There is no single best material for a flat roof, only the one that suits the roof in front of you. A plain rectangle over a kitchen extension, a dormer boxed in by tile hanging, a lead bay on a Victorian terrace and a porch canopy under an ash tree each ask something different of a covering. The choice turns on the roof's shape, how it meets the walls around it, whether anyone will stand on it, what falls on it, and how long you plan to stay. This guide works through those questions in the order we ask them when we survey a roof.
Start with the roof, not the brochure
Three coverings dominate domestic flat roofs around Basingstoke, and it helps to know what each one physically is. EPDM is a synthetic rubber membrane (ethylene propylene diene monomer) that arrives as a single large sheet, is bonded to the deck with adhesive and is turned up the walls at the edges. GRP is glass-reinforced plastic, better known as fibreglass: resin and glass matting are laid wet over a timber deck, cure into a hard shell, and get a coloured topcoat. Torch-on felt is a set of reinforced bitumen layers melted onto the deck with a gas torch, finished with a cap sheet faced in mineral chippings. Lead and liquid-applied coatings fill the gaps on smaller or older roofs.
If you want those three side by side on cost, life and repairs, our EPDM vs GRP vs felt comparison covers that ground, and the quick outline of the three types of flat roof is a shorter read. This page is about choosing. We look at four things in turn: the shape and detailing, the use, the exposure, and then budget and time.
Rear extensions: simple shapes, awkward junctions
The single-storey rear extension is one of the commonest flat roofs we price. Its open area is usually a plain rectangle, which suits EPDM well. A rectangle up to the width of a standard roll can be covered in one sheet with no field joints at all, and a sheet with no joints has nowhere obvious to leak. EPDM also stretches with the timber as the deck swells in a wet February and dries out in August, so it shrugs off the small movements that crack rigid coverings.
The weak points on an extension are the edges. Where the roof meets the house wall there has to be an upstand, the part of the covering turned up the wall, protected by a flashing (a strip of lead or membrane that laps over it and is tucked into the brickwork, or sits under a cavity tray built into the wall). Rooflights and lantern kerbs add more upstands. A soil pipe or extractor vent through the deck adds a collar. Each of these has to be formed by hand.
That is where GRP earns its place. Because it is laid as a liquid, it wraps a lantern kerb, a vent pipe or an inside corner in one continuous layer, with preformed trims at the edges. On an extension with a big lantern and two vent pipes we often lean towards GRP; on a plain roof with one wall abutment and a gutter edge, EPDM is hard to beat for the money. Felt does both jobs competently in skilled hands, and it remains a fair choice where the budget is tight.
Dormers, bays and porches: small roofs with a lot of edge
Small roofs are not simple roofs. A dormer on a 1930s semi may have a top only a couple of metres across, but it has a front edge over the window, two cheeks (the side walls of the dormer, often tile-hung) and a back edge tucked under the main roof tiles. The length of edge compared with the open area is far higher than on an extension, and the leaks we trace on dormers tend to start at an edge rather than in the middle of the covering.
For dormers we usually choose EPDM or GRP. EPDM can be dressed under the bottom courses of the main roof and up behind the tile hanging in one piece, which keeps the number of joints low. GRP suits dormers with a lead-style roll edge or a curved front. Torch-on felt is the one we are most careful with on a dormer, because working a naked flame under timber cheeks and against old sarking (the boards or felt laid over the rafters under the tiles) is a real fire risk. Where felt is right for the roof, cold-applied or self-adhesive felt layers avoid the torch.
Bay window roofs bring shape into it. A canted bay has three or more facets meeting at hips, and many Victorian and Edwardian bays in Reading and Newbury were roofed in lead from the start. Lead is heavy, long-lived and can be bossed (beaten into shape) over awkward forms, and on a period house it keeps the original look, which matters if the street sits in a conservation area. If you live in one, check with the council's conservation officer before changing the material. Where lead is not wanted, GRP follows the facets well. EPDM is possible on a bay but needs careful folding at every hip.
Porch canopies are the simplest decision of all. They are small, easy to reach and cheap to redo, so a good torch-on felt or a GRP canopy is usually the sensible spend, and the choice comes down to the look you want and whether the canopy is overhung by anything that drips.
Roofs you walk on, and roofs under trees
Use changes the answer more than almost anything else. A roof nobody stands on except a roofer once a year can have a soft, flexible covering. A roof you step out onto to clean windows, or one you hope to use as a terrace, needs a surface that copes with feet, chair legs and dropped tools.
GRP is the natural choice for occasional foot traffic because the cured laminate is hard, and an anti-slip topcoat can be added. EPDM can be walked on carefully, but a stone caught under a shoe can puncture it, so on a roof with regular access we add walkway pads or put a raised deck or paving on adjustable supports over the membrane, which protects it and spreads the load. Turning a roof into a proper balcony is a bigger step: it raises planning and Building Regulations questions about guarding, privacy and structure, so check with your council's planning team and your building control body before you buy anything. Extra weight matters too. A new covering that is 15% or more heavier than the old one needs approval and may need the structure strengthened, and paving, soil or a sedum layer can add a great deal.
Trees are the other big factor. Leaves block outlets and the gutter at the roof edge, so water sits on the roof longer, and moss takes hold in the debris. Falling twigs and the odd heavy branch can damage a soft membrane. Under an oak, horse chestnut or sycamore we tend to prefer GRP or a mineral-faced felt, which take knocks better, and we design the edges so leaves can be cleared easily. Whichever material goes on, a roof under trees needs its outlets cleared every autumn.
Budget, lifespan and how long you will stay
Typical UK ranges reported by one cost guide are £60 to £100 per m² for felt, £80 to £120 per m² for EPDM and £100 to £160 per m² for GRP, supply and fit. A trade-facing guide puts EPDM at £70 to £100 per m², GRP at £80 to £120 per m² and modified bitumen felt at £45 to £75 per m². These are national figures for orientation only, and your fixed quote follows the survey, because the price of a flat roof is driven as much by the deck, the insulation and the edges as by the covering.
Lifespan cuts the other way. Modern reinforced felts last far longer than the thin felts that gave flat roofs their poor name, but of the three they generally have the shortest life, with EPDM and GRP lasting considerably longer when properly laid and maintained. Our guide to how long a flat roof lasts goes into what shortens and stretches that life. The practical question is how long you will own the house. If you plan to sell in a few years, a sound felt roof may be the honest recommendation. If this is the house you mean to stay in, the extra spent on EPDM or GRP is spread over many more winters.
| Your roof | What usually suits it | Why |
|---|---|---|
| Plain rectangular extension | EPDM | One sheet, few or no joints, tolerates deck movement |
| Extension with a lantern or several vents | GRP | Laid as a liquid, so kerbs and pipes are wrapped in one layer |
| Dormer | EPDM or GRP | Can be dressed under tiles and up the cheeks without a torch |
| Period bay or porch | Lead or GRP | Follows facets and keeps the original look |
| Walked on or used as a terrace | GRP, or a membrane under raised paving | Hard surface, or a protected membrane with the load spread |
| Short ownership, tight budget | Torch-on felt | Lowest typical cost per m², with a shorter life |
The detailing matters more than the membrane
When a two-year-old flat roof leaks, the covering itself is rarely at fault. The leak is usually at an edge trim (the metal or plastic strip that finishes the roof edge and guides water into the gutter), at an upstand that was not taken high enough, at a flashing chased into soft mortar, or at an outlet with no proper seal. So we choose the material partly on how well it handles the details this particular roof has.
The deck and the falls matter just as much. The covering is only as good as the boards under it, and a roof laid dead flat will hold water whatever is on top. A new roof should be built with a proper fall to its outlets, usually set with tapered firrings (timber strips cut to a slope) or tapered insulation boards. Insulation is part of the decision too. Replacing a flat roof covering, rather than overlaying it, generally brings an insulation upgrade into the job under the energy rules for existing homes, and a warm roof (insulation above the deck) is the usual way to achieve it. Work affecting less than 25% of the building envelope and less than 50% of the roof does not normally need a building control application, while larger jobs do. We deal with the Building Regulations side wherever the work is notifiable.
How we make the call on your roof
Every recommendation we make starts with a free survey. We go up, check the deck for soft spots, look at the falls, the outlets, the edges and every junction with the house, and photograph what we find so you see your own roof rather than a stock diagram. Sometimes the answer is that the existing covering has years left and a repair at one upstand will do, and we say so.
When a new roof is the right call, the report sets out which material we would use and why, given your roof's shape, use and setting, plus a fixed written quote. If you want a different material, we tell you plainly what that changes. The work carries our 10-year workmanship guarantee, written down, and we clear the site and take the old covering away when we finish. You can see the rest of our domestic work on our flat roofing in Basingstoke page, or call 01256 213956 or send a WhatsApp to book a survey.
