How Should Lead Flashing Meet an Extension Wall?

Why this joint is hard

An extension is built against the side or back of a house that has been settled for decades. The new structure moves differently from the old: it may settle slightly, its timber frame expands and shrinks with moisture, and the roof covering reacts to temperature differently from the brickwork. The line where the two meet is a joint that must stay watertight through all of that movement. Water also runs down the old wall, collecting at the junction, so the flashing is dealing with whatever the house wall sheds as well as the rain on the roof. That makes it one of the most common places for an extension to leak in the first few years.

Pitched extension roofs

Where the new roof is tiled or slated, the wall above it rises in a straight line, and the roof slopes up to it. The flashing here is stepped or follows the line of the slope. In a stepped detail, short pieces of lead are fixed into the wall in a stepped pattern that follows the courses of the brickwork and are lapped over the roof covering. Soakers, flat pieces of lead that lie under each tile and turn up the wall, can be used under the flashing, as covered in our guide to soakers and step flashing. Where the extension roof meets the wall at the top of its slope, the flashing is an apron, a single strip that runs along the wall and tucks under the course above.

The top edge of each piece is wedged into the chase, which is cut in a mortar joint above the roof line. Our description of how lead flashing is fixed into brickwork explains the method, and the pointing that finishes it.

Flat extension roofs

A flat roof is detailed differently. The membrane, whether EPDM rubber, GRP or felt, is turned up the wall to form an upstand, and the lead apron flashing covers the top of the upstand and the junction with the wall. The upstand must be high enough that rain cannot splash or flow over it, and the covering should be bonded or fixed so that it cannot slip. The lead is chased into the wall above and dressed down to lap the upstand. On a flat roof, a cavity tray or a damp-proof course above the flashing is important, because moisture in the old cavity wall can run down and into the roof if it has nowhere else to go.

Insulation is a related matter: a new flat roof usually has to meet current energy standards, and the build-up thickness has to be planned so the upstand and flashing are still at the right height once the layers are in. Raising the roof edge without raising the flashing line is a common mistake.

Cavity trays, damp-proof courses and the wall above

The old wall may have a cavity, and the extension roof will sit against its inner and outer leaves. A cavity tray is a shaped strip of damp-proof material that catches moisture running down inside the cavity and sends it out through weep holes above the flashing. If it is missing or cut short, damp finds its way through the wall into the extension ceiling and shows up as a stain just inside the wall. Where the wall is solid brick, the flashing chase and the pointing do more of the work. Where the wall is rendered or clad, the lead is tucked up under the cladding or the render is cut back, and the edge is sealed so water cannot run behind it.

Timing and sequence on a new build

If you are having an extension built, the flashing should be planned with the roof, not left to the end. The chase has to be cut before the roof covering is finished, the cavity tray has to go in as the wall above is built, and the membrane or tiles have to be laid so that the lead laps them correctly. Ask the builder who is doing the leadwork and whether it is a roofing specialist. A flashing squeezed in after the plasterers have gone is more likely to be a sealant-only fix. Request photographs of the junction before it is covered, as this is the best record you can have if a problem appears later.

Common faults and checks

We often find flashings that were never chased in and rely on a bead of sealant, a rigid mortar fillet that has cracked, lengths of lead that are too long, and a missing or incorrectly fitted tray; see lead flashing against mortar fillets for why fillets fail. Stains on the extension ceiling beside the wall are the usual first sign.

If your extension joint is leaking, we inspect it, photograph each part of the junction, and tell you what is wrong. The lead roofing and leadwork hub lists related pages, the homepage covers the rest of what we do, and you can ring 01256 213956.