The short answer
Lead is deliberately laid in short lengths because it expands and contracts with every change in temperature and slowly creeps downhill under its own weight. A long piece cannot move freely, so it stretches, fatigues and splits. Permitted lengths vary with the lead code and with where it sits, so a gutter, a flat roof and a wall flashing each get different limits.
Why lead is never laid in one long piece
Lead is a soft, dense metal that reacts strongly to heat. On a bright afternoon the sheet on a south slope warms and grows; on a clear winter night it cools and shrinks. The change in a short piece is tiny. In a long piece it accumulates, and the metal has to take up the difference somewhere.
When the edges can slide a little, that movement is absorbed and nothing happens. When the sheet is pinned along its whole length by nails, sealant or a tight chase, the movement turns into stretching and ripples. Each daily cycle works the metal a little harder, and in time it cracks. Our page on why lead cracks follows that failure through in more detail.
Short bays also keep the weight manageable. A long, heavy sheet is awkward to carry up a scaffold and easy to kink while it is being dressed into shape, and a kink is a future crack.
Creep: the slow slide down a slope
Gravity pulls on lead all the time. On a steep pitch, or on a vertical face like the cheek of a dormer, the sheet creeps very slowly downhill. It does not drop off in one go. Instead it thins near the top, bunches near the bottom and eventually leaves a gap around the fixings.
The longer the piece, the more weight hangs from each nail, and the faster this happens. Dividing the run into shorter bays, each held at its head and left free at its tail, spreads the load and lets every section settle by itself. That is why the fixing method matters as much as the length. Copper nails at the head, with clips rather than nails holding the free edges, give the sheet room to behave.
Creep is also why steep work gets shorter bays than gentle work, even in the same code.
How the limit changes with the job
Published trade guidance sets maximum bay lengths by code and by use, and the figures are not the same for every detail. We work from those limits and confirm them against the actual roof, because a number quoted without the detail behind it is easy to misapply.
| Where the lead sits | What shortens the bay |
|---|---|
| Flat roof deck | A wide temperature swing on a dark, exposed surface; bays are broken by rolls or drips |
| Valley or gutter | Water flow and the fall of the gutter; drips set the bay length |
| Wall flashing | Height of the wall and how well the sheet is held at the top |
| Steep slope or cheek | Creep, so the bay is kept shorter than on a gentle pitch |
A heavier code carries more weight per metre but is also stiffer, which is why the code and the length are always chosen together. The thicknesses and weights of each code are set out on our lead code for flashing page.
Joints, drips and rolls
Short lengths only work if the joints between them are designed properly. On a flat roof, sheets meet at wood-cored rolls or at drips, which are low steps in the deck that carry water down to the next level. A drip lets one bay finish and the next begin without trapping water, and it stops a single piece running the full width of the roof.
On pitched work, bays overlap in a lap, and the lap has to be long enough that wind-driven rain cannot be pushed back up it. In a valley or flashing the upper piece covers the lower one, never the other way round. Sealant is not a substitute for an adequate lap. A joint that depends on sealant fails when the sealant dries, so we avoid leaning on it. Where two pieces must be joined with no lap at all, lead welding is the alternative.
Reading the signs on your own roof
Long runs without a break are one of the clearest clues that lead was laid to save time. Splits near the middle of a long flashing, or ripples running along a flat roof, usually mark a bay that was cut too generously. On a roof that has been robbed and patched, you often see a jumble of lengths, with the newer pieces failing first.
If thieves have stripped the roof, the replacement bays are cut to suit the new fixings rather than copied from whatever was there before. Our notes on stolen lead and insurance cover the cover side of that, and white staining on lead explains the pale film that appears on new sheet. The same thinking applies to lead flashing pointing where a strip meets brickwork.
What we do on site
At the survey we measure each run of lead, mark where the old bays ended and photograph the joints, so you can see where the metal has been stressed. A bay that has rippled or split is usually the one to start with, and the photographs make that obvious without a lecture.
The quote that follows is written, and it comes as one total price. Where a repair to a few bays would properly solve the fault, that is what we propose. Where the whole run was cut too long, we explain why patching would only push the leak along. Everything sits within lead roofing, our hub for leadwork. Our workmanship guarantee runs for 10 years and is in writing, and the scaffold, offcuts and old sheet leave with us.