Refelting a flat roof

To make sure you comply with local bye-laws, find out from your local authority what type of felt you should use and the fixing method (there are fire precautions as well as waterproofing considerations to take into account). Generally the refelting a flat roof procedure is to buy the felt in advance, cut it to size with a sharp knife, and let it lie flat for a day or two to uncurl. If you have the most common type of flat roof, one with a decking of planks, the old felt will be fixed with nails. Removing this will be a dirty job, so beware blocking the downpipes. Prise up the top layers of felt with a tough paint scraper, an old chisel, or a long-bladed knife. lf you have any trouble, try a garden spade. Its sharp blade will probe under the layers and the long handle gives plenty of leverage. The bottom layer is almost certain to be fixed with clout nails. Remove these with a claw hammer or a tack lifter (a tool that looks like a screwdriver, but with a V-shape cut in its blade).

Once the nails have been removed, you should be able to lift the felt clear. Inspect the timber decking for faults. You may have to replace missing nails; there should be two per board, per rafter. All heads, on old or new nails, should be punched well home. Some of the boards may have swelled slightly, because water finding its way through the defective felt has soaked into them. lf the swelling is slight, plane the board down flat. Where it is severe, the board may have to be replaced. Any boards that are damaged, or any areas affected by rot, should be removed and replaced. New wood should be treated with preservative; old wood may need this too.

refelting flat roofDo not use creosote, because this reacts adversely with the bitumen that is used both for the manufacture and the bonding of the felt. On a flat roofed lean to, look at the flashing. If it is sound, roll it back carefully so that it can be repositioned once the job is done. Should it be defective, remove it and install a new piece after the felt is in place. The first layer of refelting a flat roof is fixed with galvanized clout nails. Use 20 mm extra large headed nails, spaced at 150mm centres. Begin nailing in the centre of each sheet and work outwards, to make sure that it lies completely flat and there are no air pockets, otherwise bubbling might occur later. Overlaps should be at least 50mm wide and here the nailing should be at 50mm (2 in) centres. Subsequent layers of felt are bonded in place. Professionals use a hot bonding method, but a do-it-yourselfer might prefer a cold bonding method, using a suitable mastic adhesive sold at builders’ merchants. Each length is stuck down in two halves. With the length laid flat in position on the roof, take one end, and roll half back on the other half. Apply the mastic to the roof, and roll the felt back, carefully bedding it into position. Work from the centre outwards to all edges to make sure no air pockets are trapped underneath.

Now roll the second half back on this first one, apply adhesive to the roof and stick the felt down in position, using the same method. Carry on in this way with subsequent lengths, until the roof is covered. The cap sheet, as roofers call the top felt, is fixed similarly. Make sure, though, that the joins do not coincide, as described in the introduction to this refelting a flat roof section. The edges are finished off with welted aprons. The apron at the eaves needs to project well into the gutter so that rainwater will be thrown well clear of the structure. Do not try to economize by cutting these aprons across the width of a roll. Cut along the length so that the material folds easily. Finish off by bonding chippings at the rate of 100 kg per 5 sq. m (122 lb per 9 sq. ft) of roof. The chippings should be about l3 mm across and can be of limestone, granite, gravel, or calcinated flint. Do not skimp on these, for the sun will damage the felt if you do. Other decking may have been used on a recently built flat roof Plywood or chipboard is treated in the same way as planks.

Where other materials are used, however, some points are worth noting. Joints between sheets of compressed strawboard need to be sealed by tape. If, when you refelting a flat roof, you rind that the tape has been damaged, you should renew it. The tape, which is l00 mm wide, is sold at builders merchants. The surface of the board should be clean, dry and free from dust. Seal it with a special primer; do not fix this board with clout nails. Instead, nail the first layer with aluminium serrated nails; or bond it in the way described for the top two sheets on a plank decking. Top layers are bonded in this way. A concrete decking needs a thorough brushing to remove any bits that are loose, and any holes or depressions should be filled. For this you can use an exterior-grade filler, or a sand and cement mix. The concrete should then be treated with a primer, which will be sold at the outlet where you buy the roofing felt. This seals the surface and ensures good adhesion. When the primer is dry, you can fix all the layers of felt by the cold bonding method already described.