Choosing between EPDM rubber and fibreglass GRP for your flat roof in Marlow? Both are great systems, but they suit different situations. GRP is the better choice for smaller, more complex roofs with lots of angles and details, for balconies and walkways, and where you want a specific colour finish. It's a bit more expensive than EPDM but it cures as a completely rigid shell that nothing gets through.
Fibreglass flat roofing — properly called GRP (Glass Reinforced Polyester) — is a laminated system built up on site. The process involves laying sheets of chopped strand mat (CSM) onto the prepared roof deck and saturating them with catalysed polyester resin. This laminate cures into a rigid, completely seamless waterproof shell. A pigmented topcoat is then applied, which provides the finished colour and UV protection. The result is a flat roof with absolutely no joints, seams, or overlaps — every square millimetre is one continuous piece of cured fibreglass. We prepare the deck with 18mm OSB3 sterling board, which provides the smooth, flat substrate the GRP needs. Proper falls are built in using either tapered timber battens or tapered insulation boards — we aim for a minimum 1:40 fall (ideally 1:80) to ensure water drains towards the gutters and doesn't pond. Edge details are finished with purpose-made GRP trims — drip edges, angle fillets, upstand trims — which are laminated into the main roof for a watertight joint. For properties in Marlow, where a charming Georgian town on the River Thames in Buckinghamshire, with many period properties and conservation area buildings requiring sympathetic roofing work, GRP is particularly well suited to extensions and conversions where the flat roof might be visible from upper windows. The range of topcoat colours — from standard grey and charcoal to green, slate blue, and terracotta — means the roof can complement your property rather than being an eyesore. Building Regulations Part L insulation is incorporated into the build-up, using Celotex or Kingspan PIR boards.
When GRP is done badly — wrong resin mix, too much catalyst, laminating in cold or damp conditions — it can crack and delaminate. This is why it matters who does the job. We see botched GRP work across Buckinghamshire where someone's tried to save time by skipping steps or laminating in the rain. A properly laid GRP roof doesn't crack. If yours has, it was likely installed wrong.
Water sitting on any flat roof — including GRP — accelerates wear on the topcoat and adds unnecessary weight. Proper falls need to be built into the deck before the GRP goes on. If your existing flat roof ponds, we build up the falls during the replacement so water drains properly towards the outlets.
The topcoat on a GRP roof protects the laminate from UV degradation. Over time — typically 15-20 years — the topcoat can fade or develop a chalky appearance. This doesn't mean the roof is failing, just that the topcoat needs refreshing. We can recoat a GRP roof with fresh topcoat for a fraction of the cost of a new roof, extending its life significantly.
If the GRP edge trims weren't properly laminated into the main roof, they can separate over time, allowing water to track underneath. This is another installation quality issue. Our trims are bonded in with the same resin and mat as the main roof — they're part of the structure, not stuck on as an afterthought.
Fibreglass flat roofing in Marlow costs a bit more than EPDM but gives you a harder-wearing, rigid finish. A single garage roof runs £1,500-£2,500. A rear extension roof of 15-25 square metres typically costs £2,500-£5,000. Per square metre, expect £90-£140 fully installed with insulation and GRP trims. If you want a non-slip balcony finish, add about £10-£15 per square metre. Dormer tops are usually £1,000-£2,000 depending on size and access. Bay window roofs start from £800. If the existing deck needs replacing, add £20-£30 per square metre for new OSB3. All our quotes include materials, labour, insulation, edge trims, and a workmanship guarantee.
We use Scott Bader Crystic polyester resin (pre-accelerated, with MEKP catalyst) and 450gsm chopped strand mat — the same specification used for boat hulls and industrial tanks. The topcoat is Crystic topcoat in the customer's choice of colour — standard options include dark grey (RAL 7015), slate grey, mid grey, charcoal, olive green, and black, with other RAL colours available to order. The deck substrate is always 18mm OSB3 (Sterling Board) with staggered joints. GRP edge trims — drip, verge, fillet, and upstand profiles — are from Fibretech or Roofing Supplies UK. Insulation is Celotex GA4000 or Kingspan TP10 PIR boards, typically 100mm minimum to meet Part L. For non-slip balcony finishes, we use Roofing Superstore anti-slip aggregate broadcast into the wet topcoat.
We visit your Marlow property, measure the flat roof area, check the existing deck condition, and discuss your requirements — colour preference, whether you need a walkable finish, insulation requirements. We then provide a detailed written quote.
The old covering is stripped and the deck inspected. Any rotten boards are replaced with new 18mm OSB3. Falls are created so water drains correctly — this is critical for a long-lasting GRP roof. Insulation boards are laid and fixed if a warm deck build-up is specified.
This is the key stage. Chopped strand mat is laid onto the prepared deck and saturated with catalysed resin, working in sections and ensuring full wet-out with no air bubbles or dry patches. Edge trims are laminated in as part of the same process. The laminate needs to cure before the topcoat goes on.
Once the laminate has cured, the pigmented topcoat is applied evenly across the entire surface. This provides the colour, UV protection, and — if required — the non-slip finish. The topcoat needs dry, mild conditions to cure properly, which is why we schedule GRP work carefully around the weather.
We inspect every edge detail, trim, and upstand. The gutters are cleared and water flow is checked. You get a walkthrough of the finished roof, care instructions (there's barely any maintenance needed), and your written guarantee.
GRP fibreglass roofing is increasingly popular across Marlow as homeowners replace failing felt roofs on extensions and garages. Marlow is a charming Georgian town on the River Thames in Buckinghamshire, with many period properties and conservation area buildings requiring sympathetic roofing work. The local climate suits GRP well — once cured, the rigid shell handles freeze-thaw cycles without cracking, and the topcoat provides excellent UV resistance even through the hotter summers we've been getting in Buckinghamshire. On extensions where the flat roof is visible from bedroom windows, homeowners appreciate the clean, finished look of a coloured GRP topcoat compared to the rough appearance of felt or the black of EPDM. For loft conversions with rear dormers — which are popular in the terraced and semi-detached housing stock in Marlow — GRP is our recommendation for the dormer top because it's rigid, walkable for occasional maintenance, and long-lasting.
GRP cures as a single shell — no seams, no joints, no overlaps. Water has nowhere to get in. It's the most watertight flat roofing system available for domestic properties in Buckinghamshire.
Standard grey isn't your only option. We offer a full range of topcoat colours so your flat roof can complement your property rather than being an afterthought.
GRP is rigid and can be finished with a non-slip aggregate for balconies and roof terraces. You can walk on it, put furniture on it, and use it as outdoor living space.
We construct proper drainage falls into every GRP roof. No ponding, no standing water — just clean runoff to the gutters. This extends the roof's life and prevents the problems that come with pooling water.
A properly installed GRP roof will last 25-30 years before the topcoat needs refreshing, and the structural laminate beneath is good for much longer. Topcoat renewal — which costs a fraction of a new roof — can extend the life by another 15-20 years. Overall, you're looking at 40-50 years of reliable service.
A garage roof typically costs £1,500-£2,500. Extension roofs run £2,500-£5,000 for 15-25 square metres. Per square metre, expect £90-£140 including insulation and trims. These are full installed costs with no hidden extras. GRP costs a bit more than EPDM rubber but gives you a rigid, walkable surface with colour options.
Both are excellent — it depends on your specific situation. GRP is better for complex shapes with lots of detail work, balconies and walkways, and when you want a specific colour. EPDM is better for larger, simpler rectangular roofs and is more cost-effective per square metre. We install both and can advise which suits your project best.
Yes, that's one of GRP's big advantages. We can apply a non-slip aggregate into the wet topcoat during installation, giving you a surface that's comfortable underfoot but grippy when wet. GRP balcony roofs are strong enough for furniture, planters, and regular foot traffic. They're popular on dormer loft conversions across Buckinghamshire.
GRP resin needs a minimum temperature of about 5°C to cure properly, and the deck needs to be dry. That rules out very cold or wet days. We schedule GRP work around the weather forecast and won't laminate in conditions that might compromise the cure. It means GRP is best done between March and October, though we can work outside that window in milder spells.
Cracking in GRP is almost always an installation issue — wrong resin-to-catalyst ratio, laminating in damp or cold conditions, insufficient mat reinforcement, or inadequate falls causing ponding. A properly laid GRP roof does not crack. If yours has, it likely needs stripping and redoing properly rather than patching, because the underlying laminate may be compromised.
Standard topcoat colours include dark grey, mid grey, charcoal, slate, olive green, and black. Other RAL colours can be mixed to order if you want something specific. The topcoat provides both the colour and UV protection for the laminate beneath. Most customers in Marlow choose grey or charcoal, but greens are popular where the roof is visible from above.
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