The short answer
A lead roof usually leaks for one of four reasons: the sheet has split, a joint at a roll or drip has opened, water stands on it because the fall is poor, or the leak is really coming from a flashing at the edge. Telling these apart matters, because a split needs a patch or a new bay, while a bad fall needs the deck corrected.
Splits, rolls and drips
A lead roof is laid in separate sheets, each free to move, joined along and across the roof by upstanding joints. Along the slope the lead is wrapped over a timber roll, a rounded batten that joins two sheets without a gap. Across the slope, each sheet steps down to the next at a drip, a small change of level that sheds water downwards.
Each of those joints can fail. A roll can open where the lead has been dressed too tightly over it. A drip can leak if the step is too shallow or the lead has crept down it. The lead itself can split across the middle of a sheet, usually because it was laid in too large a piece and was not free to expand and contract.
A split is easy to recognise once you are looking at it: a clean, straight crack, often running across the sheet. Water from a split tends to come in directly beneath it.
Our page on why lead splits explains the movement that causes it. The repair depends on size. A short split can be patched, but a sheet with several is better replaced as a bay, which is one full length between two rolls.
Falls and standing water
Lead is waterproof, but a roof that cannot drain still leaks, because water that stays on the sheet works its way into every joint. The cure for that is the fall, the slope of the deck that sends water to the gutter or outlet.
A fall can be wrong from the start, or it can be lost over time. Boards under the lead rot, joists sag, and a roof that once drained now holds a shallow pool. You can often see this after rain, when water lingers on part of the roof while the rest has dried.
BS 6229:2025 sets a minimum 1:80 finished fall for membrane flat roofs, but that standard covers flexible membranes, not lead. Lead sheet roofs follow BS 6915, which Approved Document C cites. The membrane figure is the wrong yardstick for a lead roof.
Standing water on lead is not always a failure of the lead. Often it is the deck beneath. Re-laying lead over a sagging deck without correcting it leaves the same pool in the same place, and the leak comes back.
That is why the deck is the first thing we examine once the lead is lifted, and why we photograph the boards before anything goes back down on them.
Flat lead roof or flashing: where is it coming from?
Many leaks blamed on a lead roof are really flashing leaks at its edge. A lead flat roof has flashings at every upstanding wall or stack, and a defect there lets water in beside an otherwise healthy covering. The stain on the ceiling looks the same either way.
Some pointers help to separate them:
- a leak that shows beside a party wall, a stack or a dormer points to a flashing;
- a leak in the middle of the roof, away from any upstand, points to a split, a joint or standing water;
- a leak that appears in heavy rain only often points to a flashing or a blocked outlet;
- a leak that continues after rain has stopped often points to water trapped in a pool or the deck.
None of these is certain, which is why we look at the roof itself. Our page on signs of a leaking flashing covers the flashing side in detail, and the one on lead flat roofs describes how the covering is built.
What happens next
Start from the garden or the room below. Note where the stain is inside, what the weather was like when it appeared and whether it grows after rain. Photographs of both help us a great deal.
We then inspect the roof. The survey records the lead sheet by sheet, the rolls and drips, the flashings, the outlet and the way water moves across the deck. You see the photographs and hear what each one shows, in plain words, before any decision is made.
With water coming in today, the first visit is a temporary cover to stop it. The proper repair follows once the area has dried, because a permanent fix over wet timber is a poor one.
The repair might be a patch to a split, a re-dressed roll, a re-laid bay, a corrected fall or a new flashing. Which one it is depends on what the roof shows, and a repair is recommended wherever a repair is enough.
The written quote is one total. The lead roofing hub explains the other leadwork we cover around Farnborough and Camberley.
