Can a lead roof crack, and why?

Lead is dense and heavy, but it still responds to heat. On a summer afternoon a slope of lead is noticeably warmer than the air, and overnight it cools again. Each time, the sheet grows a little, then draws back. Over a year that is a great many small cycles, and over decades it is a vast number.

Properly laid lead is free to slide through those cycles. It is held at the top edge and left loose elsewhere, with joints that allow one piece to move past the next. The metal does not stay put; it creeps. The crack comes when something stops it creeping. If the sheet is pinned along both sides, or is simply too long to shift evenly, the stress has to go somewhere, and it goes into the weakest line, usually a fold.

That is why a crack tells you a lot. A split across the sheet, at right angles to the slope, means the metal was pulled apart. A split along a fold means it was bent back and forth until the grain tired.

The most common cause we meet is length. A long run of lead moves further in total than a short one, and its ends bear the strain. MyJobQuote's guidance is to fit the correct code in 1.5 metre lengths, with laps between them. Our answer on why lead must be fitted in short lengths explains the thinking.

Nails in the wrong place

A fixing driven through the middle of a sheet pins that spot to the deck. When the lead tries to move, the nail stays where it is, and the sheet tears around it. Fixings belong along the top edge, where the sheet hangs, and at the clips on the lower edge that hold it down against the wind without stopping it sliding.

Solder and sealant doing a job they should not

A bead of solder or a smear of sealant across a joint locks the two sheets together, which is the opposite of what a lap should allow. We see joints that were meant to flex stuck solid and then cracked beside the bond.

Walking and weight

Thin lead that gets walked on gets dented, and a dent is a place where the metal is thinner. A crack can start there months later.

Code is the number that stands for thickness, from 3 up to 8. Leadworx puts Code 4 forward for ordinary flashing work, with Code 3 reserved for soakers, which are the small hidden pieces under a stepped flashing. Use a code too thin for the position and the metal stretches and thins at the folds, then cracks. Our answer on the right code for flashing sets out the choice for one job.

Age matters too. Lead that has been through very many cycles becomes work-hardened at the places it flexes, and the oldest roofs often show fine cracks along their folds even where nobody did anything wrong. In that case the repair is not about blame. It is about whether the remaining metal is healthy enough to keep.

Poor cutting also plays a part. A sharp crease or a hammered corner gives a crack a place to begin, and lead that was bossed too hard into a tight angle thins at the bend. Careful shaping with a dressing tool, rather than a hammer, leaves the metal a full thickness at the corner.

What can be done depends on the crack, and the quote follows the survey, covering the options.

One hairline crack in a sheet that is otherwise healthy can be patched with a lead piece let in over the damage, or welded where the situation suits it. A patch holds if the cause was an isolated flaw. It does not hold if the cause is that the whole piece is too long, because the next crack opens beside the first.

Where several cracks have appeared in one run, the better repair is to cut the old piece out and replace it with shorter lengths in the right code, lapped so each can move. MyJobQuote gives per-metre prices for lead flashing replacement, and Checkatrade quotes lead flashing repairs at £50 to £70 per metre and stepped flashing at £80 to £120 per metre for Code 5, excluding VAT. Those are national guide figures, not ours.

Do not use bitumen tape or mastic as a lasting fix. It hides the split and leaves the timber behind it to soak.

Three decisions made on the first day cut the risk more than anything done later: the code, how long each piece runs and the fixing pattern. A roof laid with those right will move quietly for a long time. Our answer on how long a lead roof lasts shows what the published lifespans depend on, and the rest of our lead roofing pages cover the work in more detail.

For safety from above and below, keep people off lead where you can, and leave leaf litter to be cleared by someone who can do it without kneeling on the sheet. Questions about handling and run-off are covered in our answer on whether lead roofing is safe.

If you have noticed a crack or a ripple, photograph it from the ground or from inside and send it to us. The survey will record the code, the lengths and the fixings, and our 10-year workmanship guarantee covers the work we then carry out.