A flat roof can look like the simplest part of a modern extension. From the garden, you might see just a straight edge above the glazing. Up on the roof, there's more going on: a roof lantern, wall junctions, corners and rainwater that needs a clear route out. A GRP flat roof can bring those details together in a continuous waterproof covering. The useful question is where that helps the design, and what needs sorting before anyone opens a tin of resin. Why GRP Appeals to Architects and Designers Architects and designers may specify GRP where a flat roof needs to remain visually simple while accommodating practical details such as parapets, rooflights, wall junctions and drainage. The waterproofing can be formed on site across the main deck and around compatible prepared corners, kerbs and upstands, which can be useful on contemporary extensions with more complex roof layouts. GRP can also suit clean, low-profile rooflines. The finished covering can sit behind a parapet or terminate at discreet perimeter trims, allowing glazing, brickwork, cladding and the overall form of the extension to remain the more prominent architectural features. GRP still needs to be considered as part of the complete roof design rather than as a finish chosen at the end. The specification should coordinate the deck, insulation, falls, drainage, movement details, rooflight kerbs and relevant fire-performance requirements so that the waterproofing works with the architecture as a complete system. A Flat Roof That Doesn't Fight the Architecture On an extension with wide sliding doors, a heavy-looking roof edge can distract from the glass and brickwork below. GRP, short for glass reinforced plastic, forms a relatively thin waterproof layer over a suitable deck, finished with a protective topcoat. That suits a restrained roofline. It can sit behind a parapet or finish at a neat perimeter trim, keeping the covering visually quiet. The complete roof still needs room for its structure, insulation and drainage falls. Choosing fibreglass doesn't make those layers disappear, so agree the finished roof height early. GRP Works Well Around Rooflights and Awkward Details A roof lantern above a kitchen table brings daylight further into an extension. It also introduces four corners and a raised kerb that the waterproofing has to meet properly. GRP roofing uses resin and glass reinforcement applied on site. Following the system's details, an installer can form the laminate around prepared kerbs, corners and penetrations. That is useful on an L-shaped extension or a roof with several changes of direction. It still takes careful work. Reinforcement, transitions and terminations need the right preparation; a continuous-looking surface isn't proof of a sound installation. Choose the glazing before building its opening. Roof lanterns and flat rooflights buying guide explains the differences in kerbs, sizing and installation requirements. Parapets Keep the Roofline Clean, But Drainage Still Has to Go Somewhere A parapet can hide the roof covering and give an extension a crisp outline. Behind it, the falls must guide water towards planned low points and outlets. BS 6229:2025 guidance calls for a minimum finished fall of 1:80 across general roof areas. That's a drop of 12.5mm per metre. The designer must allow for construction tolerances and structural deflection when deciding how to achieve it. Where drainage passes through the parapet, our GRP through-wall/parapet outlet provides a component to incorporate into a compatible roof detail. Its position, capacity and connection to the rainwater system need planning together, including provision for a blocked outlet. Leave access for clearing debris, too. Extensions, Garages and Garden Rooms Are Where This Really Makes Sense Home extensions Picture a kitchen extension with large glazed doors and a roof hidden behind a low parapet. GRP can follow the perimeter and dress around a lantern kerb without becoming the main visual feature. The junction with the original house needs particular attention, including any movement between old and new construction. Garages and workshops On a garage roof, the edges do different jobs. Some keep rainwater from spilling over the sides, while the edge above the gutter lets it drain away. The photograph-style illustration below shows how those edges can give a plain brick building a tidy finish. Garden rooms and studios A low roofline can help a garden studio sit comfortably beside fences and planting. A fibreglass flat roof can suit that appearance, but don't squeeze out insulation or drainage falls to save height. For a heated office, the roof construction also needs proper condensation control. Our Roofs for All House Types article looks at other building types where flat roofing systems can be considered. The Edge Details Matter More Than Most People Expect Much of the roof is open deck. The more demanding work is often where that deck ends, turns a corner or meets something else. The components in our GRP trims and corners range serve different jobs: Drip trims carry water towards gutters. Raised edge trims contain water at non-draining edges. Wall fillets help form transitions at upstands. Compatible corners, flashings and drainage trims complete the junctions. On an irregular roof, sketch each perimeter section before ordering. It helps identify where a drip changes to a raised edge, and which corner detail is needed between them. Choose profiles that work together and suit the finished build-up. A tidy edge depends on its support, fixings and reinforcement as much as the topcoat you see. GRP Isn't a Substitute for Good Roof Design Traditional GRP is relatively rigid. If the deck flexes or movement concentrates at a junction, the covering can suffer. Our guide to GRP cracking, blistering and thermal expansion explains why those details deserve attention. Before installation, check: The deck is suitable, sound, dry and properly supported. Falls, outlets and upstands work together. The specification allows for structural and thermal movement, including expansion details where required. Weather and temperatures suit the system's application and curing instructions. The complete roof build-up meets the project's relevant fire and building requirements. For fire performance, check the specific tested or classified roofing system and its permitted build-up. The word “GRP” alone tells you very little about that. Choosing the GRP System for the Project Roof area is only part of the choice. Building use, expected access, movement and the number of awkward details also matter. A roof intended for maintenance access needs a different assessment from an occupied terrace. Don't assume a standard kit covers both. Compare our GRP roofing kits against the agreed specification, including the deck, reinforcement, finish and compatible accessories. Thinking About GRP for a Contemporary Flat Roof? Tell us what you're building, the approximate roof size and whether there are parapets, rooflights or unusual corners. You can browse our GRP roofing kits or contact us for help matching the materials to your plans. Frequently Asked Questions Is GRP suitable for a modern house extension? Yes, where the system suits the roof construction and intended use. It can provide a discreet finish around parapets, edges and rooflights. Can GRP be used around rooflights? The waterproofing can be formed around a suitable raised kerb, following compatible rooflight and roofing-system details. Does a GRP flat roof need a fall? Yes. Water needs a route to the outlets. Current BS 6229:2025 guidance calls for a minimum finished fall of 1:80 for general roof areas.
