The method behind a lead roof has hardly changed in centuries. A traditional lead roof is not one sheet but a set of narrow ones, laid side by side, each held down at its edges and free to move in the middle. The seams between them are raised, the steps across them are built into the deck, and the whole thing is designed around one fact: lead expands when it warms and shrinks when it cools, and a sheet that cannot do either will tear.
You find lead flat roofs today on porches, on the tops of bay windows, on dormer cheeks and roofs, on parapet gutters and on small extensions where the owner wants something that will outlast the house. They are heavy, skilled work, so they sit at the dearer end of the market, but they are among the longest-lived.
The three terms in the title are the vocabulary of the trade. A bay is one strip of lead. A roll is the raised seam between two bays. A drip is the stair-step in the deck that lets a long roof drop a level without letting water through. The sections below go through each, then the lead codes used, where lead suits a building, how it fails and what we do about it.
Bays: why lead is laid in strips
A bay is a single run of lead sheet, laid between two rolls and running with the fall of the roof, from the high side down towards the gutter. It is typically a fairly narrow strip, because the narrower the strip, the less it moves in total across its width as the temperature changes, and the less strain it puts on its own fixings.
That is the point of the whole system. A sheet of lead on a sunny roof grows, and when evening cools it again, the lead has to come back to where it started. If it is trapped, it stretches a little on each cycle and thins at the points of greatest stress. After enough cycles it cracks. The traditional answer is to cut the roof into bays and length limits and to fix each sheet so that it can slide a short way inside its own width. Our guide to why lead cracks goes into how that fatigue works.
Under the lead goes a layer that lets it slide: a building paper or an underlay, over a smooth deck of boards or plywood. Without it, the lead would bond to the rough surface beneath, and then it could not move. The deck itself must be flat and free of nail heads, since lead is soft and will show every ridge in the surface beneath it, and wear at the high points.
How the sheet is held
The bay is held down by clips, strips of lead or stainless steel folded over the edge of the sheet, rather than nailed through the face. Every nail hole through the surface is a hole for water. The top edge is fixed so the sheet cannot creep down the slope under its own weight, which is another reason the bays are limited in length.
Rolls: the seam between two bays
A roll is the raised seam where one bay meets the next. It is built over a length of timber, a round or slightly tapered wooden strip fixed to the deck, running down the slope. Each bay is dressed up one side of the timber and over its top, and the next bay is laid against the first and wrapped over it. The result is a distinct raised ridge, like a spine, that looks tidy and drains water away to either side.
The timber core raises the seam above the surface of the roof, which matters because water on a flat roof always finds its way to the lowest point, and the seam should not be that point. A roll lifts the join above the bay surface where it will not be sat in water.
The overlapping layers also allow movement. Each side of the roll is free to slide inside its covering, so as the neighbouring bays expand and contract, the roll absorbs the difference. In other words, the seam is a joint designed to flex, which no flat, stuck-down lap could do.
Rolls and the direction of the fall
Rolls always run with the fall, from the high point to the low, since they are designed to shed water and not to dam it. Rolls across the fall would hold water behind them. That job belongs to the drip, which is built in a quite different way and described in the next section.
On an old roof we look at the ends of the rolls first, because the lead there is bent hardest, and a split at that point lets water run down inside the roll and into the deck.
Drips: how a lead roof changes level
A drip is a change of level built into the deck, set across the fall at the point where one bay ends and the next begins at a lower level. The step does a pair of jobs. It lets the roof keep a workable slope over a length that would otherwise need the fall to carry on and on, and it breaks a long run of lead into short pieces, so that no one sheet is long enough to creep or crack.
The upper bay is turned down over the step like a waterfall, and the lower bay is dressed up under it. The overlap is built so that water, which can be blown or pulled upward by capillary action in thin gaps, cannot climb back over the step. The height of the step makes the difference. Too small, and water can be forced over it by wind. Too large, and the details become hard to dress.
Water runs down each flat, drops over the edge of the step and carries on to the gutter, so drips are the place to look when a lead roof begins to leak: the lead there is folded harder than anywhere else.
Common faults at drips are a split along the fold, where the lead has been flexed with each warm and cool cycle, and a gap at the ends, where the drip meets a roll or an upstand and the folds crowd into each other.
A split at a drip is often repairable with a patch of new lead, burned or dressed in correctly, provided the surrounding sheet is sound. If the lead around the drip has crept and thinned over many years, relaying the bay is the lasting fix, and our guide to lead flashing repairs covers how we decide between the two.
Falls, decks and heavier codes
Whatever the lead, the deck beneath it decides how the roof performs. The boards or plywood must be sound, dry and well supported, and the surface must slope enough to shed water. IKO, citing the 2018 edition of BS 6229 (since revised), advises a finished minimum fall of 1:80 on a membrane flat roof, and designing for 1:40 so that tolerances and deflection in the deck still leave 1:80 at the end. That applies to flat roofs generally, and it matters for lead because ponded water speeds up the failures described above.
A lead roof also needs a deck that will carry the weight. Lead is among the heaviest roofing metals, and the codes used on roofs are heavier than those used for flashings. Leadworx lists Code 5 for general, vertical and pitched work, Code 6 for large roofs and Codes 7 and 8 as heavy duty. At Code 6, the sheet is 2.65 mm thick and weighs 30.10 kg per m², and Code 8 is 3.55 mm and 40.26 kg per m². Our lead codes guide has the full table, with the colour used to mark each.
The code chosen for a roof is not an automatic choice, and a porch is a different job from a long run over a bay window. We name the code in the written quote and why we chose it, so you can see what has been specified and for what.
Checking the structure
On an old roof we look at the timbers before we quote. A deck gone soft from years of slow leaks, or joists that have sagged, will bring new lead down with them, and we would rather tell you at the survey than find it with half the old lead off.
Where lead flat roofs suit a building
Lead flat roofs are at their best on small, awkward, visible or valued parts of a building. A porch is the classic case. The area is small, the roof is seen from the front door, and a lead covering suits brick and stone in a way a black membrane does not. The same is true of the roof of a bay window, which is a hexagonal or curved shape that a sheet membrane struggles to follow and lead takes happily.
Dormers are another common home. The cheeks, the sides of a dormer, are often covered in lead, and the flat or shallow-pitched top can be too. On older houses, lead also lines parapet gutters, the hidden channel behind a wall at the roof edge. It is a good material for them because it can be dressed into corners and made in long, narrow shapes.
There is a cost to weigh. MyBuilder (updated 21/08/2026) gives a typical UK range of £150 to £200 or more per m² for a lead flat roof, against £90 to £130 for EPDM and £95 to £150 for GRP. Less puts the life of a lead flat roof at 50 to 100 years, and those two figures together are the whole argument. We set out the arithmetic in how much lead roofing costs.
On a coastal house, a flat roof faces strong sea weather, and the decision rests on the details and the fixing as much as on the material. We choose the code, the clips and the junctions to suit the exposure, and where the lead reaches the house wall the details are those in our guide to types of lead flashing.
Lead is not the right answer everywhere. A large, plain extension roof may be better served by a membrane, and we will say if a membrane suits better.
How lead flat roofs fail, and what we do about it
Lead seldom gives up in one go. Trouble starts in particular places for particular reasons, and the pattern tells us where to look.
Cracks in the face of a bay are the commonest. They come from fatigue, the repeated stretching of lead trapped by a fixing or a stuck-down lap. They often run across the bay and are fine as a hairline at first. Splits at drips and at the ends of rolls come from the same cause, concentrated at the places where the lead is bent hardest.
Creep is the second. A bay that was not fixed correctly at the top, or was laid in too great a length, slides slowly down the slope over the years, thinning at the top and bunching at the bottom. You see it as ripples, and then as a thin patch that wears through.
The third cause is not in the lead at all. Ponding from a deck that has dropped, a blocked outlet or a roll that has been laid across the fall will hold water against the metal, and the water will get in at any weakness. Beneath it the deck rots, and then the lead has nothing to sit on.
Repair, patch or relay
A single split in sound lead can be repaired with a patch. A roof with cracks in several bays, or a deck that has gone soft, is at the stage where relaying costs less than repeated callouts. We find the cause first and photograph it, then the quote that follows is one total for the work that cures it. Our guide to whether lead roofing is safe answers the question owners ask about handling and old lead. Our lead roofing work starts with a survey and photographs of the existing sheet, so you can see the seams and wall edges for yourself.
