The short answer
Where an extension roof meets the house wall, lead flashing seals the joint. On a lean-to with tiles or slates it is stepped up the slope and tucked into the brickwork; on a flat extension it covers the upturned edge of the roof membrane. Leaks here come from a loose chase, split lead or a flashing that is too low.
How the flashing between an extension roof and the old wall is built, where it goes wrong, and what a repair involves.
Why this joint leaks more than most
A new extension is built against an old wall, and the two never behave identically. They settle at different rates, warm and cool at different speeds and move a little against each other for years. The junction where the extension roof meets the house is called an abutment, and the flashing over it is the only thing keeping rain out of the gap.
It is also a place where water is delivered in bulk. Rain running down the house wall lands straight on the abutment, and on a lean-to, the whole of the wall above feeds that joint. A hairline fault that would be harmless elsewhere can let in a steady drip here, especially in wind-driven rain.
The damp tends to show as a stain or blistered paint where the extension ceiling meets the house wall, sometimes with a smell of mould in the corner, or as a tide mark that grows after each storm. Because the leak runs along the junction before it drops, the stain is often several feet from the actual fault. A roofer follows the line back up to the flashing instead of assuming the nearest tile is to blame.
Lean-to roofs: tiles, slates and stepped lead
A lean-to extension has a pitched roof sloping away from the wall. Along the top, where it meets the house, the flashing is a continuous strip. Along the slope the wall steps up with the roof, so the lead is cut in a stepped line, with each step tucked into a brick joint. Beneath each tile or slate along that edge sits a soaker, a little folded piece of lead, and a cover flashing laps over it, so water has two layers to get past.
Faults appear in predictable places. The wedges loosen and the mortar crumbles, or the top edge is sealed with sealant that has dried out. The lead can be too thin for its length and split at the bends. Occasionally the wrong pieces were used altogether, with a single strip of lead bent to follow the steps, which cannot expand.
Our guide to step flashing and cover flashing explains how the layers fit together, and lead soakers covers the small pieces beneath.
Flat extension roofs: upstands and the cover
A flat extension roof is covered with a waterproof membrane, either felt, rubber (EPDM) or fibreglass (GRP), and where it meets the wall the membrane is turned up to form an upstand. The lead flashing then covers the top of that upstand and goes into a chase in the wall, so water running down the brick drops onto the lead, not behind the membrane.
Leaks at this joint come from one of three faults. The upstand is too low, so rain splashes over the top. The lead has been fixed through the face with nails or screws, which let water in. Or the membrane has pulled away from the wall and left a gap that no flashing can cover. A bead of sealant along the top is a sign someone has been chasing the problem, and it often hides where the water really enters.
Flat roofs also need a fall, so water runs away from the wall rather than towards it. Where the roof sags against the abutment, water pools at exactly the joint that is hardest to seal. Our page on lead flat roofs covers lead as a covering, and the flat roof junction is one we survey with photos.
Repair, replacement and cost
Sound lead needs only the chase raked out, the wedges refitted, the pointing redone and fresh sealant where that is used. If the lead is split or too short, new flashing goes in, along with soakers where the edge is tiled. If the upstand is the problem, the membrane has to be dealt with first, because new lead over a failing upstand will not last.
As a national guide, Checkatrade's June 2026 table has repairs at £50 to £70 per metre, with Code 5 stepped flashing higher at £80 to £120. Extension roofs are usually low, which helps, though a two-storey wall above a lean-to can mean scaffold or a tower. Confirm in writing whether the total covers VAT and any scaffold.
We survey first and share photos of the joint, the wall and the covering, then put a single total in writing, with no sums left for you to do. If you are planning an extension, ask the builder who will fit the abutment flashing and what lead code is specified, since a weak abutment is easiest to avoid at the start. The main lead work page covers the rest.
More on this in Lead Work & Lead Flashing in Crawley
Two extension flashing questions
Why does my extension leak only in heavy rain?
Wind-driven rain finds gaps that light rain misses. A lifted edge, a shrunken sealant joint or a short upstand may stay dry for weeks and then let in water when the wall is soaked and the wind pushes it sideways. Note the date and wind direction, and photograph any stain, which helps trace the joint.
Can the flashing be fitted to an existing extension?
Yes. A chase is cut or an existing one cleaned out, the lead is dressed over the roof edge and wedged, and the chase is pointed. We would check the covering and fall at the same time, since a new flashing over a worn membrane gives the leak somewhere else to go.