Why does lead crack on a roof?

Every day lead warms in the sun and cools overnight. It grows a little, then shrinks again. Where the sheet can slide, it repeats this for decades without harm. Where something holds it still, the movement goes into the metal, which stretches slightly each time. Lead is soft, and repeated stretching hardens and weakens it until a hairline crack opens.

Cracks often begin where the sheet is pinned hardest: a bend, a corner, a nail, the edge of a chase. Once a crack starts, it runs along the line of greatest strain. A neat, straight split across a flashing, or a crack following the fold of a valley, almost always points to this cause.

The most common mistake is cutting sheet too long or too wide. A bigger piece moves more, and its edges find it harder to slide. Short bays, with laps or drips between them, let each section move on its own. Our page on lead sheet lengths explains the principle in full.

Roofers trying to save time or avoid joints sometimes lay one long flashing where several pieces were needed. It looks tidy on the day. Within a few winters the middle of the run splits.

Flat roofs are especially exposed because the deck is dark and open to the sky. A summer afternoon bakes the surface and a frosty night chills it, so the swing is larger than on a shaded flashing.

How lead is held matters as much as how big it is. A flashing should be held at the top with fixings that let it hang and move. Nailing it down the full length, or sealing it all round with mortar or mastic, locks it in place. Sealant on top of lead may also trap moisture underneath, which speeds corrosion. See lead roof sealant for the reasons.

The code of lead is the next point. Royston Lead and Leadworx list Code 3 at 1.32 mm as suitable for soakers only, Code 4 at 1.80 mm for general flashings and Code 5 at 2.24 mm for heavier flashings and many valleys. A code that is too thin for a bigger detail stretches and tears. The reasoning for each choice is set out on our lead code for flashing page.

Finally, workmanship: poorly dressed corners, sharp folds and thin spots where the sheet has been overworked all become starting points for cracks.

Old lead can be sound for a very long time. MyJobQuote's guide of 22 September 2026 says properly installed flashing can last over 80 years, and over 100 years for higher codes. So age alone is not usually the culprit. Cracks in a roof only 15 or 20 years old nearly always trace back to a problem at the start.

Foot traffic is another cause. Walking on lead, especially thin or old lead, can crush or tear it, and a ladder resting in a valley leaves a dent that becomes a split. Wet lime mortar and contact with certain other metals can also attack lead from below. If you wonder whether lead is a sensible choice in the first place, is lead roofing safe and how long a lead roof lasts cover the background.

A crack can sometimes be mended by cutting out the damaged area and fusing in a new piece, as described in lead welding. That works when the rest of the sheet is sound and the cause has been dealt with. It does not work if the same stress is still acting on the same spot.

If the cracking is widespread, or the original pieces were too big, the right answer is to relay the lead in smaller bays with the correct code. Patching one crack in a rigid flashing simply moves the next failure along. We look at the whole run, photograph the splits, and tell you whether a repair or a replacement properly solves the problem.

From the ground, or through an upstairs window, look for light-coloured lines across a flashing, ripples running along a flat roof and corners that look puckered. Stains on the ceiling directly below a valley or a chimney also point at a crack somewhere above.

Stay off the slope itself. Lead tears under a boot, and a fall from a pitched roof is a far bigger problem than a split flashing. A photograph taken with a zoom lens, sent with a note of where the stain appears indoors, is usually enough for us to say where to look first, and we will tell you if the picture shows something that needs attention soon.

Typical flashing prices are covered under lead flashing cost. Checkatrade's October 2026 repair guide puts Code 5 lead flashing not exceeding 150 mm at £50 to £70 per metre, ex VAT. Read it as an indication of scale, since your roof will price differently.

Once we have surveyed, the figure for your roof is set out on paper as one total, alongside pictures of the cracks and the joints around them. Ten years of workmanship guarantee are included in writing, and the garden is left tidy. The wider subject is gathered under lead roofing.