A flat roof sits in an odd place in the rules. Putting a new covering on a garage roof can look like simple maintenance, yet the same job on a kitchen extension may need an application to building control, an insulation upgrade and a change to the way the roof drains. The difference comes down to what is being done, how much of the building it touches and what the roof is made of. This guide sets out the questions that decide it in England: when approval is needed, what insulation has to do with it, why falls and weight matter, and who checks the work. It is general information, not legal advice, so confirm your own case with your council's planning team or your building control body.
Planning permission and Building Regulations are different things
People often use the two terms as if they meant the same. Planning permission is about what a building looks like and how it affects the street and the neighbours. Building Regulations are about how the building is made: structure, energy use, fire safety, drainage and ventilation. A like-for-like flat roof renewal rarely needs planning permission, but it can still fall under the Building Regulations. Changing the roof's shape, raising its height or turning it into a terrace is a different matter, and our answer on whether a new flat roof needs planning permission covers that side.
Conservation areas, listed buildings and homes covered by an Article 4 direction add their own layers. If your house is in one of them, ask the council's conservation officer before you change the roof's material or its edge details, because consent can be needed even for work that looks minor.
The test for when approval is needed
The Planning Portal gives a clear starting point for re-roofing. Repairs and re-covering usually go ahead without a building control application where the work touches under a quarter of the total building envelope and under half of the roof, which the Planning Portal writes as 25% and 50%. Both limbs of the test apply together. Go beyond either one and the job needs approval, and the roof as a whole can be expected to meet current insulation standards.
The building envelope means all the outer surfaces of the building: walls, roof, floor and openings. That is why a small extension roof can stay under the line while a flat-roofed block of garages with the same covering would not. The wording matters here. You will sometimes read that approval is needed "over 25% of the roof area", which is not the Planning Portal test. If you want the original, the Planning Portal page on re-roofing sets it out.
Why insulation comes into a re-covering job
A roof counts as a thermal element, meaning one of the parts of the building that keeps heat in. When most of its covering is renewed, Approved Document L, the part of the regulations on conservation of fuel and power, expects the roof to be improved at the same time. In practice that means insulation. A tired felt roof laid in the 1980s over bare boards, or a cold roof with a few scraps of glass wool between the joists, is unlikely to be left as it was.
The distinction we draw on site is between replacing and overlaying. Stripping the old covering and laying a new one is renovation of a thermal element, and it normally brings the insulation upgrade into play. Laying a new layer over a sound existing one is generally treated more like repair, though it brings its own risks: trapped moisture, extra weight and a covering that cannot be inspected underneath. Councils word their guidance differently, so we treat this as a point to confirm with your building control body rather than assume. Our page on laying a new flat roof over the old one explains when an overlay is sensible.
Warm roof or cold roof
Insulation can sit in two places. In a warm roof the boards go on the joists, the insulation goes above them, and the weatherproof layer goes on top, so the structure stays inside the warm envelope. In a cold roof the insulation goes between the joists, with a ventilated gap above it under the deck. The two behave very differently with moisture, and our guide to warm roof against cold roof construction goes through both.
The rule that matters for the regulations is simple. Cold flat roofs need ventilation, in line with Approved Document C and BS 5250, the standard on managing moisture in buildings. Warm roofs need a vapour control layer, a sheet that stops warm, damp air from the room reaching the cold insulation. Get either of those wrong and condensation forms inside the roof, which is how many perfectly good coverings end up over rotten decks. If you want to know how deep the insulation has to go, our short answer on flat roof insulation thickness picks that up.
Falls, drainage and the deck
Regulations and good practice agree that a flat roof is not really flat. It needs a fall, a slope towards the outlets, so water leaves instead of sitting. On a new deck the fall is built in with tapered timber firrings or tapered insulation boards. On an existing roof that has lost its fall, perhaps because the joists have sagged, a stripped re-cover is the moment to put it right. Laying a new membrane over a dipped deck simply repeats the old ponding in new material.
Drainage is also part of the picture. Outlets, gutters and downpipes have to take the water the roof collects, and the roof edge has to direct it into them. If you are changing the outlet or adding a larger area, check how rainwater will be dealt with. Our answer on the minimum fall for a flat roof goes into the slope itself.
The deck underneath matters too. Soft or delaminating boards are not something you can lay over. They need cutting out and replacing, and any structural repair to joists is part of the job rather than an extra to hope you can avoid. Water that has been getting in for years often leaves the insulation sodden as well, and wet insulation stops insulating. Because the regulations expect the finished roof to perform, we lift and replace any board or insulation that has taken on water instead of sealing it in under a new membrane. That is also why a quote written without going up on the roof is only a guess: the deck and the layers beneath it decide how much of the job is covering and how much is repair to what lies below.
Weight, structure and the 15% rule
Approved Document A covers structure. Where the new covering would weigh 15% or more above the old one, approval is needed, and the supporting structure may have to be strengthened. Modern flat roof coverings are light, which is one of the reasons they suit extensions. The weight problems come from what is added on top: paving slabs on pedestals, a layer of gravel ballast, soil and plants, or a deck for a terrace. Adding a second layer of covering over an old one also adds load that nobody has checked.
If you are thinking about turning a flat roof into a place to stand, there is more to deal with than weight. Guarding, privacy for neighbours and escape routes all arise, and our page on using a flat roof as a balcony or terrace starts with those.
Who approves the work
You have two routes for building control. One is the council's own building control team. The other is a registered building control approver, the name given to private building control bodies since April 2024. Either will take your application, inspect at the stages that matter, and issue a completion certificate at the end. That certificate is worth keeping, because a buyer's solicitor can ask for it when you sell.
If the job is notifiable, Beacon deals with the paperwork and the inspection bookings on your behalf. A typical sequence is a survey, a decision on whether the job is notifiable, the application or notice, inspections while the work is open, and sign-off. Inspections matter on a flat roof because the layers that count, such as the vapour control layer and insulation, are covered over by the final membrane within a day or two.
| The job | Is approval likely? | What to check |
|---|---|---|
| Patch repair or local re-covering | Not normally | Stays under a quarter of the building envelope and half of the roof |
| Full strip and re-cover of the roof | Often yes | Insulation upgrade and ventilation or vapour control layer |
| New covering much heavier than the old | Yes at 15% or more | Whether the structure needs strengthening |
| Overlay on a sound roof | Treated as repair, confirm locally | Trapped moisture and added load |
| Terrace or balcony use | Raises planning and building control questions | Guarding, privacy and structure |
What a survey should tell you before you decide
Few homeowners can judge from the ground whether a re-roof sits under the threshold. So the first visit is a survey, and it costs you nothing. Someone climbs up with a tape and a camera, measures the roof, records the existing build-up, prods the deck for soft boards and looks along the surface for dips that hold water. Every finding is photographed, so the report shows your own roof rather than a stock picture. From that we can say whether the job is likely to be notifiable, which insulation and ventilation details it would bring in, and whether a repair at a single upstand, outlet or edge would do instead. Plenty of flat roofs have a few good winters left, and when yours does we will tell you that plainly.
The report comes with a fixed written quote, and our workmanship is guaranteed for ten years, and the guarantee comes on paper. When the job finishes the site is left clear and the old covering leaves with us. Our main flat roofing in Basingstoke page shows the range of roofs we cover, and you can book a survey on 01256 213956 or by WhatsApp.
