felt in wide strips which run along the length of the roof. The strips should be cut and laid out for at least hours before fixing. Start fixing at eaves or gutter level and overlap adjacent strips by 75mm, finishing off at the ridge or apex. Nail them down using 13mm galvanized clout nails at about 50mm intervals around the edges. Fold under exposed edges at the eaves and verges before nailing. For extra weather protection, secure the felt overlaps with roofing felt adhesive. Seal the ridge with a 300mm wide strip of felt fixed with adhesive and clout nails along the edges.

BITUMEN STRIP SLATES These are fixed in place with galvanized clout nails. Start at the eaves and work up the roof to the ridge, making sure the strips are laid with staggered joints. With some types of strip slates, you should stick down the exposed part of each strip by melting the underside of the strip with a blowlamp: check the maker’s instructions.


Plastic, glass fibre, asbestos-cement, galvanized steel and aluminium are frequently used for corrugated rooting. In all cases, small cracks and holes can be repaired by patching with self-adhesive foil-backed flashing strip.

PATCHING Use a wire brush to clean the damaged area; cut the patch so it overlaps this area by at least 50mm all round and press it on. YOU should prime an asbestos-cement surface first with flashing strip primer to ensure the patch adheres firmly. As an alternative method, you can repair holes using the glass fibre matting repair kits sold 3C for car bodywork repairs. When the repairs are complete, resurface the entire roof with two coats of heavy duty liquid bitumen proofing. This will seal pin-prick holes or porous areas on most surfaces, but cannot be used on thin plastic sheeting. REPLACING CORRUGATED SHEET Where there is extensive damage you should replace the damaged sheets. Use a claw hammer to remove the nails from an old sheet, then ease up the flashing and pull out the sheet. Cut the new sheet using the old sheet as a pattern. Put wood blocks under the third corrugation of a sheet next to the gap which the new sheet will fill and slide the new sheet under the raised sheet and over the sheet on the opposite side, checking it is properly in place. Remove the blocks and drill holes to take the fixing screws which should penetrate the high points of the corrugations, not the valleys. If necessary, replace the flashing with self-adhesive flashing strip.

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