Walk onto any flat-roof job in the UK and you'll see the same handful of products coming out of the van. GRP (Glass Reinforced Plastic) has become the default cold-applied flat roof system for a reason: stock the right materials and use them correctly, and you finish with a seamless, leak-free membrane that comfortably outlives most other systems on the market. This guide cuts through the marketing and looks at what working roofers actually rely on day to day, what each component does, where the quality differences hide, and how to put a kit together that won't let you down in year fifteen. Why GRP Has Become the Trade's Go-To Flat Roof System GRP has not been new to boat builders for decades, but the flat-roof formulation has been worked out to such a degree that a well-laid roof is now expected to last 50 years or more. Cold applied (no torches, no hot-works permits) bonds to itself, and is immune to UV, foot traffic and standing water after curing. It's also forgiving on awkward shapes. Dormers, balconies, dormer cheeks, valleys, parapet walls, anywhere felt would need countless joints, GRP wraps the lot in one continuous membrane. That's why it's now dominant for both new-build extensions and re-roofs. The Core GRP Roofing Supplies Every Roofer Uses A complete GRP system has five pillars. Skimp on any one and the whole roof underperforms. 1. Fibreglass Matting (CSM) - 450g or 600g Chopped Strand Mat is the structural backbone of the laminate. The choice comes down to wear and traffic: 450g CSM - the workhorse for standard domestic flat roofs: garages, extensions, porches. 600g CSM - the heavier-duty option for balconies, walk-on roofs, green roofs and anything seeing regular foot traffic. If in doubt, the trade default has shifted toward 600g for residential work because it adds resilience without dramatically changing the install. APEX's 600g and 450g GRP roof kits cover the full range. Heavy Duty · 600g GRP Roofing Kit – Dark Iron Grey Ideal for balconies, green roofs & walk-on surfaces. 50+ year lifespan. View product → Standard · 450g GRP Roofing Kit – Dark Iron Grey Perfect for garages, extensions & standard flat roofs. 25+ year lifespan. View product → 2. GRP Roofing Resin and Catalyst A single waterproof skin is made of the resin and the MEKP hardener, which controls how long it takes to cure. There are two important things here: Cure window. Quality resins cure cleanly between 5°C and 30°C, giving you usable working days year-round in the British climate. Styrene content. Lower-styrene-emission resins are easier on the lungs and less hassle on confined sites. The HSE specifically recommends them where possible in its styrene control guidance. Stocking up on bulk resin? Order from the resin range here. 3. Topcoat Topcoat is your UV barrier and your finish. It's pigmented (Dark Iron Grey is the most-specified colour, with slate and tile reds for visible roofs) and goes on as the final layer once the laminate has cured. Cheap topcoats fade and chalk within a few years; a quality polyester topcoat keeps the roof looking right for decades. 4. GRP Trims, Edges and Corners Trims are where good roofs are won and lost. The laminate handles the field, but the edge details handle the water - and water always finds the weakest joint. The standard kit: A170 / A200 drip trims - straight edges where water falls clear B260 / B300 raised edge trims - gable ends and parapet returns Internal and external pre-formed corners - clean, leak-free transitions Wall flashing trims - where the roof meets brickwork A full breakdown is on the GRP roofing trims page, and any decent supplier should size trims to your roof rather than making you guess. 5. The Right Tool Kit You can have the best resin in the country and still ruin a roof with the wrong rollers. A working GRP tool kit needs resin rollers, topcoat rollers, a consolidation roller (this is the one that pushes air out of the laminate - non-negotiable), measuring buckets, mixing sticks, acetone for cleaning, and a telescopic handle for reach. Safety: What the HSE Expects on a GRP Job GRP involves styrene vapour, MEKP catalyst and acetone - all manageable, none to be casual about. The Health and Safety Executive's guidance on working safely with plastics and resins sets the benchmark, and the headline points are: Ventilate. Styrene is heavier than air, so it spreads out on open roofs. However, you should never use GRP in a small space without using forced extraction. PPE every time - nitrile gloves, safety glasses, and a respirator with organic-vapor cartridges if you're going to be doing a lot of work. Catalyst is corrosive. Never let it contact your eyes; keep eyewash on the roof and a spare bottle at the mixing station. Building Regulations You Need to Tick Off Any flat-roof replacement covering more than 25% of the surface triggers thermal upgrade requirements under Approved Document L (Conservation of Fuel and Power). The current target U-value for replacement flat roofs is 0.18 W/m²K, typically hit with a warm-deck build-up of PIR insulation above the deck. Part B (fire), Part C (moisture) and Part F (ventilation) also apply. Your local Building Control or an Approved Inspector signs the work off, and you'll want that certificate on file, it's a legal document buyers will ask for when the property next changes hands. Buying Smart - Complete Kits vs Individual Components For most roofers, a complete GRP roofing kit is the smarter buy. The mat, resin, hardener, topcoat and tape are matched to each other and proportioned to the m² of the roof - which kills the two most common waste sources: leftover resin and "I'll grab another roll on the way." Order trims separately once you've measured the perimeter, and add a tool kit if you're new to the system or replacing tired gear Need a Kit Specced for Your Specific Roof? Tell us the dimensions and we'll size the materials for you - no guesswork, no overstock. Browse the full APEX GRP range or call our team directly, 25+ years of roofing knowledge on the other end of the line. No upsell, no condescension, just the right kit for your roof. Not sure GRP is right for the job? We also stock EPDM rubber roof kits for jobs where rubber suits the spec better.
