Why Has My Lead Flashing Cracked?

Lead looks dead and still, but it breathes with the weather. On a bright day a sheet on a south-facing slope warms and stretches. By the small hours it has cooled and drawn back. The movement is tiny on any one day, yet it repeats every day for the life of the roof.

What happens when the sheet cannot move freely?

If the lead is free to slide, nothing is harmed. If it is held rigidly at both ends, or stuck down along its length, the stretch has nowhere to go. The metal takes the strain at its thinnest or most bent point, usually a fold, a corner or a nail hole, and a hairline crack opens. Each hot-and-cold cycle then pulls the crack a little wider.

That is why a crack often runs across the sheet in a straight line, or starts from the corner where a flashing steps round a chimney. The line shows you where the movement was trying to happen. A flashing that has split cleanly at a fold has usually been working against a fixing for many winters.

Two mistakes in the original work are common causes of cracks, and both are easy to spot once you know to look.

Why does a long length crack?

The longer a piece of lead, the more it grows and shrinks across a day. A length cut too generously, so that it runs the whole width of an abutment, the line where a slope runs up to a wall, in one go, builds up more movement than its fixings can allow for. Good practice is to break a long run into shorter overlapping pieces so that each can move on its own. Where a roofer has saved time by using one long strip, the crack is waiting for the first hard summer.

Can too much fixing do harm?

Yes. Lead should be held firmly enough that wind cannot lift it, and loosely enough that it can still slide. Extra clips, a heavy bead of sealant along the whole edge or a flashing nailed at several points all pin it. We have written separately on sealant on lead flashing, because a filler that sets hard is a common reason a repair fails again.

Even well-made lead tires. Metal that flexes thousands of times slowly work-hardens and loses its give, and a sheet that was once soft and forgiving becomes brittle at its folds. In its Sep 2026 guide, MyJobQuote writes that lead flashing "can last for over 80 years", a floor and not a pledge. A flashing that has served for longer than most of us have been alive has earned a retirement, and a crack at its folds may simply mean it has done its job.

Does the weight of the lead matter?

It does. Sheet lead comes in numbered codes, and a lower number is a thinner, lighter sheet. Royston Lead's brochure lists Code 3 for soakers only, with Codes 4 and 5 used for flashings. A flashing cut from a sheet that is too thin for the job will crack earlier than one cut from the right weight. Our page on lead codes explains the range in plain words.

If the crack is the only sign of age and the remaining sheet lies flat and sound, the trouble may be local. If the whole run is crazed with hairline splits, the lead has reached the end of its working life.

A crack in lead is not something to smear over. A coat of bitumen or a blob of sealant may hide it for a season, but the movement that opened the split is still there, and the repair will open again.

Can a cracked flashing be patched?

Sometimes. A short, clean split in an otherwise healthy flashing can be repaired by a lead patch, fitted so the water runs over it and not under it. That suits a small, isolated crack. It does not suit a flashing cracked along most of its length, or one cut too long in the first place, because the patch cannot cure the cause.

When the length is the problem, the fix is to lift the lead and replace it with shorter pieces that overlap and are free to move. When the fixing is the problem, we take away the extra pins and filler. Either way, we survey the run first, show you photos of the crack and the joints near it, and quote one total in writing for the work. If you can see damp on a chimney breast ceiling below, say so, and we make the roof safe before the repair. The full range of jobs is on our lead work page.