Flashing, the lead or metal-backed plastic strip which seals the joint between a wall and slate or tile roof, is a common point of water entry on older houses and those with flat roofs. Flashing which was fitted when the house was built is usually embedded into a row of mortar between the bricks above the joint of wall and roof. Flashing which has been added after a house has been completed (such as on a porch or flat-roofed extension) might be a metal-backed strip which is stuck over the joint with strong, waterproof adhesive. Both types are relatively easy to replace should they become corroded, split or loose.
Removing Lead Flashing
If lead flashing has become corroded, it is best to remove a complete strip of it, rather than just cutting out the corroded section. You don’t need to replace all the lead flashing on your house just because one area is bad, but check the surrounding areas carefully if you do discover corrosion. To remove lead flashing, chip out the mortar holding it in place using a Plugging Chisel and Club Hammer. Remember to wear gloves and protective goggles when chipping out old mortar.
When you are able to, strip out the section of old flashing and clean out the row of mortar where it sat using a wire brush. Repoint the joint with a standard mortar mix and allow it to dry overnight. You don’t need to try to fit a new lead flashing in place of the old one. It would be tricky to do and would probably not offer the greatest of seals anyway. Instead, buy a roll of metal-backed flashing strip and some flashing primer.
Applying A Flashing Strip
Brush down the area where the new flashing strip will go to get rid of any dust and debris. You should aim to make sure that the new flashing strip will at least cover the same area as the old lead flashing did, if not slightly more. Paint the area on the wall and roof where the flashing is to go with flashing primer and allow it to dry for the recommended time (usually 30 mins to 1 hour).
Whilst the primer is drying, cut two strips of flashing to the full length of the area you are replacing. Peel off the backing paper from the first strip and carefully apply it to the joint. Make sure that it is laying equally on the roof and wall before pressing it down and smoothing it out with a wallpaper seam roller. Paint another strip of primer above the flashing strip you just laid, but only paint the strip of primer half as wide as the flashing strip (if the flashing strip is 200mm wide, paint a strip of primer 100mm wide). Let this dry again and then peel the backing off the second strip of flashing. Apply this so that its top edge is 100mm (or 50mm depending on the width of your flashing strip) above the top of the previous strip. This will seal the top joint of the first flashing strip. Smooth the second strip down with the seam roller.
Dealing with Corners
Corners in the flashing are always going to be the weak point as you will need to cut it to make the turn. Be careful when making your cuts and apply additional exterior-grade sealant if you are not totally happy with the joint.
For internal corners, make a cut in the lower edge of the flashing strip and overlap the cut edges to form the turn. The adhesive on the back of the flashing should be enough to seal the join, but it is a good idea to apply some exterior sealant over the top.
External corners are more difficult. Cut a square from the excess flashing strip and make a cut from one corner to the middle (Fig. 1). Place the patch so that the centre is at the point where the angle of the wall meets the roof, with the cut in the patch facing upwards (Fig. 2). Press the patch into the angle of the wall and the cut in the patch will allow the top corner to splay out. Lay the main flashing strip up to the corner, cut into the bottom edge so that it can make the turn and continue alone the adjoining wall. The patch will cover where the main strip has been cut. Apply some exterior sealant over the top for added waterproofing.
If your flashing is cracked or corroded, and you do not want to replace it completely, it is perfectly possible to make repairs to it instead. Of course, this depends on just how cracked or corroded the flashing is.
Small Holes – Small holes caused by corrosion can be patched over using a square of self-adhesive flashing strip. Clean the area around the hole with a wire brush and use some abrasive paper to roughen up the surface where the patch will sit. Paint Flashing Primer over the area, extending the primed area at least 50mm past where the edge of the patch. Allow this to dry for the recommended time (usually 30 mins) and then cut your patch. Peel off the backing and press the strip over the hole. Smooth it down with a seam roller, taking care to get the edges nice and flat.
Fine Cracks – Fine cracks in lead flashing can be resealed using a bituminous sealant or other roof and gutter sealant. Choose a sealant which matches the colour of your flashing if possible. Inject the sealant into the cracks and allow it to fill them completely. Leave the sealant proud of the crack for extra waterproofing.
Replacing Lost Mortar – If small sections of the mortar holding the flashing in place has come loose, it may be easier to replace the mortar in that section rather than replacing all of the flashing. Make sure the flashing is pressed back into the gap between the bricks and mix up some exterior mortar (you can see more information on mortar mixes in out Concrete and Mortar Guide). Repoint the joint and let the mortar harden. If the flashing keeps springing back out of the joint before the mortar can set, cut some small wedges of wood and use them to hold it in place. Once the mortar has set, remove the wedges and fill the small holes they leave with more mortar.