A flat roof suits an extension for good reasons. It keeps the roofline low, it leaves room for a rooflight over a kitchen or dining space, and it avoids tying into the existing roof slope. On a Worthing bungalow with a rear extension, or a Victorian terrace with a kitchen at the back, a flat roof is often the neatest answer.
The same roof, though, has no spare capacity for poor detailing. A pitched roof throws water away by gravity and has many overlapping joints, each of which gives a little. A flat roof depends on one waterproof layer, and water stays on it for longer. If the edge is awkward, the fall is marginal or a rooflight is fitted without a proper upstand, the problem is built in, and no covering can fully cure it later.
Most of the choices are made on the drawing board, long before a roofer arrives. This guide goes through them in the order they come up: the edge, the fall, the trims, the rooflight, the build-up and the choice of finish. It is written for homeowners talking to an architect or builder. Our flat roofing page covers everything else we do on flat roofs.
Parapet or overhang: choosing how the roof ends
The first design decision is how the roof meets the wall at its edges. There are two broad answers. In a parapet design the walls continue above the roof surface, so the roof sits in a shallow tray. In an overhang design the roof extends beyond the wall, with a fascia and often a gutter at the edge.
A parapet gives the cleanest look and hides the roof edge, which is why architects favour it. It also brings a detail to get right: where the membrane meets the wall, it is dressed up the inner face and finished under a coping or flashing at the top, so water running down the wall cannot get behind it. Outlets must pass through the parapet or sit within the roof, because water cannot run over the edge. Those outlets, and any overflow through the wall, are the points to check first, since a blocked outlet in a parapet roof fills the tray instead of spilling harmlessly.
An overhang is more forgiving. If an outlet blocks, water runs over the edge and is seen, rather than collecting. The edge trim and gutter are accessible from a ladder, which makes routine cleaning easier. The trade-off is a more visible fascia line and a gutter that must be kept clear and correctly fixed.
Which suits your house
A parapet works best where the wall below is solid and weatherproof, as it exposes the top of the wall to more weather than an overhang protects. An overhang suits roofs that need easy access for upkeep. What matters is that the drawing shows every junction, because a vague note such as 'flash to wall' is where leaks start. If you plan to use the roof as outdoor space, read our guide to a flat roof terrace first, since a parapet is also the safest way to provide an edge barrier.
Falls: the quiet detail that sets the roof's lifespan
No flat roof is truly flat. It has to slope so water leaves, and the slope is a design decision that is easy to skimp. IKO's summary of the 2018 edition of BS 6229 (since revised) gives 1:80 as the finished minimum and 1:40 as the design figure, because tolerances and sag in the structure eat into the slope. Design 1:40 to achieve 1:80.
The gap matters because joists sag a little, decks are never perfectly level and trades work to tolerances. A roof drawn with a 1:80 fall may end up flat in places, and flat patches hold water. Standing water on a roof is a leading cause of early failure, because it keeps the surface wet, collects grit and algae, and puts every seam under constant pressure. Our guide to the minimum fall on a flat roof explains the figures in more detail, and flat roof ponding shows what happens when they are missed.
There are two ways to form the fall. Timber joists can be set sloping, or tapered boards can be laid over a level structure; in an insulated warm roof, the insulation itself is often supplied as tapered boards. Either way, the fall should be shown on the drawing as a direction and a figure, with the outlet position noted, not left as something to be 'sorted on site'.
On a wide extension a single-direction fall can leave one edge much higher than the other and eat into headroom or window heights. The cure is usually a central ridge or a fall towards a corner, formed with extra tapered boards.
Edge trims, drips and upstands that survive coastal weather
The edges of a flat roof are where wind gets under the membrane, where water finds its way behind the trim, and where the roof is least forgiving. Edges and upstands are one of the first places we look when a flat roof has failed. A good design gives them as much attention as the main surface.
At a drip edge, the membrane should finish over a metal or rigid trim that throws water clear of the wall or fascia. The trim must be fixed securely, with laps that shed water, and the membrane must be properly stuck to it. A trim fixed with a few clouts and a line of sealant is a weak point, and sealant cannot be relied on to hold for years against sunlight, movement and the salty air on this coast.
Upstands, the upward turns of the membrane at walls and around rooflights, need a minimum height above the finished roof surface so that splashing and standing water cannot reach the top edge. The membrane is dressed up the upstand and covered at the top by a flashing, a cover strip or a coping. If insulation is thick, the upstand is easy to make too short. Check the dimensions on the drawing against the finished level once the insulation and the covering are on.
Wind at the edges
Wind lifts a roof from the perimeter, so edge trims and fixings do real work in a gale. We cover the reasons in our guide to flat roofs on the coast: wind, edges and trims. For a new extension, ask the roofer to confirm how the edge trim is fixed and how the membrane is held down at the perimeter, because those answers show whether the edge was designed or merely finished.
Where the metal trim is zinc, salt deposits form on surfaces that rain does not wash, such as the underside of a fascia or soffit, so these need an occasional rinse with clean water.
Rooflights and lanterns: getting the upstand right
A rooflight over a kitchen extension changes how a room feels, but it also adds a hole in the roof and a new junction. Approved Document L separates two kinds of product. A rooflight is a glazed unit installed out of plane with the roof surface, on a kerb or upstand, and a roof window lies flush with a pitched slope. A flat roof extension almost always has the first kind, so the kerb is part of the roof's construction.
The same document sets limiting U-values for new or replacement elements in existing dwellings, which is the heat-loss measure for each part of the building. For a rooflight the figure is 2.2 W/(m²·K). A note to that table caps a site-built upstand at 0.35 W/(m²·K), so the kerb needs insulating and cannot be left as a bare timber box. A cold kerb is a place for condensation to form on a winter morning, and the staining that follows is often mistaken for a leak. Our guide to telling condensation from a leak shows how we separate the two.
The kerb has to be tall enough that the membrane can be dressed up it and finished under the rooflight frame with room to spare, and it must be level and square, since the glazed unit sits on it. Many leaks around rooflights come from kerbs built too low, or from a membrane that stops short of the frame and relies on a bead of sealant.
Rooflight position also affects the fall. Placing a large rooflight in the middle of the lowest part of the roof blocks the path of water, so the design needs a small saddle or cricket upslope to divert water around it. If the glazing is large, ask whether the structure has been designed for its weight and for the opening it needs.
The build-up under the membrane and what it does to height
The layers beneath the waterproof covering decide the roof's thermal performance, its condensation risk and its overall depth. A warm roof puts the insulation on top of the structure and under the membrane, which keeps the timbers warm and avoids the need for ventilation in the roof void. A cold roof puts the insulation between the joists, with a ventilated gap above and a vapour control layer below. Our guide to warm and cold flat roofs compares them. For most extension roofs today a warm roof is the simpler build.
The target is set by the Building Regulations. Table 4.2 of Approved Document L caps heat loss through a newly built or replaced roof on an existing home at 0.15 W/(m²·K). Meeting that with a flat roof takes a substantial thickness of insulation, and that depth has to be allowed for in the design: it raises the roof edge, lifts the upstands and can affect the height of a window head or a door. If the extension is joined to an existing roof, the junction must be planned so that the new roof does not trap water against the old wall.
Your architect and building control body confirm the build-up for your extension. As a roofer, we would ask for the specification early, including the type of insulation, the vapour control layer, the deck and the membrane, so that the roof can be priced and built as drawn.
Deck and structure
The deck is the layer the membrane is stuck or laid on. It might be plywood, OSB or timber boards. It must be dry when covered, sound and securely fixed, because a loose or damp deck causes blisters and wrinkles. Moisture trapped under the covering has nowhere to escape and will not simply dry out.
Matching the covering to the design, and to the years you want
The covering is the last decision and the one many people start with. It should follow from the design, not lead it. A roof with many penetrations and awkward junctions suits a liquid-applied or GRP system, which forms a continuous skin around odd shapes. A large, simple roof suits a single-ply EPDM sheet laid in one piece, with fewer joints to maintain. Our guides to EPDM against GRP and flat roof materials cover the choice in detail.
Published lifespans give a rough guide. EPDM is quoted at 20 to 30 years or more, GRP at 25 to 40 years and torch-on felt at 10 to 20 years, with sources disagreeing on how well a good felt job holds up. These are typical ranges rather than promises, and each assumes good falls and sound detailing. A top-quality membrane on a roof with a flat spot and a short upstand will fail early. A modest membrane on a well-drained, well-detailed roof can last.
MyBuilder (updated 21/08/2026) gives a typical UK range of £90 to £130 per m² for EPDM, £95 to £150 for GRP and £70 to £100 for felt, and MyBuilder's re-roofing guide lists £2,500 to £6,000 for a semi-detached home's flat roof. Price is fixed in writing after the survey as one sum, and turns on size, access, the number of junctions and what lies beneath the old layers.
If you inherit an extension roof that already has problems, the right question may be what to do with it now. Our guide to repairing, overlaying or replacing a flat roof sets out how to decide, and a close look at the original design often explains why it has failed.
