The roof is the last big structural job on a single-storey extension, and it is the one that decides whether the new room stays dry. Most kitchen and living room extensions end in a flat or low-pitch roof, and the shape is deceptively simple: a rectangle, a wall on one side, a gutter on the other. Behind that simplicity sits a run of decisions about the joists, the falls, the insulation, the covering, the edges and any rooflight. This page follows an extension roof in the order it is built, so you know what to ask your builder or architect and what we look at when we are called in to roof one.
Start with the structure and the brief
A flat roof is only as good as the timbers under it. The joists are sized by whoever designs the extension, and they carry the deck, the insulation, the covering, the weather and anything that might stand on the roof, including snow and maintenance visits. Before we price a covering we check the deck is level enough to be given a fall, that the joists have not been notched or sagged and that the wall plate at the house wall is fixed and dry.
If the extension is still on paper, settle three things early. First, whether anyone will use the roof other than for maintenance. Second, whether it will take rooflights or a lantern. Third, how it meets the house wall, because that junction usually decides the covering. A new extension needs Building Regulations approval for the structure and its insulation, so speak to your building control body, whether that is the council or a registered building control approver, and check with your council's planning team about the extension itself before you start.
Falls: getting water off the roof
A flat roof is not flat. It needs a slope, called a fall, so rain runs to an outlet or a gutter instead of resting on the surface. Standing water adds weight, ages the covering faster and finds any weak spot. The fall can be built into the joists, which are cut or set at a slight angle, or created on top of them with tapered timber firrings, which are long strips cut to a wedge. A third method uses tapered insulation boards, which build the slope into the insulation layer.
On an extension that adjoins the house, the fall usually runs away from the wall towards a gutter or a rear outlet. It is worth checking this on the drawings, because a fall running towards the house wall is a recipe for trouble. We check by eye and with a level before any covering goes on, and where the deck is dead flat we tell the builder what is needed to correct it.
Insulation: warm roof or cold roof
The extension has to be insulated, and the position of the insulation decides how the roof is built. In a warm roof the insulation sits on top of the deck, under the covering, which keeps the timbers on the warm side and avoids condensation in the joist space. In a cold roof the insulation sits between the joists with a ventilated gap above it. The warm roof is the usual choice on a modern extension because it is simpler to get right, and it needs a vapour control layer beneath the insulation to stop moist household air reaching it.
Cold roofs need ventilation at the eaves to carry moisture away, and that is hard to guarantee on a roof with a wall on one side. Our page comparing warm roof and cold roof construction explains the trade-offs. The insulation depth is a design choice for the person drawing the extension and the building control body, so we build to their specification and fit the covering and upstands to suit.
Choosing the covering for an extension
Three coverings dominate. EPDM is a rubber sheet that can cover a plain rectangle with no joint across it. GRP is fibreglass laid wet, which wraps around lanterns and vent pipes in one skin. Torch-on felt is layered bitumen, the cheapest outlay and the shortest life. Our guide to the best flat roof material sets out how the choice depends on the roof's shape, use and setting.
On an extension the shape is usually the answer. A plain rectangle with a single wall abutment and a gutter edge suits EPDM. A roof with a large lantern, a flue or several vents suits GRP, because each of those is a place where rubber needs hand-formed corners. Typical UK ranges from third-party cost guides run from £45 to £75 per m² for modified bitumen felt through to GRP at the top end, £100 to £160 per m², with EPDM between at £70 to £120 per m² depending on the guide. They are national figures for orientation. The fixed quote comes after the survey.
Edges, abutments and the house wall
Most extension leaks start at one of three places. The first is the junction with the house wall, where the covering is turned up as an upstand and sealed under a flashing. A flashing is a band of lead or membrane that overlaps the upstand and is tucked into the brickwork or sits beneath a cavity tray, the stepped waterproof layer built into the wall. If the upstand is too low or the flashing is stuck on with sealant alone, water gets behind it in driving rain.
The second is the roof edge, where a metal or plastic edge trim finishes the covering and guides water into the gutter. The third is any hole through the roof, such as a soil vent, an extractor or a flue, each needing a proper collar. When an extension roof leaks within a couple of years, one of these three is nearly always the cause, not the covering in the middle.
Where the house wall is old brick with soft mortar, we sometimes cut a slot, called a chase, into a sound course and set the flashing into it, then repoint. Where there is a cavity tray, the flashing is dressed to sit beneath it. The right method depends on what the wall turns out to be, which is why the details are photographed at survey.
Rooflights, lanterns and access
An extension often has a rooflight or lantern to bring in daylight. Each sits on a kerb, a short timber upstand, and the covering is turned up that kerb like a miniature wall. The work is best coordinated with the roofer, because a kerb built an inch too low leaves no room for a proper upstand. For planning and installation points on the units themselves, see our guide to roof windows and rooflights.
Think about access, too. If somebody will step out onto the roof to clean a window or reach a flue, say so before the covering is chosen, because it changes the finish. Walkway pads or a boarded path protect the surface, and a roof that is meant to be used as a terrace is a different job with its own regulation questions.
Timing, weather and the order of trades
The roof goes on at a busy point in the build, so the order of trades matters. Ideally the deck is boarded, the falls are set and the insulation is laid within a short window, then covered at once, because insulation left open to rain soaks up water that is then sealed in beneath the membrane. We try to make the roof watertight before plasterers and electricians start inside, and we agree with the main builder who cleans the deck before the covering starts.
Winter is not a bar to the work, but it does tighten the window. Adhesives, resin and bitumen all behave differently in the cold and damp, and a wet deck cannot be covered. We wait for a dry spell instead of covering over moisture, and we protect the deck with a temporary sheet if the roof must be left overnight.
After the covering is finished, keep other trades off it where you can. Scaffold boards, ladders and dropped tools do more damage to a new roof in the first few weeks than weather does. If a rooflight installer, an aerial fitter or a plumber must reach the roof, a protective path of boards avoids punctures.
Handover, paperwork and aftercare
When the roof is complete we check the falls with water, look at every junction and photograph the finished edges, outlets and upstands. You should be given the specification in writing, the guarantee in writing and details of any building control sign-off that applies. A look at the roof a year later, ideally after the first autumn and before the second winter, catches a blocked outlet or a lifted trim before it leads to damp inside the new room.
What to settle before the builder leaves
Agree who is responsible for the roof from deck to finished covering, so that nobody assumes somebody else is doing the flashing. Ask for the specification in writing: covering, insulation, vapour control layer, fall and trims. Ask how the wall junction will be formed. Check the outlets are accessible for clearing leaves. Keep the paperwork for the insulation and any building control sign-off, because a buyer's solicitor will ask for it one day.
If you are not sure about an existing roof, we offer a free survey with photographs of your own roof, and tell you plainly if it has years left. When a new covering is the right answer, you receive a fixed written quote, we put a ten-year guarantee on the workmanship in writing, and the site is left clear of rubbish. Our flat roofing in Basingstoke page covers the rest of what we do, and call 01256 213956 or use WhatsApp to arrange a visit. Tell us the size of the extension, how the roof meets the house and whether a rooflight is planned, and we will know what to look for when we arrive.
