Project at a glance Difficulty: Intermediate DIY project Time: Allow a settled, dry weekend for a small, straightforward roof; preparation or structural work may take longer Best conditions: Work within the application limits for your Apex system. The deck, materials and roof must be completely dry. Core materials: Approved roof deck, compatible Apex GRP roofing kit, GRP trims, fixings, outlets/details and suitable PPE Key rule: Use the catalyst, coverage and cure instructions supplied with your Apex kit—not generic online ratios or timings. Thinking about fibreglassing your own flat roof? GRP (glass-reinforced plastic) is cold-applied, with no roofing torches or hot bitumen. This guide covers a standard timber-decked installation—from checking the roof structure to applying a Dark Iron Grey topcoat. Read the full guide before starting and keep the product labels, Safety Data Sheets (SDS) and kit instructions available throughout the work. Safety and regulations Roof work presents a serious fall risk. Plan the job, use suitable access and edge protection, and do not treat a ladder as a general working platform. HSE guidance requires work at height to be planned, carried out by competent people and controlled using appropriate equipment; ladders are generally for low-risk, short-duration work only. Read the HSE roof-work guidance and its step-by-step guide to working at height. Before starting: Read the labels, SDS and instructions for the resin, catalyst, topcoat and additives being used. Wear the chemical-resistant gloves, eyewear, clothing and respiratory protection required by the SDS and risk assessment. Keep resin, catalyst and acetone away from flames, sparks and smoking. Work in good ventilation and never use acetone to clean skin. Keep bystanders away and stop if rain, frost, ice, high winds, condensation or unsafe conditions are expected. In England, Building Control approval may be necessary if more than 50% of the roof is re-covered, more than 25% of the total building envelope is affected, structural alterations are made, fire performance changes significantly or the replacement covering adds 15% or more weight. Thermal-insulation requirements may also apply. Requirements differ elsewhere in the UK, so check the Planning Portal guidance for work to an existing roof and confirm the requirements with the relevant local authority before work starts. Before ordering your GRP roofing kit A GRP roof only performs as well as the structure below it. Check the following first: Sound structure: Replace rotten, wet, soft or moving timber before waterproofing. Falls and drainage: Water must run toward the intended gutter or outlet; GRP cannot correct a badly formed fall. Roof build-up: Check insulation, vapour control, condensation and fire performance—not only the waterproof layer. Approved deck: Timber GRP systems commonly use clean, dry 18mm OSB3 tongue-and-groove roofing board, subject to the selected system and board instructions. Compatible components: Use resin, catalyst, matting, topcoat, trims and additives approved for the same system. Safe access and weather: Start only when access is protected and conditions are suitable for preparation, laminating and topcoating. If the structure, insulation strategy, drainage or expansion detailing is uncertain, seek advice before ordering. For a more accurate quote or kit selection, prepare a simple roof sketch showing the overall dimensions, drainage edge, raised edges, wall junctions, outlets, rooflights and penetrations. Choose the matting weight Kit Best suited to Important note 450gsm GRP kit Standard domestic roofs such as garages, extensions, sheds and porches without routine foot traffic Suitable for maintenance access only where the complete roof structure and system allow it 600gsm heavy-duty GRP kit Roofs requiring greater reinforcement or wear resistance, including specified maintenance-access roofs, balconies beneath tiles or decking, and designed green-roof systems Matting weight alone does not make a balcony, terrace or green roof structurally suitable Explore the 450gsm GRP Roofing Kits for standard domestic flat roofs or the 600gsm Heavy-Duty GRP Roofing Kits for appropriate heavier-duty applications. Use the Apex GRP Kit Calculator to estimate the kit size, then check the required trims, outlets and upstands. Measure the finished roof deck—not only the building’s external dimensions. Materials and tools Materials Correctly sized 450gsm or 600gsm Apex GRP roofing kit Approved 18mm OSB3 tongue-and-groove boards, unless the system specifies another substrate Corrosion-resistant board fixings Compatible drip trims, raised-edge trims, wall details, corners, outlets and pipe collars Taping mat for relevant details and joints Approved PU trim adhesive and trim fixings where required Catalyst or seasonal additive specified for the Apex system and conditions Tools and PPE Resin rollers, topcoat rollers, brushes and paddle/consolidation rollers Clean mixing buckets, measuring equipment and scales where required Mixing sticks or an approved paddle mixer Knife or scissors for matting Drill/driver and board-cutting tools Acetone only for permitted tool cleaning or specified surface preparation Required PPE and suitable fall-prevention/access equipment Step 1: Check conditions Curing depends on the temperature of the air, deck and materials—not simply the weather forecast. Check the application-temperature range stated for your exact Apex kit. Confirm that conditions will remain within that range through laminating, topcoating and initial cure. Account for morning dew, shade, cooling temperatures and direct sunlight. Ensure the deck and materials are fully dry. Cut and dry-fit matting and trims before catalysing resin. Store and condition products exactly as directed. Cold conditions can change working and curing behaviour. If temperatures are close to the lower application limit, read the Apex guide to curing GRP resin in cold weather and follow the instructions supplied with the product. Step 2: Remove and inspect Remove the old felt, failed GRP or other covering as necessary to inspect the roof structure. Check joists, firrings, fascias and supporting timber for rot, moisture, movement and damage. Repair defective structural components before fitting the new deck. Confirm the designed fall leads toward the correct gutter or outlet. Confirm insulation, vapour control and ventilation before closing the roof build-up. Keep the exposed roof protected from rain without creating unsafe temporary access. Do not laminate over old felt, chipboard, contaminated surfaces, wet timber or an unstable deck. Step 3: Install the deck For a standard timber-decked GRP roof, use good-quality 18mm OSB3 tongue-and-groove roofing board approved for the system. Lay boards in a staggered arrangement and orient them across joists as required by the board manufacturer and roof design. Fix boards into every supporting joist at the required centres. Keep fixing heads flush without damaging the board face. Provide movement gaps at walls, rooflights, chimneys and other fixed abutments as specified. Do not create arbitrary gaps in every tongue-and-groove joint. Protect the deck from rain and laminate as soon as practical. Sweep and vacuum thoroughly: the surface must be stable, dry and free of dust, oil, silicone and loose fibres. Large or unusually shaped roofs can require designed expansion details. Confirm this before laminating. Step 4: Fit GRP trims Trims form drainage edges, raised boundaries, wall junctions and corners. Use a compatible drip trim at the drainage edge. Use suitable raised-edge trim where water must be directed away from the perimeter. Dry-fit, cut and plan all trims before mixing resin. Fit wall fillets, flashings, outlets, corners and pipe details appropriate to the roof design. Fix or bond trims using the compatible method, adhesive and spacing specified by the system. Leave designed movement sections free where the trim profile requires expansion and contraction. If unsure which profile belongs on each edge, use the Apex guide to GRP roof trims and flashings or send Apex a roof sketch before proceeding. Step 5: Reinforce details Before laminating the main roof area, reinforce the required trim joints, corners, outlets, penetrations and other high-stress details. Treat board joints only as required by the selected board and GRP system. Cut and dry-fit the taping mat. Mix a small, controlled batch of resin according to the supplied chart. Apply resin to the prepared detail. Embed the matting and add only enough resin to wet it out fully. Consolidate until fibres lie flat and trapped air is removed. Do not leave dry white fibres, sharp ridges, loose strands or bridging across internal corners. Step 6: Mix resin correctly Catalyst starts the curing reaction. The correct amount depends on the resin, catalyst, material temperature and working conditions. Never substitute a generic catalyst ratio for the chart supplied with the product. Use the Apex chart supplied with the kit as the controlling instruction. Measure with the specified method and mix thoroughly from the sides and base of the bucket. Mix only what can be applied and consolidated within the product’s working time. Work in planned roof sections rather than catalysing material for the whole roof at once. Allocate resin according to the specified coverage rate. Never add acetone, solvent or unapproved additives to thin resin or topcoat. Never pour catalysed material back into the original container. Do not leave catalysed resin in a deep bucket, where the reaction can generate heat and accelerate rapidly. If the mix behaves unexpectedly, stop and follow the relevant SDS and product instructions. Step 7: Wet out matting Work in manageable, pre-planned sections. Apply the specified amount of catalysed resin evenly to the prepared deck. Lay pre-cut chopped-strand fibreglass matting into the wet resin without stretching it. Apply additional resin as required by the system, working from the centre outward. Allow the resin to soak through before assuming more is needed. Continue until the matting changes from dry white to evenly translucent. Form overlaps at the width stated by the installation instructions. Too little resin can leave dry fibres. Too much can create a resin-rich laminate, reduce consistency and leave insufficient material for the rest of the roof. Follow the specified coverage rate. Step 8: Consolidate the laminate Immediately use a clean paddle roller to consolidate the wet laminate. Roll in more than one direction using controlled pressure. Focus on overlaps, corners, trim edges and changes of level. Remove every bubble and air pocket. Check again for dry fibres, wrinkles, bridging and resin-starved areas. Stop before the material begins to gel; overworking a curing laminate can damage the finish. Step 9: Apply topcoat Do not apply topcoat simply because a generic number of minutes has passed. The laminate must reach the cure stage stated in the system instructions. As a practical check, reinforcement should be set and should not lift or move under light pressure from a gloved finger. Surface tackiness can vary between resin systems, so it is not a universal test. Confirm the laminate is ready and weather conditions remain suitable. If the laminate has exceeded the recoat window, become wet or contaminated, follow the exact product-specific preparation method. Mix topcoat with the catalyst specified by its own chart. Apply at the stated coverage rate—not too thin and not too thick. Cut in edges and details with a compatible brush. Roll the main roof evenly while maintaining a wet edge. Do not thin topcoat or add wax or other additives unless the product instructions expressly require it. Avoid repeated rolling once topcoat starts curing. The topcoat provides the finished colour, weathering surface and UV protection, so its coverage and mixing are as important as the laminate below. Step 10: Cure and inspect Keep people, tools, standing water and debris off the roof until the topcoat has cured for the period stated in the kit instructions. Cold conditions can lengthen curing and walk-on times. Inspect the roof in good light: Check for missed or thin areas, pinholes, dull patches and contamination. Confirm corners, penetrations and trim joints are fully sealed. Check that gutters and outlets are clear. Confirm drainage runs in the intended direction. Keep product batch details, instructions and installation photographs with property records. Do not apply an improvised patch or extra coating over a suspected defect. Identify the cause and use the approved repair method. Common mistakes to avoid Laminating onto a damp, dusty, contaminated or unstable deck. Mixing materials or instructions from different GRP systems. Guessing catalyst quantity instead of using the supplied chart. Mixing batches too large to apply within working time. Ignoring resin and topcoat coverage requirements. Leaving dry fibres, bubbles, wrinkles or bridging. Thinning materials with solvent or assuming acetone is a universal cure-recovery solution. Missing movement or expansion details. Topcoating outside the allowed recoat window. Working through unsuitable weather. Treating safe access as an afterthought. Quick troubleshooting What you notice Likely cause What to do Matting remains white Fibres are not fully wetted out or air is trapped Work resin through and consolidate while the laminate remains workable Visible bubbles or bridging Air is trapped or matting is not following the detail Consolidate immediately; do not hide defects under topcoat Resin gels in the bucket Batch is too large, too warm or beyond working time Stop using the batch, follow SDS guidance and reduce future batch size Laminate does not cure as expected Measurement, temperature, compatibility or contamination issue Do not topcoat or add catalyst to the surface; protect the roof and contact Apex Rain or dew reaches laminate Product-specific recovery may be required Assess safely and follow exact system instructions before abrading, wiping or recoating When to use a professional Use a competent roofer, builder or relevant specialist when: Joists or structural support need repair or redesign. The warm-roof build-up, insulation, vapour control or condensation strategy is unclear. Building Control approval, fire-performance evidence or major thermal upgrades are required. The roof includes complex rooflights, outlets, abutments or expansion details. The project is a balcony, terrace or green roof requiring structural and drainage design. Proper safe access and edge protection cannot be provided. Moisture is present and its cause cannot be identified. The weather window is too short. You are not confident measuring, mixing and applying materials accurately. Frequently asked questions Can I fibreglass a flat roof myself? Yes, if you are competent with practical building work, have safe access, understand the full roof build-up and follow the exact system instructions. Use a professional when structural, insulation, drainage or complex-detail work is involved. What board goes under a GRP roof? A standard timber-decked GRP roof commonly uses approved 18mm OSB3 tongue-and-groove roofing board. Board type, orientation, fixings, movement gaps and preparation must match both the board manufacturer’s instructions and the GRP system specification. Can GRP go over existing felt? Not with the standard installation described here. Remove the failed covering, inspect the structure and use an approved clean, dry substrate. Select an overlay system only when it is specifically tested and approved for that existing surface. How much catalyst should I add? Use only the catalyst chart supplied with the Apex resin or topcoat. Do not rely on a percentage copied from another brand or system. If you are working in colder conditions, check the Apex cold-weather GRP guide before starting. Can I topcoat the next day? Only when the system instructions allow it and the required preparation has been completed. A laminate that is delayed, wet or contaminated may require product-specific abrasion or solvent preparation before topcoating. How long does it take to fibreglass a flat roof? Allow a settled, dry weekend for a small and straightforward DIY roof. The actual time depends on preparation, repairs, roof complexity, temperature and the curing instructions for the selected kit. Do not shorten a required cure stage to finish before rain or darkness. Can I fibreglass a roof in the rain? No. The deck, matting and materials must remain dry during installation and initial cure. Rain, dew or condensation can affect adhesion and curing. If the laminate becomes wet, protect the roof and follow the product-specific recovery instructions before continuing. How do I calculate the kit size? Measure the finished roof area and account for upstands, details and the waste allowance required by the system. Enter the measurements into the Apex GRP Kit Calculator, then check that the selected trims and outlets match the roof sketch. Can I walk on a GRP roof? A standard 450gsm domestic roof is not intended for routine foot traffic. Limited maintenance access may be possible where the complete roof structure and selected system permit it. Balconies, terraces, walkways and green roofs require an appropriate specification; choosing heavier matting alone is not enough. How long does a GRP roof last? A correctly installed GRP roof commonly lasts 20–30 years or more, and heavy-duty 600gsm systems can last 50+ years. Actual service life depends on the specification, installation quality, weather exposure and maintenance — check the expected service life and warranty terms for the specific Apex kit you choose. Start your project Measure the main roof area, upstands and details, then include the required waste allowance. Use the Apex GRP Kit Calculator to choose the appropriate kit size, and select compatible trims, outlets and tools before ordering. Before installation day, review the Apex fibreglass roofing video tutorials alongside the written product instructions. For typical domestic flat roofs, explore the 450gsm GRP Roofing Kits. For correctly designed heavy-duty applications, explore the 600gsm Heavy-Duty GRP Roofing Kits. If any part of the structure, roof build-up, access or product choice is uncertain, contact the Apex team before starting. Technical and safety references HSE: Roof work HSE: Working at height—step-by-step guidance Planning Portal: Building Regulations for work to an existing roof Product labels, Safety Data Sheets and application instructions supplied with the selected Apex roofing system
