Yes, both GRP and EPDM can be used to waterproof flat and low-slope building rooftops. They suit a range of domestic and commercial projects, provided the chosen system is right for the roof beneath it. GRP, or fibreglass roofing, uses resin and glass matting to form a continuous waterproof surface. EPDM is a rubber membrane fitted with compatible adhesives, trims and detailing products. Choosing between them starts with a closer look at your roof. What's it made from? Is the existing covering sound? Will anyone use the space regularly? Those answers matter before you order a kit. What types of rooftop can use GRP or EPDM? You don't have to stop at sheds and garages. Suitable GRP and EPDM systems can also cover extensions, flat-roofed homes, workshops and commercial premises. The building's name tells you very little about the work involved, though. Two garage roofs can need completely different preparation if one has a sound timber deck and the other has damp concrete beneath several old coverings. Rooftop What needs checking House or extension Deck condition, insulation, drainage and details where the roof meets walls Garage or workshop Existing covering, structural condition and whether the space is heated Concrete rooftop Moisture, surface preparation and the system's approved substrates Larger commercial roof Wind exposure, movement, attachment design and maintenance access Roof intended as a terrace Load capacity, waterproofing protection, walking surface and guarding Treat these as starting points. The final specification needs to suit the actual roof. What needs checking before choosing a system? The roof deck and existing covering A new waterproof layer won't put rotten boards or a weak roof structure right. Soft timber, loose surfaces and trapped moisture need investigating before they're covered over. Conventional GRP roofing generally needs a suitable timber deck prepared to the system's instructions. Flexible GRP overlay systems are different, and some are designed for additional surfaces. Don't assume the same preparation works for both. Concrete rooftops need particular care. The concrete must meet the chosen system's requirements for condition, moisture and preparation. Depending on the product, that may involve a primer or an approved intermediate layer. EPDM also needs a compatible surface and adhesive. Being flexible doesn't mean it can bridge a failing deck or stick reliably to whatever happens to be underneath. Drainage, insulation and roof details Before choosing the finish, look at how rainwater gets off the roof. Low areas, blocked outlets and poorly positioned drains deserve attention while the work is being planned. Then consider what's below the waterproofing. Over a heated room, the roof design needs to address insulation and condensation control. GRP and EPDM provide waterproofing; they don't replace insulation. Rooflights, pipes and adjoining walls need their own details too. Allow for properly formed upstands, compatible flashings and accessible outlets. A large, neatly finished roof can still leak through one badly fitted junction. Roof size, movement and wind exposure On a larger roof, movement joints and the arrangement of the waterproofing need planning around the chosen system. Existing structural movement joints must be dealt with properly. Wind exposure affects the fixing or bonding specification as well. A sheltered extension and an exposed commercial rooftop may need different approaches, even when the surface area is similar. For large or complicated projects, get the roof specification agreed before buying materials. A kit sized by square metres alone can't account for every detail. When might GRP or EPDM suit your roof? GRP is worth considering where you want a firm, continuous finish on a suitable deck. Its liquid application allows the reinforced waterproofing to be formed around roof details, provided those details are prepared correctly. Our GRP roofing kits include different specifications, so choose according to the project rather than simply picking the heaviest matting. EPDM can be particularly practical on straightforward roof shapes. Where the dimensions allow, a single sheet can cover the main roof area, reducing field seams. Edges, outlets and wall junctions still need careful sealing. Our EPDM roofing range includes membranes, kits and accessories for putting the system together. Neither option wins every job. Preparation, access and the number of roof details can affect the labour involved as much as the material itself. For an idea of material costs, our roofing kit price comparison is a useful starting point. Budget separately for any deck repairs, insulation and access equipment. Can you walk on the finished rooftop? Occasional maintenance access is different from using a roof as an outdoor room. Where the structure and roofing specification permit access, a roof may accommodate someone checking an outlet or inspecting a rooflight. Repeated routes can need extra protection, such as compatible walkway pads on EPDM. A terrace needs more planning. People, furniture, paving and planters add loads, and the waterproofing needs protection from the use above it. Suitable access and guarding also need designing in. Some GRP systems are specified for balcony or terrace applications, but an ordinary roof kit shouldn't automatically be treated as a finished walking surface. Likewise, exposed EPDM isn't a substitute for terrace flooring. If you want to use the rooftop regularly, include that in the brief from the beginning. Adding decking later can change much more than its appearance. What about Building Regulations and fire performance? Depending on the project, reroofing can bring requirements for insulation, structural performance and fire safety. The rules also vary across the UK, so check what applies locally before work starts. Fire performance relates to the tested roof construction and its conditions of use. A product described as fire-retardant doesn't establish that any combination of deck, insulation and waterproofing will meet your project's requirements. Ask for the relevant system documentation and have the proposed construction checked by the person responsible for the roof design or building control submission. This deserves particular attention on larger buildings and occupied roof areas. Frequently asked questions Can I install the roof myself? A straightforward roof may suit a competent DIYer with safe access and suitable conditions. Our beginner's guide to GRP roofing kits explains the basics. Complex details or structural concerns need professional input. Does a roofing kit include insulation? Don't assume so. Check the contents carefully. Insulation, decking and vapour control layers need specifying separately where required. Does the old roof covering always need removing? Not always. An approved overlay system may suit a sound, compatible existing roof. Wet, loose or deteriorated layers need assessing first, even if the top surface looks reasonable. Before you order your roofing materials Have your roof measurements ready, along with photographs of the surface, edges and outlets. Let us know what’s on the roof at the moment, if it’s leaking anywhere and what you’ll be using the space for. Get in touch with us with those details and we can help you work through the material options. It's much easier to sort out the right kit before the roof is opened up.
