Replacing a flat roof looks like a single job from the pavement, but it is really a chain of stages, and each depends on the one before. The old covering has to come off before anyone can see the deck. Insulation can only go on once the deck is dry and sound. The insulation has to be laid to the right falls before the waterproof layer, and the waterproof layer has to be finished and sealed before the roof is left overnight.
Knowing that order helps you plan. It tells you when the house will be noisy, when a tarpaulin may be needed, why a dry spell matters and what questions to put to a roofer before work begins. The stages below follow a typical domestic job on a garage, extension or dormer roof. Exact timings depend on the size and the weather, and a roofer who promises a fixed number of days before seeing the roof is guessing. For the other end of the process, see flat roof maintenance, which keeps a new roof in shape.
Before anyone climbs up
Everything begins with the survey, well before a ladder goes up. Our flat roofing service starts the same way every time. We inspect the existing covering, look at the deck from above and below where we can, and look over the outlets, edges and upstands to see how rainwater leaves. Pictures of the roof at your address come with the written quote, which says what is being replaced, with what, and how the roof will drain.
Three decisions are settled at this stage. The first is the covering: felt, EPDM, GRP or another system, covered in our guide to flat roof materials. The second is the build-up: whether insulation goes above the deck as a warm roof or between the joists in a cold roof, which our page on warm and cold flat roofs explains. The third is whether the job needs Building Regulations approval. Under Approved Document L it often does, because the guidance lists a fresh membrane on a flat roof under renovating a thermal element. If the work must be notified, we talk you through the consent route and make the Building Regulations notification.
Do I have a cooling-off period?
Yes. Under the Consumer Contracts Regulations 2013 a contract signed at your home has a 14-day cancellation period. We only begin inside it if you tell us to, on paper or by email, and anyone who cancels after a start pays for what has been done so far.
Stage one: access, protection and strip
Access comes first. Scaffolding is often needed on a first-floor roof or a tall garage, and sometimes only a ladder and a hoist are enough. MyBuilder's scaffolding guide, updated on 21/08/2026, puts a standard two-storey setup at around £850 to £1,200, so ask whether any quote includes it.
Before stripping, the crew protects what lies beneath and around the roof: sheets over plants and patios, dust sheets or boards over adjacent glazing, and a plan for where waste goes. Rooms below may need clearing of precious items, since vibration shakes dust from ceilings.
Stripping then begins. Felt is cut and peeled in sections, EPDM is lifted and the glue scraped, and old GRP is chiselled or lifted. Old trims, flashings and outlets come off too. Materials are bagged or loaded into a skip as the work goes, so the roof is never left open while waste lies about. On a pre-2000 building the possibility of asbestos is checked first, particularly in old soffits or fibre-cement sheets, because HSE says to assume it is there in any property built or refurbished before 2000.
Nothing should be stripped unless it can be made watertight again the same day. A roof opened in the morning needs to be covered by evening, or the weather forecast has not been read properly.
Stage two: the deck check
With the covering off, the deck is exposed for the first time. This is the moment the survey's guesses become facts. The crew looks for rot, softness, delamination and boards that move underfoot, and checks that the deck is dry. Damp boards cannot simply be covered over, because trapped moisture rots the timber and blisters the new membrane.
Sound boards stay. Rotten boards come out and are replaced, and the supporting joists are checked where there is any sign of trouble. Where the roof is a concrete or screeded deck, the surface is checked for cracks and level. If the deck needs more than minor repair, the cost changes, which is why we flag the possibility at the survey and tell you straight away if it turns out worse than expected.
What happens if the deck is worse than expected?
We pause, photograph the damage and call you before carrying on. A rotted section found on site is a reason to revise the scope, not to quietly cover it. Our page on spongy flat roofs explains what soft boards usually mean.
The deck is also where the falls are set out. The rule is to design for 1:40 so the finished roof achieves at least 1:80, according to IKO citing BS 6229:2018. A level or back-falling deck is corrected now, with tapered insulation or timber firrings, because a covering cannot cure ponding on its own. See flat roof minimum fall for more.
Stage three: vapour layer, insulation and falls
On a warm roof, the build-up goes in this order from the bottom: the deck, a vapour control layer to stop warm, damp house air reaching the cold insulation, then rigid insulation boards, then the waterproof covering on top. The vapour layer is laid with sealed laps and turned up at the edges. The boards are laid tight with staggered joints, mechanically fixed or adhered to suit the system, and tapered boards form the falls to the outlets.
Why does this matter? Because Approved Document L sets the standard for thermal performance. Table 4.2 gives 0.15 W/(m²·K) for a new roof element, and Table 4.3 gives 0.16 for an existing element that is being renovated. Insulation is therefore not an optional extra when the roof is replaced. How thick the insulation must be varies with the product, and the build-up can also affect door thresholds, window cills and rooflight upstands, which are checked at the survey.
A cold roof keeps its insulation among the joists under the deck, with a ventilated void above. That arrangement needs a clear air gap and vents at opposite edges, and it has to be kept free of blockage. Many installers prefer the warm design for replacements because it avoids the condensation risk of a poorly ventilated void, and a roofer should explain which they are proposing and why.
Do rooflights and outlets get changed at this point?
Usually yes. Outlets are renewed so they sit in line with the new covering, and rooflight kerbs are checked, re-lined or replaced. Part L limits rooflights to 2.2 W/(m²·K), so an old single-glazed unit may need replacing rather than reusing.
Stage four: the waterproof layer
This is the stage that depends most on weather. Each covering has its own method.
Torch-on felt goes down in layers, with a gas torch softening the underside so each sheet fuses to the one beneath. Laps are staggered, edges are dressed up the upstands and the cap sheet finishes the top. EPDM is rolled out in one piece, glued to the deck or insulation with a contact adhesive, folded into the corners and sealed with purpose-made tapes and edge trims. GRP is laid as resin over glass matting, then rolled out to remove air, and finished with a pigmented topcoat once the base has cured.
Rain stops all three. Adhesive will not stick to a wet surface, felt will not fuse to damp boards, and GRP resin will not cure in damp or in the cold. Strong wind is a nuisance for large EPDM sheets and hot-work, and standing in direct summer heat can soften felt and make a topcoat go off too fast. A competent crew watches the forecast, covers up each evening and will stop rather than lay on a doubtful surface. That sometimes costs a few days, and it is the right trade.
When the waterproof layer is complete, the outlet is checked by running water across the roof and watching where it goes. Any pools that stay behind are marked and discussed before the crew leaves.
Stage five: trims, flashings and clear-up
The last details do much of the long-term work. Edge trims are fixed and sealed so water drips into the gutter, not behind the fascia. Flashings, usually lead, are dressed over the top of the upstands where the roof meets a wall, tucked into a chase in the brickwork and pointed. Leadworx says Code 4 has become the usual choice for general flashing, with Code 5 for heavier duty. Our page on flat roof trims shows how each edge is formed.
Drainage is the other item. Gutters are cleaned, joints re-sealed and brackets checked for a gentle fall. If the old gutters sag, the new roof will not behave well, and replacing them at the same time saves a second scaffold. Our guide to flat roof drainage covers the options.
Then the site is cleared. Offcuts, packaging and old covering go into the skip or the van, and the yard or garden is swept, with roof debris collected from the gutters and ground. Household asbestos waste, if any turns up, is a separate matter and needs a consignment note.
What do I get on completion?
A walk-through, your 10-year workmanship guarantee in writing, and any manufacturer paperwork for the product used. If building control was involved, the sign-off follows.
What the homeowner needs to do
The job goes more smoothly with a little preparation on your side.
- Clear the access route and the area where materials will be stored. Move cars off the drive or street frontage.
- Tell neighbours, especially if scaffolding crosses their boundary or hot-work is planned near a shared wall.
- Empty shelves and wardrobes on walls below the roof, and cover furniture, since vibration drops dust.
- Agree where the crew can park, make tea and use a toilet, and where the electricity and water are.
- Keep pets and children away from the working area, and talk to the foreman if you are unsure.
Please do not ask the crew to carry on in the rain to save time. Ask instead how the roof will be made watertight each evening. If the work is on a flat roof above a bedroom, think about sleeping in another room for the first night or two.
What if a stage needs a decision from me?
Rotten boards, a failed outlet or a rooflight that needs replacing are the usual ones. We stop, show you photographs and set out the options before any extra money is spent, and anything that changes the job is agreed in writing. Our guides to flat-to-pitched conversions and EPDM against GRP help with the bigger choices before the day arrives.