Flat roof installation is a job where most of what decides the result is out of sight by the time the roofers leave. The deck, the falls, the vapour control layer and the insulation all sit under the membrane, and they matter more to how long the roof lasts than the covering you can see.
This guide sets out what a flat roofing installation actually involves: how a flat roof is constructed, which system suits which job, and what happens at each stage from the survey to the final sign-off. Whether it is a new flat roof on an extension, a replacement on an existing home or a commercial building, the core sequence is the same.
First: Is a Flat Roof the Right Choice?
Flat roofs suit certain situations better than others. They are the standard choice for single-storey rear extensions, garages, dormers, porches, and outbuildings. They also work well for commercial units and larger properties where a pitched alternative would be impractical or unnecessarily expensive.
If you are weighing up flat against pitched for an extension, we have written a detailed comparison for extensions and a general flat vs pitched guide that cover the decision in full. For this post, we are focusing on what happens once you have decided a flat roof is the way forward.
Which Flat Roofing System Suits Which Job?
There is no single best flat roofing system. The right one depends on what the roof has to do: whether anyone walks on it, how much detail is packed into it, how big it is, and whether it can be seen from the house. As a guide to how the choice usually falls:
- Extensions, garages and dormers: built-up felt, EPDM and GRP are all common choices here, and the decision usually comes down to the finish, the detailing and how the roof will be used.
- Balconies, terraces and roofs that get walked on: GRP, because it cures into a hard-wearing, walkable surface.
- Roofs crowded with pipes, rooflights and plant: a liquid applied membrane, which goes on wet and takes the shape of every upstand and penetration without cut joints.
- A tired covering on a sound deck: in the right circumstances, a liquid coating over the existing roof, where the survey shows the surface underneath is fit to take it.
- Large commercial and industrial roofs: single ply membrane, which is light, quick to lay over big areas and welded at every lap.
- Roofs in view from the house: a green roof over a suitable waterproofing system, where the structure can carry the extra weight.
How long each system can be expected to last, and how the materials compare in more detail, is covered in our guide to flat roof materials. This guide deals with what happens once a system has been chosen.
If you would rather we quoted for the work, our flat roofing service covers felt, EPDM, GRP, liquid and single ply, and we will tell you which of them suits your roof.
Flat Roof Construction: What Goes Into the Build-Up
A modern flat roof is a stack of layers, each doing one job. On a typical warm roof, working from the inside out:
- Structure: the joists that carry the roof, usually timber on domestic buildings.
- Deck: the boarding fixed over the joists, usually plywood or OSB on domestic roofs, which everything else sits on.
- Falls: the slope that takes water off the roof, built in with tapered firrings under the deck or with tapered insulation above it.
- Vapour control layer: a membrane that stops warm, moist air from inside the building reaching the cold parts of the roof and condensing.
- Insulation: rigid boards laid over the vapour control layer, sized to meet Part L of the Building Regulations.
- Waterproofing: the membrane itself, whether felt, EPDM, GRP, liquid or single ply, bonded or fixed to suit the system.
- Details and finish: the upstands, flashings, edge trims and outlets that close the roof off at its edges, plus any surface finish or green roof build-up.
That arrangement, with the insulation above the deck, is called a warm roof. It is the standard for new flat roof construction because it keeps the structure on the warm side of the insulation, which is where condensation cannot form. A cold roof puts the insulation between the joists and relies on a ventilated gap above it, which is harder to get right and carries a greater condensation risk. An inverted roof puts the insulation on top of the waterproofing, held down by ballast or paving, and is mostly seen on larger roofs and terraces.
The steps below follow the same order. The build-up goes on from the bottom, and each layer depends on the one beneath it being right.
The Flat Roof Installation Process, Step by Step
Every flat roof installation follows a logical sequence. Here is what happens at each stage.
1. Survey and Specification
Before any work begins, your roofer should carry out a proper site survey. This is not a quick look from the ground; it means getting up on the roof (or inspecting the deck from inside if it is a new build) to assess:
- The condition of the existing roof structure and decking
- Whether the deck boards need replacing or can be retained
- The size, shape, and any penetrations (pipes, vents, soil stacks, skylights)
- Access requirements and scaffolding needs
- Drainage: where the water currently goes and whether that needs to change
- Insulation requirements under current Building Regulations
From this survey, the roofer produces a specification: which system, which insulation, how the edges and upstands will be detailed, and how long the work will take. This is the point to ask questions, because once installation starts, the key decisions are already made.
2. Scaffolding and Site Setup
Most flat roof installations need scaffolding or another form of edge protection, even on single-storey buildings. The Work at Height Regulations 2005 apply to any work where someone could be hurt by a fall, whatever the height, and a roof edge is exactly that.
Before the roofing team arrives, you can help by:
- Moving vehicles, bins, and garden furniture away from the building to give clear access
- Letting your neighbours know that work is starting (there will be noise, and scaffolding may be visible)
- Making sure there is somewhere reasonably close for materials to be stored and waste to be skipped
The scaffolding usually goes up before the roofing work starts.
3. Stripping the Old Roof
If this is a re-roof rather than a new build, the first job is removing the existing roof covering. This means stripping off the old membrane, felt, or whatever system is currently in place, right back to the deck boards.
This stage is important because it exposes the condition of the timber underneath. Old flat roofs, particularly felt roofs that have been patched repeatedly, often hide problems: rotten deck boards, damp timbers, failed previous repairs, or inadequate falls that have been causing ponding for years.
A good roofer treats the strip as a diagnostic step, not just demolition. What they find underneath determines whether the deck can be re-used or needs replacing.
4. Deck Inspection and Repairs
With the old covering removed, the structural deck is inspected thoroughly. The deck is the timber base that everything else sits on, typically 18mm plywood or OSB (oriented strand board) on a framework of timber joists.
The roofer checks for:
- Rot or water damage: any soft, discoloured, or delaminated boards need replacing. There is no point putting a new roof over a compromised deck.
- Structural integrity: are the joists sound? Is there any sagging or deflection?
- Falls: a flat roof is never truly flat. BS 6229, the British Standard for flat roofs, calls for falls to be designed at 1:40, so that once deflection and building tolerances are allowed for, the finished roof still drains at 1:80 or better rather than holding water. If the existing falls are inadequate, this is the stage to correct them using firring strips (tapered timber battens) or tapered insulation boards.
On a new build, the deck is constructed to the correct specification from the outset, but the same checks on falls and structural adequacy still apply.
5. Vapour Control Layer
Before insulation goes down, a vapour control layer (VCL) is installed across the deck. This is a thin membrane that prevents warm, moist air from inside the building rising into the roof structure and condensing.
Condensation inside a flat roof is one of the most common causes of premature failure. The VCL stops it at source. It needs to be continuous, with all laps sealed, and dressed up around the edges and any penetrations.
This step is often invisible in the finished roof, but skipping it or installing it poorly leads to damp, rot, and a roof that fails years before it should.
6. Insulation
Part L of the Building Regulations sets the thermal standard the new roof has to meet. Under the current Approved Document L, a replacement flat roof on an existing home generally needs to reach a U-value of 0.18 W/m²K, and a roof on a new extension 0.15 W/m²K. In practice, this means a significant thickness of rigid insulation board, typically 100mm to 150mm depending on the insulation type and the overall roof build-up.
On a warm roof, which is the standard for new installations, the insulation boards go down over the vapour control layer and the waterproofing goes on top of them, as set out in the build-up above.
If most of an existing flat roof is being replaced, the regulations generally require the insulation to be brought up to that standard at the same time, whatever was there before. The exceptions are cases where doing so is not technically, functionally or economically feasible, and that is a case to make to building control rather than one to assume.
Tapered insulation boards can also be used to create or improve the roof falls, solving drainage issues and meeting insulation requirements in a single layer.
7. Membrane Installation
This is the core of the job: the waterproof layer that keeps the weather out. How it is installed depends entirely on the system specified.
For GRP: The deck (or insulation boards) are covered with an OSB overlay if needed, then a resin and fibreglass mat system is laid by hand. The resin is mixed on site and rolled over the mat, which wets out and cures to form a rigid, seamless shell. Corners, upstands, and edges are reinforced with additional layers of mat. The whole surface cures as one piece, with no joints, no laps and no seams.
For EPDM: The rubber membrane is dry-laid or bonded to the substrate with contact adhesive. The sheets are positioned, folded back, adhesive is applied to both surfaces, and the membrane is rolled into place. Seams (where sheets overlap) are sealed with specialist tape or adhesive. The membrane is then dressed into the edge trims and up the upstands.
For liquid applied: The liquid is applied by roller or spray in multiple coats, with a reinforcement fleece embedded into the wet coating at critical points (upstands, edges, around penetrations). Each coat needs to cure before the next is applied.
For single ply: The membrane sheets are positioned, and overlapping edges are heat-welded using a hot-air gun to fuse the material together. The sheets are either fully adhered to the substrate, mechanically fixed with fasteners, or held in place with ballast (gravel or paving).
For torch-on felt: Layers of modified bitumen felt are heated with a gas torch and pressed onto the substrate. The heat softens the bitumen, which bonds the felt to the layer beneath. Multiple layers build up a robust, waterproof covering.
Regardless of the system, the membrane installation needs dry weather. Rain, heavy dew, or freezing temperatures during application will compromise the bond and the long-term performance of the roof.
8. Upstands, Flashings, and Edge Details
This is where the quality of the installation shows. The flat expanse of the roof is relatively straightforward; it is the edges, abutments and penetrations that separate a roof that lasts its full expected life from one that leaks within a few years.
Upstands are where the roof membrane turns up vertically against a wall or parapet. The membrane must continue up the wall face by at least 150mm above the finished roof level. This prevents water from getting behind the membrane at the most vulnerable point.
Flashings are then dressed over the top of the upstand and chased (cut) into the mortar joint of the brickwork. Lead is the traditional material for this, and it remains the best option for longevity and weathertightness. The flashing must be properly chased, wedged, and pointed into the brickwork, not just stuck on with sealant.
Edge trims define the perimeter of the roof where it does not meet a wall. These are typically aluminium or GRP profiles that provide a clean, weather-tight edge and direct water into the gutter.
Penetrations such as soil vent pipes, extractor fans and skylight frames each need individual weatherproofing collars or detailing to maintain the integrity of the membrane around them.
9. Drainage
Water needs to get off the roof and into the guttering or drainage system efficiently. The falls built into the roof (from stage 4 or 6) direct water towards the drainage points, but those outlets also need to be properly detailed.
On most domestic flat roofs, water drains to external gutters via a drip edge. On larger or parapet-walled roofs, internal drainage outlets (sometimes called roof gulley outlets) are used, which need leaf guards to prevent blockages.
Your roofer should test the drainage before final sign-off, either by visual inspection of the falls or, on larger roofs, by flood-testing to confirm water flows to the outlets without ponding.
10. Final Inspection and Sign-Off
Once the installation is complete, the site supervisor carries out a thorough inspection:
- Membrane integrity: no wrinkles, blisters, voids, or unsealed laps
- All upstands at the correct height with flashings properly dressed
- Edge trims secure and aligned
- Drainage flowing correctly
- All penetrations sealed
- Site cleared, scaffolding down, waste removed
If the work requires building control approval (which most full re-roofs and new builds do, primarily for the insulation upgrade under Part L), the building control officer will also visit to inspect the installation and issue a completion certificate.
You should receive a warranty for the completed roof. At Cornerstone, our flat roofing installations carry a 10-year insurance-backed warranty on workmanship, which stays protected even if the installing company ceases trading, alongside the manufacturer’s guarantee on the membrane fitted.
How Long Does a Flat Roof Installation Take?
Timescales depend on the size of the roof, the system being installed, access, the weather, and what turns up once the old covering is off. Smaller roofs such as porches, dormers and garages are often finished in a few days, larger domestic roofs take longer, and commercial roofs or jobs that need the deck or insulation rebuilding can run for a few weeks. We give you the timeline in writing alongside the quote.
All of that assumes dry weather. Rain delays are a reality of roofing in the UK, and Sussex weather can turn quickly. A good roofer will not rush the work in marginal conditions, because applying membranes, resins or adhesives in the wet compromises the whole installation. A day lost to the weather is always better than a roof that fails early.
Scaffolding goes up before the roofing work starts and comes down once the roof is finished and checked, so it is on site a little longer than the roofers are.
Building Regulations and Compliance
Most flat roof installations require some level of building control involvement:
- Full re-roofs bring in Part L (energy efficiency). The insulation generally has to be upgraded to the current standard for a renovated roof, a U-value of 0.18 W/m²K. Building control will inspect and issue a completion certificate.
- New builds and extensions are covered under the broader building control process for the project.
- Repairs (patching a split or renewing a small area of covering) generally do not need building control, but once the work amounts to replacing most of the roof, it does.
If your property is in a conservation area or is listed, you may also need planning permission for changes to the roof appearance. Your roofer should flag this during the survey stage.
For more detail on roofing regulations, see our guides to Part L compliance and British roofing standards.
Commercial Flat Roof Installation
Commercial flat roof installation follows the same principles on a larger scale, with two differences that shape the job. The roof is usually bigger and more exposed, which is why single ply membranes such as PVC are the standard covering on commercial and industrial buildings: they are light, quick to lay over large areas, and welded at every lap. And the building is usually still in use underneath, so access, phasing and keeping the roof watertight at the end of each working day are planned before the work starts.
If you are pricing a commercial re-cover or a new roof on a commercial building, the single ply flat roofing section of our flat roofs page explains how we approach it, and we can quote from drawings or the roof area.
How to Prepare Your Property
You do not need to do much, but a few things help the job go smoothly:
- Clear access around the building: move cars, bins, garden furniture, and anything stored against the walls where scaffolding needs to go
- Protect items inside: roof stripping creates vibration and dust. If you have items on shelves directly below the roof, move them or cover them
- Let your neighbours know: scaffolding, noise, and a skip are all part of the process. A quick word beforehand avoids any tension
- Keep children and pets away from the work area during installation
- Be flexible on dates: weather delays are normal. A roofer who pushes ahead in the rain to keep to a schedule is not doing you a favour
After Installation: What Comes Next
A new flat roof is not entirely maintenance-free. To get the full lifespan from your roof:
- Inspect it visually twice a year: once in spring after winter weather, and once in autumn before it
- Clear leaves, moss, and debris from the surface and drainage outlets
- Check that flashings and edge trims remain secure
- Look for any signs of ponding (standing water more than 48 hours after rain)
- Do not store heavy items on the roof or drag furniture across it
For a full seasonal breakdown, see our flat roof maintenance checklist.
If you spot anything that concerns you, such as a lifted edge, a crack or water where it should not be, get it looked at quickly. Small problems caught early are straightforward fixes. Small problems ignored become expensive ones.
Flat Roof Installation Across Mid Sussex
The difference between a flat roof that fails early and one that lasts its full expected life comes down to three things: the right system for the building, correct detailing at every edge and junction, and proper compliance with building regulations. The flat surface itself is the easy part; it is everything around the edges that matters.
Cornerstone Roofing installs and replaces flat roofing across Mid Sussex from our base in Scaynes Hill, in built-up felt, EPDM, GRP, liquid, single ply and green roof systems, on homes, extensions and commercial buildings alike.
If you are planning a flat roof installation or need an existing flat roof replaced, get in touch for a no-obligation survey. We will assess the roof, recommend the right system, and give you a clear picture of what the work involves. You can also book a free visit and we will come to you.