A flat roof covered in lead looks nothing like the felt and rubber roofs on modern extensions. It is a grid of narrow metal strips, divided by low ridges, and it has been used on parapet roofs, porches, bays and dormer tops for a very long time. Lead is heavy, expensive and slow to lay, and in return it can last far longer than any other flat roof covering.
The secret is movement. Lead expands in warm weather and shrinks in cold, and a single big sheet would tear itself apart. So the roofer cuts the covering into bays of limited size, joined by details called rolls and drips that let each piece shift. This guide takes you through that system, then looks at falls, ventilation, life, cost and the sort of house where you find it. Patching a damaged roof is a separate job, covered on our page about lead flat roof repair. The service as a whole is described at lead roofing.
What lies under the lead
The lead is only the top layer of a built-up roof. At the bottom is the structure: joists or rafters carrying a timber deck, often boards or sheets of plywood. Over the deck goes a separating layer, a thin sheet that lets the lead slide a fraction as it expands and stops it sticking to the timber or being marked by resin and nail heads. Then comes the lead, in sheets cut to the size of each bay.
Between deck and ceiling there is also a void or an insulation layer, depending on the age of the roof. How that part is built decides whether the roof is warm or cold. In a cold roof the insulation lies along the ceiling, leaving the deck chilly above an air space. In a warm roof the insulation is laid directly beneath the lead, so the deck stays close to room temperature.
Why the deck matters so much
Lead is unforgiving of a poor surface. A deck that dips holds standing water, which sits on the lead for days and speeds up wear. A deck with sharp edges or loose boards puts stress on the metal where it bends. Many failures that look like lead faults are deck faults, and a good survey looks at the timber from beneath before anyone writes a figure on a quote.
When the covering is lifted the deck is exposed, which is the moment to replace any boards that have gone soft and to fix the falls. It is far cheaper to correct the deck then than to relay the lead later.
Bays, rolls and drips: how the sheets are laid
Think of the roof as a set of long strips running down the slope. Each strip is a bay. Bays are separated by rolls, which are raised ridges that run in the direction of the fall. Across the slope, where one bay ends and the next begins at a lower level, the deck steps down in a drip, a small vertical lip that lets water run off the upper sheet onto the lower one.
Rolls
A roll is a timber core, shaped to a rounded profile and fixed to the deck, with the lead from the two neighbouring bays folded over it. The upper sheet is dressed over the lower edge so that water, which can only run downhill, never meets an open seam. The roll also gives the metal somewhere to move. When the lead expands, it tightens against the roll rather than buckling in the middle of the bay.
Drips
A drip does the same job across the fall. It limits the length of any sheet, because the longer the piece, the more it moves. The upper sheet runs over the step and the lower sheet tucks under it, so the joint sheds water outward. Without drips, a long bay would creep downhill, pulling away from the top.
Fixing
Lead is held at its top edge with clips and fixings, never nailed through the field of the sheet. A hole through the middle of a bay is a leak in waiting. The flashing round any chimney or wall that interrupts the roof then dovetails with the same rolls and drips, which is why lead flashing repairs on a flat roof need to follow the bay layout.
Falls, so water leaves the roof
Even the flattest roof has a slope built in. BS 6229 is the flat roof code of practice, and IKO and Rockwool both describe its two numbers. The target is a design fall of 1:40, which still leaves a finished fall of no less than 1:80 after the deck has deflected and the builders' tolerances have been used up. Put in plain terms, 1:80 is a drop of 12.5 mm for every metre, and 1:40 is 25 mm for every metre.
Those figures are the reason a lead roof is laid on a deck built to a fall rather than on a level surface. Water that stands on lead does not simply sit there. It leaves marks, carries grit into folds and finds any weak seam. Water that runs away takes the dirt with it and leaves the metal clean and dry.
Where the water goes
On a bay or porch roof the water usually runs to a gutter at the front edge. On a larger roof, or one behind a parapet wall, it runs to a parapet gutter or a box gutter and out through an outlet. Those are the same channels covered on our page about lead valleys, where the heavier lead codes are normally used.
When a lead roof is replaced, the falls are checked against the deck, and any improvement to the fall is itemised as its own line. It is much cheaper to taper new timber while the deck is open than to live with a pond that was there before.
Water still lying on the roof a couple of days after rain points to a deck problem, and new lead laid over an unchanged deck will simply repeat the fault.
Ventilation and insulation under lead
Water is not the only threat to a lead roof. Moist air from the house can rise into the roof structure and condense on cold timber, and the underside of the lead can sweat too. BS 5250 deals with moisture in buildings, and it sits behind the ventilation advice that manufacturers publish for cold pitched roofs. For a flat roof the arrangement is different, and we work it out from the construction we find at survey.
Cold decks
On a cold deck, the air space between the ceiling insulation and the timber above it needs an outlet for damp air, normally through gaps or vents along the edges. If those routes are blocked by later insulation, paint or a clumsy repair, the timber stays damp and rots from below long before the lead wears out.
Warm decks and Part L
A warm deck puts the insulation above the structure, so the void is not needed. The thermal rules come into play once the roof is reworked. Under Approved Document L, stripping the waterproof layer off a flat roof and laying a new one is renovation of a thermal element, which is the roof itself. If the work touches over 50% of its surface, the entire roof should reach 0.16 W/m²K, and a wholly new or replacement roof element has a limit of 0.15 W/m²K. How all this applies to your roof is for your building control body to decide; at survey we explain the position and deal with any notification.
VAT on roof or loft insulation fitted in a house is nil until 31 March 2027, whereas the lead and labour carry the normal 20%, and each line of the quote shows its VAT.
How long lead lasts, and what it costs
Less Ltd's flat roof guidance gives a lead flat roof a life of 50 to 100 years. MyBuilder's flat roof guide gives flat roofs generally a life of 15 to 30 years, and Less puts built-up felt at 10 to 20 years and GRP at 25 to 40. Those are typical ranges, based on good laying and reasonable care, and not promises.
The price comparison
| Covering | Typical cost per m² |
|---|---|
| Bitumen felt | £70 to £100 |
| EPDM | £90 to £130 |
| PVC single-ply | £95 to £140 |
| GRP | £95 to £150 |
| Lead | £150 to £200 or more |
The figures are MyBuilder's, from its flat roof cost guide updated in August 2026. They are national and typical, and they exclude whatever your own roof needs in the way of scaffold, deck repair or insulation.
Lead costs the most to lay and lasts the longest. Whether that trade is worth making turns on how long you plan to keep the house and on whether the building has features, such as a parapet or a bay, where a different material would look wrong or be hard to detail. We set out the lead and the alternatives side by side if you ask, and the written quote shows the cost of each stage: access, stripping, deck repair, lead, flashings, waste and making good.
A lead flat roof is also repairable in sections, so a damaged bay need not mean a whole new roof.
Where you find lead flat roofs on period homes
Lead belongs to a particular kind of building: older houses with bay windows, porches, parapets and dormers, where a small area of flat or shallow roof has to be made weathertight at a detail that would be awkward in any other material. Stroud District Council lists 41 conservation areas, and some changes to a house inside one need planning permission that would not be needed elsewhere.
The usual places
- Bay window roofs, where the covering folds round several angled faces.
- Porches over front doors, usually small and shallow.
- Parapet roofs, hidden behind a stone or brick wall at the edge, with a gutter along the foot of the wall.
- Dormer tops and cheeks, where the lead meets the main roof at a flashing.
- Flat sections of older extensions, behind a gable or between two pitches.
Listed and protected buildings
On a listed house the protection covers the entire building, outside and in, whatever the grade, according to Stroud District Council. Putting lead back as lead is one thing. Changing the material, or the profile of the roof, is another, and the conservation team can tell you before work starts which of the two you are planning. On older houses we put lead back as lead, and stone as stone.
Your survey photographs show the metal, rolls and flashings as they stand on your own house, and the quote is built from them. The job carries our written 10-year workmanship guarantee, and we clear everything away when we finish.