A dry morning in January can look like the perfect chance to get a roof done. Then the sun drops behind the house, the boards turn cold and the job suddenly has a much smaller window. For a straightforward flat roof, EPDM can be the quicker winter option, provided the deck is dry and every adhesive and detail product is suitable for the conditions. GRP can work too, but its resin and topcoat need enough warmth and time to cure. In rain or freezing conditions, neither standard system is a sensible default. Cold and Dry: check more than the forecast EPDM: the adhesive sets the limit Rubber can stay flexible in temperatures that are too low for its adhesive. That’s why a membrane’s cold-weather rating doesn’t tell you whether you can fit it today. ClassicBond EPDM flat roof kits include water-based deck adhesive and contact adhesive for the perimeter and details. Flex-R’s winter guidance says not to use its water-based adhesive if temperatures may drop below 5°C within 24 hours. A mild afternoon followed by a frosty night therefore isn’t a suitable window. Check the contact adhesive, primer and tapes as well. They have their own instructions, and swapping adhesives isn’t a shortcut around them. The deck must be clean, dry and suitable for the specified bond. Our guide to the EPDM roofing range explains where the different kit components are used. GRP: allow time for both stages GRP is formed on the roof by wetting fibreglass mat with resin, then finishing the cured laminate with topcoat. Colder conditions slow the cure, so leave time for both stages.Apex stocks the RESTEC GRP Roof 1010 kit. The Grp Roof 1010 application manual specifies dry application conditions between 5°C and 30°C. Without the approved accelerators, the system should not be used below 5°C or where the temperature may fall below 5°C before the resin has cured. Its product-specific low-temperature procedure allows application down to 2°C, using separate accelerators for the base coat and topcoat. These figures apply only to the GRP Roof 1010 system. They are not a temperature guide for every fibreglass roofing kit. Always follow the instructions supplied with the exact resin, topcoat, catalyst and accelerators being used. The accelerator must be thoroughly mixed into the resin or topcoat before the catalyst is added. Accelerator and catalyst must never come directly into contact, and the accelerator must not be treated as a replacement for catalyst. Never add extra catalyst beyond the manufacturer’s stated limit to force a faster cure. GRP Roof 1010 is not intended for consumer use and should only be installed by professionals. Cold or wet: when should you stop? Cold and dry Clear skies don’t make a frozen roof ready for bonding. Check the roof surface for frost, ice and condensation as well as measuring its temperature. The boards can be colder than the air. At 0°C or below, postpone the standard bonded EPDM or GRP installation described here. The 2°C GRP example above still does not permit freezing conditions. Any different cold-weather method needs explicit approval for the complete system and the actual site conditions. Cold and wet Neither a conventional GRP laminate nor bonded EPDM should go onto a wet deck. Rain, dew or frost must be dealt with before work starts; drying the surface briefly isn’t enough if the boards remain damp. Wait for a dry window long enough to prepare, install and protect the roof while the products set. If rain arrives mid-job, stop and follow the system’s weather-protection and recovery instructions. Don’t cover a suspect layer and hope the next one will put it right. Store winter roofing materials correctly Cold materials can behave differently even when the air temperature appears acceptable. Follow the storage instructions for every membrane, adhesive, resin and topcoat used. The GRP Roof 1010 application manual recommends storing the resin at approximately 15°C for 24-48 hours before use. This does not permit application onto a deck outside the specified temperature range. Never use naked flames or improvised direct heating to warm roofing materials. EPDM and GRP in winter: quick comparison These are planning pointers for standard bonded EPDM and conventional GRP roofs. The instructions for the actual products take priority. Conditions EPDM GRP Cold and dry Possible within all adhesive limits. Do not use ClassicBond water-based adhesive if temperatures may drop below 5°C within 24 hours. Possible within resin and topcoat limits. GRP Roof 1010 normally requires 5–30°C and a suitable curing window. Its approved accelerated procedure allows application down to 2°C. Freezing and dry Postpone standard bonded installation. Dry-looking boards can still be frosty. Postpone standard installation. The cited 2°C accelerated method does not cover freezing conditions. Cold and wet Wait for a dry, suitable deck and an adequate weather window. Wait for a dry deck and conditions suitable for both installation stages. Short daylight A single sheet can save fitting time, but detailing and adhesive cure still matter. Allow enough time for reinforcement, lamination, cure checks and topcoat. Frost overnight or strong wind Check the following 24 hours when using water-based adhesive. Do not handle large sheets in unsafe wind. Check temperatures throughout the curing period. Stop if wind makes access or material handling unsafe. Does EPDM save time on short winter days? It can. On a simple roof, a single EPDM sheet removes much of the on-site joining and avoids the separate GRP laminating and topcoating stages. That can help when daylight is limited. It isn’t a guarantee that every EPDM job will be faster. Leave time for the membrane to relax before bonding. Flex-R recommends allowing the membrane to relax for at least 30 minutes, depending on conditions. Outlets, corners, wall junctions and edge trims still need proper attention, even when the main sheet goes down quickly. A small roof with a prepared deck may be fitted in a day by a suitably equipped team. Stripping the old covering, replacing rotten boards or working around rooflights can change that completely. Finishing the fitting also doesn’t mean the adhesive has fully cured. How large can an EPDM sheet be? There isn’t one maximum size for all EPDM. Flex-R’s current ClassicBond range lists rolls up to 9.15m wide by 30.5m long. Larger commercial membranes exist: Elevate lists RubberGard sheets up to 15.25m by 61m. These are manufacturer range examples, not a promise of Apex stock or a one-day installation. For a domestic roof, order the right size with the required allowances for edges and details. A huge sheet is harder to lift and control, especially in wind. Ask us to confirm the available size and thickness before ordering. Will a winter-installed roof crack in summer? Installing a roof in winter doesn’t, by itself, make it crack when summer arrives. What matters is whether it was fitted within the system’s limits and detailed to accommodate movement. EPDM is flexible and can accommodate normal building movement. Let the sheet relax and follow the fixing and detailing instructions rather than pulling it tight to make it reach. Flexibility doesn’t compensate for a weak bond or an unfinished corner. Conventional GRP is more rigid, so the supporting deck and movement details matter. The GRP Roof 1010 manual, for instance, specifies expansion provisions at abutments and additional expansion trims for larger roofs. Follow the design for your chosen system rather than assuming every GRP roof needs the same layout. Our GRP and EPDM roofing FAQs cover the wider differences between the materials if you’re still deciding which suits the roof. Our advice before you start Choose the roof system around the job and the whole weather window. Before opening up the roof, check: The air and deck temperatures, including the overnight low. Whether the deck is dry, sound and free from frost or condensation. The application and curing instructions for every product in the system. Enough daylight, safe access and manageable wind conditions. A suitable plan to protect the building if work has to stop. EPDM can be convenient for a straightforward winter replacement. GRP remains an option where its specified conditions can be met. If either needs you to bend the instructions to get it finished, wait. Tell us your roof size, what’s underneath the existing covering and when you hope to start. Call us on 0800 612 7903 or use our contact page, and we’ll help you choose from our EPDM roofing range and fibreglass roof kits.
