A lead flat roof looks like a single silver-grey surface, but it is nothing of the kind. Underneath the metal is a plan: the roof is divided into bays, each bay is a separate sheet, and every sheet is held down by its edges and left free to move in the middle. The joints between the sheets are rolls and drips, and they are what make the whole thing last or fail. If you own a lead-covered flat roof on a porch, a bay window, a dormer cheek or a small extension, this guide explains how the bays are set out, why the rolls run the way they do, how the drips deal with the fall of the roof, and what the deck underneath has to be like. It also covers typical cost and lifespan, and what to look for when a patch of the roof starts to look wrong. Nothing here replaces a survey, but it will help you read what a roofer tells you.
Why sheet lead is laid in bays
Lead is soft, heavy and very good at keeping water out, but it moves. When the sun heats a lead sheet, the metal expands. When the night is cold, it shrinks. On a roof that sees full sun on a summer afternoon and frost on a winter morning, that is a great deal of movement across a year, and the metal never goes back exactly to where it started. This is called thermal movement, and it is the reason a lead roof is never one big sheet.
The answer is to break the roof into bays. A bay is a single sheet of lead, laid between two joints, and sized so that it can move without being pulled against its own fixings. Each bay is fixed along its top edge with lead or copper clips and left free elsewhere, so the sheet can slide a little as it warms and cools. Where two bays meet, the joint is either a roll or a drip, and the choice depends on whether the joint runs with the fall of the roof or across it.
The size of a bay depends on the thickness of the lead, and that is why the lead code comes first when a roof is designed. Heavier sheet has a little more resistance to creep and so can run further before it needs a joint. Our page on lead codes sets out the thickness of each code and where it belongs. Manufacturers publish maximum bay sizes for each code on flat roofs of 10 degrees or less, and a skilled roofer works to them rather than guessing. Past those limits the sheet ripples, the ripples work-harden, and the metal splits along the line of the worst fold.
The outline of the roof also drives the layout. A rectangular porch roof divides cleanly. A bay window with angled sides is trickier, because the bays must meet at the angles without leaving a long run of lead unsupported. A good layout drawing, or at least a chalked plan on the deck, is the first sign that the roof will be properly built.
Rolls: the joints that run with the fall
A roll is a joint that runs down the slope of the roof, from the high side to the low. It is built over a shaped timber batten, called a roll core, fixed to the deck. The first bay is turned up the core and across its crown, and the next bay is then wrapped over that, so the finished joint looks like a rounded rib running down the roof. Because the joint stands proud of the surface, water running down the flat sheets cannot get across it, and it cannot run underneath.
There are two common types. A conical roll is a tapering core with an under cloak, which is lead laid beneath, and an over cloak dressed over the top. It is the neater of the two and takes more skill. A wood-cored roll uses a straight rounded batten, and the two sheets are tucked in and closed over it. Both are accepted methods and both rely on the lead being dressed neatly and without kinks, because every kink is a place where a crack can start.
The rolls also set the width of the bays. The distance from one roll to the next is the width of the lead sheet that lies between, and it has to be small enough for the lead to move but large enough that the sheet is not cut into strips. The details sit on a drawing and in the manufacturer's guidance, and for a given code they are not a matter of taste.
On an older roof, a roll that has been walked on or knocked is one of the usual sources of a leak. The lead flattens, the cover splits at the crown, and water finds the gap. If you have to cross a lead roof for any reason, step on the flat bays rather than on the rolls, and keep to soft-soled shoes. Our short page on the lead roll goes into the detail of how one is built.
Drips: how the roof gets across a fall
A drip is the joint that runs across the fall of the roof, formed as a small step in the deck so that the upper bay finishes a little higher than the lower bay begins. The lead on the upper side is dressed down over the step and the lead on the lower side is tucked up beneath it, which means the water leaves one bay and runs on to the next without any need for a flat lap that could draw it backwards.
A drip does two jobs. First, it keeps the bays to a size the lead can handle, since without drips a long slope would need one very long sheet. Second, it lets the joint sit above the line of the water, so that the water falls past it, not through it. A flat lap at the same level would rely on the lead being tight to the deck, and the lap would inevitably open as the sheet moved.
The roof has to be built for drips. The deck is laid in steps from the start, and the boards are cut to give a clean vertical face at each drip. Putting a drip into a flat deck later is a repair that involves lifting the lead and re-laying the boards beneath, which is why a drip that has been left out is such a stubborn fault. The dimensions of the step are set by the manufacturer's guidance and the roof's fall. On a very shallow roof a drip may need to be deeper, and on a steeper roof a shallower one will do.
When you hear a roofer talk about a roof with "no drips", the trouble is usually that the original builder chose to run the sheets too long. Those roofs tend to show the same pattern: ripples, then splits across the sheet, usually on the sunny side. We look at the small bay above a gutter on our page about the lead gutter drip.
Falls, decks and the layers beneath the lead
Lead is a good waterproof layer, but it can only work if water leaves the roof. BS 6229 puts the recommended minimum fall at 1:80, and manufacturers advise designing to 1:40 so that after tolerances and deflection in the timbers the finished roof still drains at 1:80 or better. A lead roof that has settled and now holds a pool of water is the typical sign of a fall that was too shallow at the start or has moved since.
The deck under the lead is usually exterior-grade board or plywood, fixed to joists. It must be smooth and flat, because any ridge or nail head will press through the lead over time and wear a hole. Between the deck and the lead there should be a separating layer, often called a lead underlay. This stops the lead sticking to the board, cushions it against rough surfaces, and allows the sheet to slide as it moves. A bay laid straight onto bare boards is a bay waiting to tear. The lead underlay page covers what is used and why.
Beneath the deck sits the roof's construction. A warm roof has insulation above the deck, so the structure below stays warm. A cold roof has insulation between or below the joists and needs a ventilated void so moisture cannot collect and rot the timbers. The type matters when you have lead renewed, because relaying the lead is the point at which the deck can be inspected, and Building Regulations may come into play if the work goes past the thresholds for re-covering. Check with your building control body if you are in doubt.
The edges of the roof are the next detail. Lead is turned down over a timber upstand or into a gutter, and is usually held with a lead strip or clips. At a wall, the bays finish with a flashing, which our lead flashings guide explains in full.
Cost, lifespan and what ages a lead roof
Lead is more expensive to lay than felt, EPDM or GRP, and it is priced by the square metre. Typical UK ranges from MyBuilder's flat roof cost guide (2026) put a lead flat roof at £150 to £200 or more per square metre, against lower figures for the other membranes. Your fixed price follows the survey, because the real cost depends on the code, the number of rolls and drips, the amount of flashing, how sound the deck is and how easy the roof is to reach.
The reason to pay for it is the life. Less's flat roof cost guide (2026) gives a lead flat roof as lasting 50 to 100 years, and MyJobQuote's leadwork guide (2026) puts correctly installed lead flashing at over 80 years, and over 100 years for higher codes. Those are typical figures and they vary with exposure and workmanship. Neither is a promise. In practice a lead roof that fails early has usually been undermined by one of a handful of causes.
Common causes of early failure
- Bays laid longer than the code allows, so the sheet creeps and then cracks.
- Fixings that are too rigid, or clips missing, so the sheet cannot move.
- Missing underlay, so the lead wears against the deck.
- Standing water from poor falls, which stains, corrodes and softens the surface.
- Foot traffic and tree debris, which score and trap damp.
A lead roof also weathers on the surface. It forms a grey patina that protects the metal underneath, and a pale powdery white coating can appear early on, especially where new lead sits damp. It is normal and washes off, and our page on lead patination explains what is going on.
Looking after the roof, and the problem of theft
A lead flat roof asks for very little, but it does ask. Clear the outlets and the gutter once or twice a year, and after storms. Leaves and silt trap water against the lead and speed up corrosion. Keep the roof free of moss and soil, and avoid stacking anything on the surface. When a gutter, hopper or downpipe clogs, the overflow seeks the weakest joint on the roof.
Keep off the roof unless you need to be on it. If you must go up, step on the flat bays between rolls, avoid the crowns, and keep sharp objects and stiff-soled boots away from the surface. A cracked sheet is usually the result of something that happened years earlier, not of what you did that morning.
Use binoculars from the pavement, or look from an upstairs window. Look for ripples that run across a bay, splits at a roll or drip, patches where the colour changes, and any place where the sheet has lifted from the deck. A small split can sometimes be repaired with lead, and the question of when patching stops being sensible is the one a survey settles. For a quick view of your options, see our page on how we repair split lead.
Lead is also valuable as scrap, and exposed roofs are targeted for theft. If lead has been stolen from your roof, make the building watertight first and tell your insurer. Our guide to stolen lead and insurance lists what to record before you make a claim. A theft that leaves the roof open is a leak waiting to happen, so the first job is making it safe.
When the survey is done we put the fixed price in writing, with the code, the bay layout and the flashings named, and if theft is a concern, ask about forensic marking of the new sheets and a roof alarm before the lead goes up. For the wider picture, see our lead roofing service.