Flat Roof Cost in Horsham

A new GRP fibreglass flat roof on a typical Horsham extension costs £900–£2,500 with a 25-year guarantee; an integral garage roof £900–£1,400; and larger flat roofs £2,000–£6,000 depending on area. A felt-to-GRP upgrade runs £1,200–£3,000. All prices include labour, materials, edge trims, outlets and VAT. The main variables are the area in square metres, the system chosen, and the condition of the timber deck once the old covering is stripped — which is why every honest quote includes deck inspection. See our GRP vs EPDM comparison for how system choice affects cost.

For most Horsham extensions and garages, GRP fibreglass is the better choice — a seamless, jointless cured system with no laps to fail and a 25-year guarantee. EPDM rubber comes into its own on larger roofs (over about 80m²) or those with multiple pipes and rooflights, where its flexibility and single-sheet coverage are an advantage. Traditional felt is now mainly fitted only where budget is severely constrained. We advise on the right system for your specific roof during the survey. Our EPDM vs GRP guide weighs up both systems in detail.

A correctly installed GRP fibreglass roof lasts 25–30 years; EPDM 25 years or more; and traditional felt 15–20. In West Sussex’s damp climate, seamless systems consistently outlast jointed felt because they have no laps or seams to fail — which is where felt roofs almost always go first. The deck matters as much as the covering: a good membrane over a sound, dry deck reaches its full life, while the same membrane over a wet or soft deck fails early. Our guide to how long a roof lasts covers all roof types.

Because a new membrane laid over a soft or rotted timber deck fails early regardless of how good the membrane is — it delaminates, traps moisture, and the money spent on the new covering is wasted. On Horsham’s 1970s–90s extensions and garages, the deck has often taken on water through failing felt for years, so soft boards are common. We strip the old covering completely, inspect every board, and replace failed sections before laying the new roof, quoting deck replacement as a clear per-square-metre rate rather than hiding it. Skipping this is the single most common reason a new flat roof fails prematurely.

Most integral and detached garage flat roofs are replaced in a single day, including deck inspection and any board replacement. Larger or more complex roofs — those with multiple outlets, awkward abutments or a deck in poor condition — take longer, and we confirm the programme in the quote. A GRP garage roof done properly in a day gives you a seamless, 25-year covering with no joints to leak. Our garage roof replacement guide covers what’s involved and what to expect.

Yes — GRP installations carry a 25-year manufacturer-backed guarantee on the system, plus our own written 10-year workmanship guarantee, with documentation provided on completion. The two together matter: the product warranty covers the material, and the workmanship guarantee covers the installation — and on flat roofs, poor installation of outlets, upstands and falls is the usual failure point, not the material. We form those details as a single seamless piece in GRP, which is where its advantage over jointed felt really shows in a wet climate like West Sussex.

It depends on the roof’s condition and age. If the leak is isolated and the system is otherwise sound, a repair may be viable and cost-effective. But if the felt is blistered or cracked across the surface, ponding water sits after rain, or the deck feels soft underfoot, replacement is the better investment — patching an end-of-life felt roof buys months, not years. A felt roof over 20 years old showing any of these signs is usually better replaced. We diagnose honestly on site. Our leaking flat roof guide helps you tell which situation you’re in.

Yes — detached garages, outbuildings, bay-window tops and porch roofs as well as extensions. Small garage roofs are often completed in a single day including deck inspection. These smaller flat roofs are frequently the most neglected part of a property and the first to leak, because they’re out of sight, but the same GRP system and 25-year guarantee apply. Whether it’s a single garage or a run of outbuildings, we survey, quote a fixed price, and replace with a seamless covering built to last. See our garage roof replacement guide for detail.

We cover flat roofing across Horsham and the surrounding West Sussex towns — Southwater, Billingshurst, Pulborough, Storrington and Henfield — and all Horsham neighbourhoods including Roffey, Broadbridge Heath, Littlehaven, North Heath, Holbrook and Southgate, with no travel premium. Flat roofs on extensions and garages are common right across this stock, particularly on the post-war and later estates where rear extensions and integral garages are the norm. Wherever you are in the district, the free survey, deck inspection and 25-year GRP guarantee are the same.

Yes — where a flat roof covers a habitable room we can upgrade it to a warm-deck build-up, placing rigid insulation above the deck and below the membrane. This keeps the structure warm, meets current Building Regulations, and avoids the interstitial condensation that rots poorly built cold-deck roofs from within. Over an unheated garage a simpler build-up is fine, but over a room used as a kitchen or bedroom — common on the Southwater and Broadbridge Heath estates — the insulation detail isn’t optional. Our roof insulation guide explains warm versus cold deck.

On Horsham’s ageing felt roofs, lap and seam failure is the most common cause — the joints where sheets of felt overlap open up as the material ages and moves, and water gets in. After that come cracked outlets, failed upstands where the roof meets a wall, and ponding that accelerates wear at low points. GRP eliminates laps and seams entirely, forming the whole surface and its details as one seamless piece, which is precisely why it outlasts felt. Our leaking flat roof repair guide covers diagnosis and when repair versus replacement makes sense.

We strip rather than overlay. Overlaying a new covering onto an old one traps moisture in the existing felt and deck, and any soft boards underneath keep failing — so the new roof inherits the old roof’s problems and fails early. Stripping lets us inspect and replace the deck, correct the falls, and lay the new membrane onto a sound base so it reaches its full 25-year life. It costs a little more up front but avoids paying twice, which is what overlaying a failing roof usually leads to. This is the same deck-first principle that determines whether any flat roof lasts.