Will sealant fix a leaking lead joint?

Lead is not still. It warms in the sun and contracts overnight, and it does this every day of its life. The lengths it is laid in are limited for that reason: MyJobQuote notes flashing is fitted in 1.5 metre lengths for the correct code, so each piece can move without tearing. A joint is designed to let the metal slide, with overlaps that shed water however the sheet shifts.

Sealant works the other way. It glues two surfaces together and expects them to stay put. Each time the lead moves, the bond is pulled, and sooner or later it lets go at one edge. By then, water has found its way behind the bead and is travelling along the back of it. The sealant is now holding the water in as well as keeping it out.

Most leaks at a lead joint come from something specific: a crack where the lead has been folded too hard, a lifted edge where a clip or wedge has failed, a stepped flashing whose cover has pulled out of its joint, or a wall mortar line that has crumbled. A smear of sealant sits over the top and leaves the cause untouched, so the cause carries on growing. By the time the leak returns, timber behind the lead may be wet.

That is the real cost of a quick fix. The roof looks sealed, the stain on the ceiling dries, and nobody looks again until it is worse. When we survey, we lift or photograph the joint, work out whether the problem is in the lead or the surface beneath it, and say which. A bead of mastic that has gone brittle or pulled away tells us at once that the joint has been moving.

Sealant on lead is not neutral. A proper repair means clean metal, and mastic is difficult to remove completely. Where lead is to be patched, burned or dressed again, contamination can stop the new work bonding, and the old sheet often has to be cut back further than if it had been left alone. Our answer on whether a lead flat roof can be patched explains what a sound repair involves, and a clean surface is the starting point.

It can also affect the price. On MyJobQuote's 2026 figures a chimney flashing patch runs £150 to £250 while a full replacement averages £570, with the lower end applying without scaffold. A joint that has been smothered with sealant tends to push a repair towards the larger job, since more has to be stripped to reach sound metal.

There are times when sealant or a temporary patch makes sense. A roof is leaking over the weekend, it is raining, and nobody can work on the lead safely. A short-term cover keeps the water out of the house until the weather clears. A leak is dealt with before routine work, and stopping the water comes first, which may well mean a temporary measure before the proper repair is planned.

The test is what happens next. If a stop-gap is followed by a survey and a written plan, it did its job. If it is the plan, the leak is only waiting to come back. Most leaks at an old lead joint will not stay stopped, so write down the date you applied anything, in case you need to point it out later.

From the ground you can often see the difference. A sound lead joint is clean metal, with a neat fold or a lap and no filler. A sealed one shows a ridge or a smear along the edge, often grey or black, sometimes with a slight sheen where it has been touched in again and again. Look for cracks in the bead, gaps at either end, and dark streaks below where water has been getting out.

Inside the loft, check the underside of the timbers by the joint with a torch. Staining, a damp smell or lifting felt tell you water has been arriving for longer than the sealant has been there. Photograph what you see before anyone covers it over. Pictures taken before and after are exactly what we take on a survey, and they let you see for yourself what we describe on the quote.

A durable repair deals with the cause. That might mean replacing a cracked section of flashing, re-dressing a stepped flashing into a fresh mortar joint, fitting new wedges, or lifting slates to renew a lead slate. Done well, lead has an exceptionally long life: MyJobQuote says flashing can last over 80 years when correctly installed, which is an indicative figure, not a promise. Our answer on how long a lead roof lasts goes through what shortens it.

If cracking keeps returning, the metal may be the wrong choice for that spot, and our comparison of copper and lead flashing looks at the alternative. Whichever route, you receive photographs of your roof, a written quote and, on the finished repair, a 10-year workmanship guarantee in writing. Our lead roofing page covers the wider work.