If you plan to use the same glue or resin for both GRP and EPDM roofing, do not continue. You can't switch between these two systems. Using the wrong product won't just void your warranty, it will crack your roof and cost you thousands in repairs.
The real problem is that GRP and EPDM work on opposite principles. One is chemistry that hardens. The other is elasticity that flexes. Understanding how each is applied, what they're made of, and what happens when you mix them will save you serious money.
What About Bitumen, Sealants, or ‘Quick Fix’ Products?
Also no, and actually, this causes the most damage of all.
Bitumen, general mastics, and “emergency” sealants:
- Prevent proper GRP bonding later
- Trap moisture beneath the surface
- Accelerate cracking, blistering, and delamination
In many cases, these quick fixes leave the roof worse than the original leak, which is why the right repair kit is so important.
How They're Applied
The moment you start a GRP project, you're mixing chemistry on site. To begin with, combine liquid polyester resin and a catalyst known as MEKP. then pour the mixture on fiberglass matting. The catalyst initiates a chemical reaction which transforms the liquid into a solid plastic shell. You apply it in layers, let it set for 3-5 days, and you get a strong, smooth surface that stays the same forever.
When you choose from our selection of complete GRP fibreglass roofing kits, you're getting 7Kg Resin, 250g Hardener, 7m Fibreglass Matting 450g, and 5KG Topcoat all measured for chemical bonding together.
EPDM is the complete opposite. The rubber membrane arrives pre-made. Nothing gets mixed. Roll it out, put on some glue that works with contact, let it get sticky, press it down, and seal the edges. The work will be completed within one to two days. The membrane will not be hard, it is designed to be flexible to be able to stretch and shrink, without breaking.
Our EPDM roofing kits come with pre-calculated membrane sizes and the correct matching adhesives, so you have everything balanced for your roof size.
Why They Don't Mix
Knowing what each product is made of helps us see why they don't mix well at a tiny level.
GRP resin is an unsaturated polyester mixed with MEKP catalyst. When combined, free radicals trigger a chain reaction that links polymer molecules together, hardening the liquid into solid plastic. This is permanent. The resin becomes rigid, strong, and doesn't flex. According to research on thermal expansion of polymers published by the National Center for Biotechnology Information, cured GRP becomes non-linearly stable and maintains rigidity even when temperature changes significantly].
EPDM adhesive works differently:
-
Base material: This is a form of synthetic rubber (mostly halogenated butyl rubber) blended with organic solvents.
-
Here's how it works: When it is drying, the solvents are evaporated to give a flexible bond that is able to stretch and shrink with the rubber.
- Key difference: It doesn't change into something hard, it remains flexible.
Unlike GRP resin, EPDM adhesive is designed to flex forever, not harden permanently. One is engineered to become rigid. The other is designed to stay elastic. That's not a small difference. That is the primary reason why they cannot work together. Learn more about EPDM contact adhesive options.
What Happens When You Use the Wrong Product
Real failures follow predictable, expensive patterns.
If you apply GRP over existing EPDM
You're gluing a rigid plastic shell to a flexible rubber sheet. Three failures happen in sequence. First, adhesion fails because EPDM rubber resists bonding, it's waterproof precisely because things can't stick to it easily.
Second, the materials fight each other. EPDM expands and shrinks with temperature. GRP hardens into a rigid shell that won't move. When summer arrives, the rubber expands underneath while hard GRP stays put. The stress cracks the GRP. Water seeps through, finds your original leak, and makes it worse. You've made the problem worse.
Third, moisture gets trapped. Water vapour can't escape through rigid GRP, causing blistering and failure. Stripping and repairing costs £2,500–£5,000+.
If you use EPDM adhesive on a GRP project
The adhesive never hardens enough to support the rigid laminate. It stays soft or tacky instead of achieving structural strength. Ponding water finds weak spots within weeks. Edges lift. You remove the entire roof and start over £3,000–£7,000+ in unnecessary costs.
Pick Your System and Stay With It
Whether you're starting with our extensive range of GRP fibreglass roofing kits or moving forward with EPDM membrane systems, the rule remains the same: pick one and commit to it completely. Don't mix products between the two. Use one system exclusively with its matching products. It’s cheaper to get it right the first time than to fix a hybrid system that didn’t work.
Check out Apex's helpful video tutorials for easy installation tips. They demonstrate how to do all aspects such as the deck preparation up to the final topcoat. Select a system, stick with it and you will have a roof that will last years.