The short answer
A sealant repair can hold a lead flashing for a short while, but it seldom lasts. Lead swells and shrinks as the weather warms and cools, and the bead cracks or peels away from the metal, letting water behind it again. A lasting fix deals with the cause: a split, a lifted edge, loose fixings or poor detailing, put right by redressing, patching or replacing the piece.
Why sealant and lead do not get on
Flashing is lead sheet bent to cover the junction between a roof and an upright surface: a chimney stack, a party wall or a dormer side. Its job is to shed water over the joint, not to seal it. A well-made flashing is lapped, so water runs down and over the edge of the lead below it, and the joint stays dry because of the overlap rather than because of anything stuck into a gap.
Lead is a soft metal with a high rate of thermal movement. On a sunny afternoon the sheet warms and grows; on a frosty night it cools and shrinks. That movement is small in a short piece and large in a long one, which is why the sheet is cut to modest lengths. A lead manufacturer's brochure gives a maximum piece length of 1500 mm for flashings in the commonly used codes, and the Lead Sheet Association limits the spacing of wall wedges on a cover flashing to about 500 mm.
Sealant bonds to the surface of the lead and, as the sheet moves, the bond is pulled again and again. Add ultraviolet light, rain and the dust of a roof, and the bead stiffens, shrinks and lets go. It can look sound from the ground while water is finding a path behind it.
What a sealant bead usually hides
When someone offers a sealant repair, the useful question is what the bead is covering. A leak at a flashing is often a symptom of something else.
- A split in the lead. Metal fatigue from movement can crack a flashing across a bend. Our page on why lead cracks sets out the usual causes.
- A lifted or slipped edge. If the top edge of a flashing has come out of the joint in the wall, wind and rain get behind it.
- Failed pointing above the lead. The lead sits in a chase, a groove cut in the masonry, and when the mortar in the chase crumbles, water runs in over the top of the flashing rather than off it.
- Poor laps or wrong lengths. A piece that is too long, or a lap that is too short, is bound to fail sooner or later.
Sealant over any of these changes how the fault looks, not how it works. It can also hide a growing problem, since the damp is now behind the bead, in the brickwork and timber, where you cannot see it.
When a little sealant is reasonable
This is not a rule without exceptions. A small amount of the right sealant is sometimes used as a temporary measure, for example to hold a leak at bay for a few days before the proper repair can be fitted, or as part of a make-safe after storm damage. A temporary fix is straightforward work when it is called temporary and its limits are explained.
The difficulty comes when a bead is sold as the repair. A flashing can have a decent amount of life left in it, and a skilled roofer will reuse it where the lead is sound, by redressing a lifted section and re-wedging it, or by patching a local split in the proper way. That is repair, not sealing. Where the lead has thinned, split in several places or been patched repeatedly, replacing the flashing is more sensible than adding another bead.
Lead can also be welded, and for small splits on suitable pieces a lead-welded repair is a lasting solution. Heating old lead produces fume, so read our note on handling lead and fume before you let anyone put a flame to it.
What a lasting repair involves
A proper flashing repair begins by establishing how the water gets in. That means looking at the lead, the chase above it, the mortar, the tiles or slates beside it and the timbers below. Inside, the stain often appears well away from the flashing, since water travels along rafters before it drips.
The repair then follows the cause. Lead that has lifted is redressed and fixed back with lead wedges or clips. A chase with failed mortar is raked out and repointed, using lime mortar where the building calls for it. A split piece is removed and a fresh length in the right code is lapped in its place. Where the whole run is tired, we strip it and lay new lead in suitable lengths, usually code 4 or 5 for flashings, depending on the position. Code 4 is 1.80 mm thick and code 5 is 2.24 mm.
This is slower than squeezing in a bead, and it costs more on the day. Typical UK figures reported by MyJobQuote in September 2026 put a patch on chimney leadwork at £150 to £250 and a full replacement at around £570 on average, depending on scaffold. Your own fixed quote follows the survey.
Alternatives to lead, and what to ask
If the appeal of sealant is cost, the real alternative is not a bead but a different material. Some homeowners are offered lead-free flashing strips, self-adhesive bitumen types or GRP and zinc. They have their place, and lead alternatives are weighed against each other in our guide. For a listed property, or one in a conservation area, swapping the material can raise a consent question, and B&NES planning is the body to ask.
Before agreeing to any flashing repair, a few plain questions help. Where exactly is the water getting in? Is the lead being reused or replaced, and why? What code is going in? Will the chase be repointed, and in which mortar? What does the guarantee cover? A clear answer to each counts in a firm's favour. A quote that says only sealant, with no cause given, is a reason to ask more.
A long-running leak also deserves a check on the timbers behind it. When comparing quotes, our page on valley leaks shows how the same thinking applies elsewhere on the roof, and our lead roofing work takes a repair from survey to finish, starting with the cause of the leak and ending with the chase repointed in a mortar that suits the wall.
