Can You Repair Lead Flashing With Sealant?

Why sealant fails on lead

Lead flashing is the strip of lead that seals the joint where a roof meets a wall, chimney or other upstand. It is soft, and it expands and contracts with changes in temperature far more than brick or tile do. A flashing that is bonded rigidly across its width cannot move without cracking. Sealant that is spread over a crack, a lifted edge or a corroded patch cannot hold the lead still, so it peels away at the edges, and water gets in behind it. The surface of old lead also carries oxide and dirt, which makes it hard for sealant to stick for long. After a season or two, you have a patched flashing that still leaks, and the sealant makes a proper repair harder because it must be cleaned off.

Many roof leaks at chimneys and walls are tackled in this way, and then come back. Our guide on whether roof leak sealants work covers the wider picture.

Where sealant does belong

A good leadwork repair uses sealant in a specific place: the joint at the top edge, where the lead is turned into a groove cut in the brickwork, called a chase. The lead is held in with small lead wedges, spaced along the run, and the gap is then closed with pointing mortar or a suitable flexible sealant. Used like that, the sealant is a weather seal, not the thing holding the flashing in. Our page on how lead flashing is fixed into brickwork describes the method, and lead flashing versus mortar fillets shows why a fillet of mortar is no substitute.

Sealant is also a fair temporary measure. If a leak is active and the weather is bad, a bead over a small gap can slow the water until a proper repair is possible. Treat it as a bridge, and tell your roofer it has been applied so they can allow for cleaning it off.

What a lasting repair looks like

The right repair depends on the fault. A small split in an otherwise sound flashing can sometimes be repaired by cutting out the damaged area and fitting a patch, or by lead welding, which fuses the metal with a flame without any solder. A flashing that has lifted from its chase is re-dressed, re-wedged and re-pointed. A flashing that is thin, corroded or full of old repairs is replaced, because patching worn lead only moves the problem along. Our guide to lead flashing repair or replace sets out how to decide, and the signs of failing lead flashing page shows what to look for.

Length matters too. Long, single pieces of lead are more prone to creep and cracking than shorter lengths with laps, so a replacement is normally done in sections with the right overlaps. And at the roof edge, soakers or stepped flashing are used where tiles meet a wall, which we cover under soakers and step flashing.

Cost and timing

A sealant bead is cheap, which is why it is tempting. But a repeated stopgap can cost more than one good repair, because each visit costs a call-out and the leak keeps wetting the timber behind. A flashing repair is often a modest job when caught early, and the price depends mainly on length, height and access. Typical UK ranges for chimney leadwork are reported at several hundred pounds, and scaffold adds to that where the stack is high, but your fixed quote follows the survey. See lead flashing replacement cost for the cost drivers.

Timing matters as well. Lead is best worked when it is dry and the weather is settled, so a temporary seal in a wet spell followed by the proper repair in better weather is a sensible plan. Make sure the roofer knows what has been applied before they start.

How we decide

We go up, look at the lead, the chase, the pointing and the brickwork, and photograph each part so you see what we see. If sealant is all that is needed to finish a sound flashing, we say so. If the lead has failed, we say that too, and we price the repair or the renewal in writing. We would rather fix the cause than cover it. You can return to the lead roofing and leadwork page for more, start from the homepage, or ring 01256 213956.