How Is Lead Flashing Fixed Into Brickwork?

Why not simply stick it to the wall?

Lead flashing seals the joint between a roof and a vertical surface such as a chimney or a side wall. The joint is hard to keep weathertight because the roof and the wall move at different rates, and rain runs straight down the wall into it. A strip of lead stuck flat to the face of the brickwork with adhesive or a bead of sealant is easily lifted by the wind and torn at the edge by movement. A fixing that is built into the wall, on the other hand, has the wall around it to hold it in place and the groove above it to stop water getting behind. That is the reasoning behind the chase.

Cutting the chase

The roofer picks a mortar joint, not a brick, usually the first one above the point where the flashing will finish. The old mortar is cut out along the joint with a mortar rake or a grinder with a dust shroud, to a depth that will hold the lead, and cleaned out. Cutting into a joint rather than a brick keeps the wall stronger and the repair neater. On a chimney the chase runs round the sides and across the back, in steps where the roof slopes. A lead flashing that runs along a sloping side wall is cut in a stepped pattern so each step is fitted into its own bed joint, following the courses of the brickwork. The dust from cutting is hazardous, so a careful roofer uses extraction or damps the work, and the area is cleared afterwards.

Fitting the lead and wedging it

The lead is cut to size and dressed to the shape of the wall and the roof covering. The top edge is bent to a small return, which is pushed into the chase. It is then held in place with lead wedges, small strips of the same metal folded and driven into the groove at regular intervals along its length. Some roofers use a few copper clips as well, which hold the lower edge down against wind lift. The wedges are important: they keep the lead tight into the groove before the pointing sets, and they take the load, so the pointing is not asked to do it.

The lower part of the flashing hangs down over the roof covering, tiles or slates, and is dressed to follow their profile, with a lap that throws water onto the roof. At the corner of a stack, the lead is folded rather than cut so that no open gap remains. Our page on soakers and step flashing shows how stepped sections are formed.

Pointing the chase

Once the lead is wedged, the chase is filled. The traditional choice is a sand and cement mortar, or a lime mix on older buildings where a softer mortar is needed to avoid damaging the brick; the council's conservation officer may specify it on a listed building. Modern practice also uses a flexible sealant designed for the job, applied into a clean, dry joint after the wedges are set. The point is that the filling is a weather seal, and not the structural fixing. Our page on whether lead flashing can be repaired with sealant explains the limits of sealant, and the comparison with a cement fillet is in lead flashing versus mortar fillets.

Choosing the right lead and spacing

The thickness of the lead is chosen to suit the position. Flashings that are exposed and have to cope with a lot of movement are cut from a heavier code than those tucked under a tile. Wedges are spaced so that no part of the lead is left to flap, close together on exposed gables where the wind is strongest, and a little wider apart in sheltered spots. Each stepped piece along a sloping wall is cut as its own lap with a generous overlap on the next, because a long run is more likely to creep. When the flashing is pointed, the filling is pushed in tight and finished slightly back from the face of the wall so that rain runs past it and is not caught on a ledge.

It is worth knowing what can be spotted from the ground. A good flashing finishes in a straight line along a joint in the brickwork. Look for pointing that has gone dark and crumbled, or a gap above the lead, or lead hanging away from the wall.

What goes wrong

The most common faults are a chase that is too shallow, lead that is not wedged and hangs loose, pointing that has shrunk away from the brick, and a flashing that runs too far in a single length so it creeps. Where the brickwork has crumbled, the chase may need rebuilding. Old flashings that were simply plastered over with mortar often hold water behind them. A survey photographs each of these, and we describe them on the page so you can see the failing joint for yourself.

The lead roofing and leadwork hub lists related work. You can reach us from the homepage or on 01256 213956.