The short answer
Lead moves. It expands as the sun warms it and contracts as it cools, and a long or wide sheet cannot slide back smoothly. Over many seasons it creeps downhill, thins at the folds and cracks. Roofers keep each piece to a limited size, and join bays with rolls or drips, so the movement is shared out.
A large sheet of lead looks tidy on day one. The limit on size exists because lead does not stay still once the sun and frost get to it.
How lead moves
All metals change size with temperature, and lead more than many people expect. On a bright afternoon the sheet on a south slope is much hotter than the air. By dawn it has cooled. Each cycle moves the metal a tiny amount.
Why does that matter on a roof?
Lead is soft, so when it expands it does not push back hard. It bulges or slides a little. When it cools, it does not snap back to its first position. Over thousands of cycles, the sheet creeps slowly down the slope. That creep is why old flashings end up wrinkled and stretched, and why the lead at the top of a long run can look thin.
Fixing alone does not stop it. Clips and nails hold a sheet in place, but a sheet that is too long builds up more movement than the fixings can take. Something has to give, and it is the metal at its weakest point, usually a fold or a fixing hole.
That is why a roofer cuts a roof into sections, each small enough to move without tearing. The same thinking applies to a flashing along a wall, a valley, or a flat gutter, wherever lead runs for any distance.
What happens when a piece is too big
An oversized piece fails in predictable ways. Cracks appear across the folds, where the metal is thinnest. Fixing holes stretch, so clips pull through. The sheet buckles in the middle, forming a ridge that collects water. Our answer on cracked lead flashing shows what those cracks look like.
Is thicker lead the answer?
Thicker lead resists tearing better, which is why the codes run from thin to heavy. Code 4 is 1.80 mm and code 6 is 2.65 mm in the Royston Lead brochure. But thick lead still moves, and a thick oversized sheet cracks later instead of sooner. Thickness helps, but size limits do the real work.
This is also why a repair made with one big replacement piece, where several smaller ones were used before, can fail early. The old arrangement was not accidental. Someone had divided the roof to share out the movement.
The right size depends on the position, the pitch and the code. We set it out at survey and name the lead code in the written price, so the lengths on your roof are recorded.
Bays, rolls and drips
A bay is one section of lead sheet, laid between joints. On a flat or low-pitched lead roof, bays are separated by rolls or drips. A roll is a rounded timber batten that the lead is dressed over, forming a raised seam. A drip is a small step in the roof surface, which lets one bay run out over the next at a lower level.
Why do rolls and drips help?
Both let the metal move. A roll absorbs expansion along the seam. A drip lets water pass from one bay to the next without a flat joint that could back up. Together they divide a roof into pieces the lead can manage.
On a pitched roof the same idea applies to flashings and valleys. A long run is split into lapped lengths, each held with clips, so no single piece carries the whole movement. Our answer on lead parapet and box gutters shows how this works on flat lead gutters, where drips matter most.
If a quote offers one continuous piece over a long wall or gutter, ask how the movement is being dealt with. A roofer who has planned it can explain the laps and fixings at once, while a vague reply suggests the detail was never worked out.
Reading the bays on an existing flashing
When we survey lead, we look at piece length as well as thickness. A flashing that has crept, split at the same point repeatedly or buckled between fixings often points to a run that was laid too long.
Can a long flashing be repaired?
Sometimes a split can be repaired, but if the problem is the length, the repair will fail again. The better answer is to lay the run again in shorter laps. That costs more up front and less over time, which is the usual choice between a patch and a proper job.
Lead can last for over 80 years when it is laid in sensible sections and kept clear of debris. Our answer on how long lead flashing lasts goes into the factors involved.
We show you photos of the lead on your roof, point out the joins and explain how a repair would divide the run. After the survey you get a single figure on paper for the whole job. Our lead work page covers the other leadwork we handle, and a photo of the area is a good place to start.
More on lead movement
Does lead shrink back after hot weather?
Only partly. It expands when warm and contracts when cool, but it does not return perfectly each time. Small amounts of creep add up over many seasons, which is why sheet length is limited and why joins are built into every run of lead.
Can a surveyor tell if lead was laid too long?
Often, yes. Repeated cracks at the same fold, stretched or thin lead near the top of a run, and buckles between fixings all point that way. We photograph those signs so you can see them yourself before agreeing to any work.
The full picture: Lead work in Taunton ยท More short answers