Why Does Lead Crack on a Roof?

Thermal movement is the root cause

On a sunny day, a sheet of lead on a south-facing roof can get quite hot. By night it has cooled again. Each cycle the sheet grows a tiny amount and then shrinks. Over a metre or two this is trivial. Over a long run, it adds up to a visible movement at each end.

Lead is soft, and when it expands against something that holds it, it squashes slightly rather than springing back. When it cools, it contracts from that new, slightly shorter position, so a stretched section is slowly left in tension. Repeat the cycle for years and the metal fatigues along the line of greatest stress, which is usually a fold or a corner.

The common causes of cracking

Sheet length is the first. Roof lead is laid in limited lengths, with rolls, drips or laps between them to take the movement. A run that ignores those limits will crack at some point along its length. Our guide to the maximum length of a lead sheet covers the principle.

Fixing is the second. Lead should be held by clips and wedges that let it slide, not nailed through the face or bonded with sealant at every edge. A sheet pinned on all sides cannot move at all, so any shift in temperature goes straight into the metal.

Thickness is the third. A lead code that is too light for the span or the exposure will fatigue sooner. The fourth cause is wear: boots on soft lead create dents that become stress points, and a sheet laid directly on a rough surface can be chafed through by the movement against it. Ask us about lead codes explained for the thickness question.

Why the crack often starts at a fold

A fold in a sheet is a place where the metal has already been stretched on one side and squeezed on the other. Every temperature cycle works that same stretched strip, so the fold is where fatigue shows first. Sharp, hammered folds with no radius are the worst, because the stress concentrates in a thin line. A softly rolled fold, formed with a dresser, spreads the stress and lasts much longer.

Corners are the same. A tight corner folded from a single sheet tends to tear, which is why leadworkers cut a corner and weld or lap it rather than forcing a sheet round it. Where the corner is a flashing against a chimney, the lead is cut to a pattern and each section is lapped, so it can move independently.

The role of the surface beneath

The surface under the lead matters too. Lead laid straight onto rough boards or exposed nail heads is chafed by every movement. A layer of geotextile or building paper gives the sheet a slippery bed, which lets it slide and prevents wear. Hidden damage from the underside is easy to miss, and a leak from it often appears as a pinhole instead of a straight split.

How to recognise a crack

Some splits are obvious: a straight line along a fold, with daylight showing or a stain below. Others are hairline and only visible up close, or from below when you see a faint line of light through the board. Rippled lead, with a corrugated look along the length of a sheet, is a warning of movement. The ripples are the early sign of creep, which leads to splits.

Inside the house, a brown ring on the ceiling under a gutter or beside a chimney often points to a split. The pattern on the stain usually shows which flashing is at fault, though the water can travel along a timber and appear metres from the crack.

Repairing a split

Sealant and bitumen tape are the common first aid, and they rarely last. The usual choices are to weld a small, clean split with lead burning, to let in a new section of lead lapped properly, or to replace the whole piece. We do not weld a sheet that has cracked in several places, because the stress that caused the first split will crack the next weak point. Our page on what lead welding is sets out when a weld is right.

The best repair also fixes the cause: shorter sheets, free-moving clips and a proper lap. We photograph the split and the surrounding lead during the survey, so you can see whether a local repair is enough. For the whole picture, see lead work we carry out in Basingstoke.