Repairing Flashings

Flashings are the watertight seals between the roof covering and the walls, chimney stacks or parapets. Flat roofed houses generally have more than the pitched roof sort, so there's more to go wrong.

Older flashings are usually zinc or lead. Modern houses and ones where the flashings have been renewed at some point make use of aluminium alloy, semi-rigid bituminized asbestos and even roofing felt. It is also quite common to find flashings made simply of mortar, particularly on outhouses and garages. In this case it's the profile of the flashing that keeps water out, rather than the material itself.

Whatever your flashings are made of, there is one repair material that you will find indispensible — self adhesive bitumen backed aluminium flashing, or flashing tape for short. This comes in rolls in a variety of widths — generally between 100mm and 250mm — and is often sold together with its own bitumen based primer. You cut the tape with a trimming knife, press it onto the primed surface and then consolidate it with a seam roller, old rolling pin or even a hammer handle.

Cracks and tears:

These are easily mended with flashing tape, whatever the existing flashing material, but if the run as a whole has deteriorated you may be wiser to replace it rather than trying to patch it up.

Clean the area around the tear thoroughly, using wire wool or a damp cloth as appropriate.

Paint on the special primer, using an old paintbrush, to cover an area at least 75mm all around the crack. Leave to dry.

Cut a piece of tape, using a trimming knife and straightedge, to extend about 50mm all around the crack. Peel off the backing paper.

Lay the patch in the centre of the primed area and press home. Use your smoothing tool to smooth out any air bubbles. Continue consolidating until the adhesive backing just begins to squeeze out around the edges of the patch.

Large Sections:

1. If a run, or part of a run, of metal flashing needs replacing, remove it and substitute flashing tape.

2. The old flashing is probably mortared into the brickwork. Hack out the lip with a hammer and cold chisel, then pull it away.

3. Check the wall behind the old flashing. Rake out, clean and then repoint where the flashing was — and any other defective mortar joints — using 1:3 repair mortar mix with PVA adhesive added (see 'Parapet walls').

4. Clean the corresponding area on the roof itself using wire wool or a wire brush.

5. When the mortar has dried, coat the whole of the area where the new flashing is to go with flashing tape primer. Leave to dry. (The extent of the area will be governed by what width of tape you are using.)

Although you can make the new flashing with a single length of tape, pressed into an L shape, a better scheme is to use two overlapping lengths. In this case you must fit the lower length before the top so that the joint between the two is protected from water run-off (see right).

If the run is longer than a manageable length of tape — say, lm — arrange for the joints between adjoining lengths to be staggered between the bottom layer and the top layer.

Cut, prepare and fit the lengths of tape as you would for a small patch. Press the lower layer into the angle between the wall and the roof so that it runs about 50mm up the wall.

Smooth it with your rolling tool until all air bubbles are removed. When the lower layer is complete, fit the upper layer in the same way; arrange it so that it overlaps the lower one by about 50mm.

Note that flashing tape can be used to patch most types of hole and crack found on flat roofs. But it is expensive, so unless the defects are very minor it's more economical to use felt and roofing mastic.