Repairs to flashings | roof repair

The repairs to flashings for the main roof are intricate. Where a two-way roof meets an end wall, for instance, a stepped flashing is required to follow the slope of the roof. Much more complicated is the arrangement round a chimney stack, with an apron flashing on the lower side, stepped flashing on perhaps two others, a back gutter, and perhaps even soakers – small pieces of lead, copper, zinc, or bituminous felt used to weatherproof the joints between these or bituminous felt. Given all these complications, and considering the difficulty of access, it is perhaps better to employ a professional when renewal is necessary, although you might feel confident about tackling the minor repairs of flashings. Renewing a straight, horizontal flashing, especially on the roof of a small lean-to building where access would be easy and is certainly much simpler. For the new repairs to flashings, you could use lead, but bituminous felt is less expensive. Remove the old flashing, and rake out the mortar joint to a depth of about 25 mm. Clean the brickwork and tiles, a wire brush is suitable for this.

Roof FlashingIf there is a small mortar fillet where roof and wall meet, inspect it for damage, and repair it if necessary, as described in the next section. Bend the flashing over then push it into the join and wedge it there. Next bend it over the tiles, sealing it with mastic to stop it from curling. Then repoint the mortar joint. You may prefer to use a proprietary flashing, which is sold in home improvement stores and builders merchants. The method of applying such flashing strips varies, so always follow the manufacturer’s instructions. These strips usually have a backing tape, which you peel off to reveal an adhesive. Some are in grey plastic, to simulate lead; others have an aluminum foil covering. The fault in a flashing may not be so drastic as to call for complete renewal. For instance, the repairs to flashings itself may be sound, but the mortar in the joint might have deteriorated, causing the flashing to fall away. In that case, rake out the joint, put the flashing back into place, and re-point. Cracks in an old flashing can be sealed with bituminous mastic. The method for doing this is as for dealing with cracks in a valley.


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.