Why Does Lead Flashing Crack?

Lead looks solid, but it responds to temperature more than most roofing materials. In summer sun the sheet warms and grows a little, and on a frosty night it shrinks back. Each cycle is tiny, yet the sheet does it day after day, and the metal slowly work-hardens and tires where it bends. The trade calls the result thermal fatigue.

A flashing built to allow movement copes for decades, which is why a cost guide can quote lead flashing as lasting for over 80 years. A flashing that cannot move is a different story. Instead of sliding gently, the strain gathers at the weakest point, usually a fold, a corner or the place where the sheet is fixed. There the lead stretches, thins and finally splits, often in a straight line across the grain of the sheet.

You may see a fine hairline crack along a fold, or a rippled, crinkled surface where the lead has been working. Either tells you the same story: the sheet has been under strain for a long time, and the next cold snap may open it. Cracks that run in line with a fold or across a long flat length are the ones to watch, because they suggest the whole piece has been working, not just one spot.

The most common design fault is a single piece of lead cut too long. The longer the sheet, the more it grows and shrinks, and the more the movement piles up at its ends. Good practice is to keep each piece to a limited length and to lap one piece over the next, so each section can move on its own.

An over-long flashing along a gable wall or a flat roof parapet is a classic. The lead may be perfectly well fitted on the day, and then slowly buckle and crack over several winters. The same applies on a flat lead roof, where the answer is to break the area up with rolls and drips, as our answer on lead drips and rolls explains.

If your cracked flashing is one long unbroken strip, ask your roofer whether it should be replaced in shorter, lapped lengths. Patching a crack in a strip that is too long only moves the strain somewhere else, and the next split will not be far away. Shorter lapped lengths cost more in time up front, but they are the cure rather than the cover-up.

Two other causes are the thickness of the sheet and the way it is held. Lead that is too thin for the size of the piece tires faster and tears more easily. The Royston brochure puts flashings in Codes 4 to 6 and keeps Code 3 for soakers, so a flashing cut from soaker-grade sheet is a warning sign. Our what code lead for flashing answer has the thicknesses.

Over-fixing is the opposite fault. Lead should be held at its top edge and left free to slide below. When it is nailed, screwed or sealed down on all sides, it has nowhere to go, and the fixings become stress points where cracks start. Sealant spread across the whole face has the same effect. The right method is to fix the top edge firmly and leave the lower edge free to slide, so the sheet can ride through each season without being pinned in place anywhere along its length.

Poor forming matters too. A corner that was hammered rather than bossed, or lead that was folded sharply back on itself, starts life already thinned and weakened. Cracks then show up there first.

A single clean crack in otherwise healthy lead can sometimes be repaired, by fusing it with a lead weld or fitting a patch, as long as the cause is dealt with at the same time. Our answer on can lead flashing be repaired says when a patch is sensible. A repair that ignores the cause tends to crack again.

If the lead is crazed with fine cracks, rippled, too thin for the job or too long in one piece, the better answer is new lead in the right code and in lapped lengths, fixed so it can move. Checkatrade's June 2026 table gives £50 to £70 per metre for lead flashing repair work as a national guide, while the figure for your own flashing comes out of the survey.

Until it is fixed, a cracked flashing is letting water in, so look inside the loft beside the chimney or wall for damp. We survey first, photograph the crack and the length of the piece, and give one written total price. See our lead work page for the wider trade.