All three materials that dominate the domestic flat roof market – GRP, EPDM and TPO – are subjected to BS EN 13956, the European testing standard for flexible waterproofing sheets for roofing, which covers issues like tensile strength, elongation, tearing and thermal cycle resistance. What this standard doesn’t provide is which of the three materials is more likely to last 20 years on the type of roof typically found on extensions and outbuildings in the UK. To answer that, you first need to understand the nature of each material, how it is likely to fail, and how much a contractor is likely to warrant against that failure.

GRP — explanation and applications of it

GRP is an acronym for glass reinforced plastic, which is more commonly known as fibreglass. The process entails the application of a liquid resin which is reinforced with fibreglass matting and is finished with a topcoat. This results in a rigid, single membrane with no seams. The absence of seams also means that in the case of multiple membranes/ layers, such membranes/ layers are not likely to experience failure at the seams, lifting of overlapping pieces, or peeling at the edges. Therefore, GRP is seen to be a reliable option for simple flat roof designs with relatively simple detailing at the upstand and outlet interfaces, and as such is the first choice for most flat roofing contractors in the UK.

The limiting factor is flexibility. GRP is more rigid than elastic and therefore does not deal well with significant structural movement. On large roofs like timber decking over an older structure for example – that rigidity will cause over time cracking typically along the lines of joins in the decking below. The topcoat also has a finite lifespan due to UV degradation and is due for inspection every 8 – 10 years. When a topcoat refresh is due, it is simple and less costly than a full membrane replacement.

For domestic extensions, we are looking at installed costs of approximately £70 – £100 per square metre for the roof including the decking substrate.

EPDM — the case for rubber

EPDM, or ethylene propylene diene monomer, is a type of synthetic rubber membrane that is delivered in large sheets, usually factory cut to the roof dimension. While GRP is rigid, EPDM is elastic — it can stretch more than 300% before failure — it will not crack under structural movement or thermal expansion. In colder temperatures, when most other membranes become brittle, EPDM remains flexible. This makes it especially ideal for exposed roofs in the north.

The main weakness is adhesion and detailing. EPDM on its own is designed to be adhesive bonded at edges, penetrations, and upstands. A poorly bonded edge can allow water to get in. Detailing around outlets, pipes, and parapets is tricky and requires experience — a contractor who does GRP for a living and fits EPDM now and then is more likely to get lazy on those joints than a real specialist. When looking for contractors, inquire about their volume of EPDM installations specifically.

Installed cost runs a tad cheaper than GRP at £60–£90 per square metre, which is partly due to the fact that installation is quicker on basic roofs.

TPO — the new kid on the block

TPO — short for thermoplastic polyolefin — is a heat-welded membrane that has been used as a standard on commercial flat roofs and is now being used more and more in home applications as well. With TPO, the seams are welded instead of bonded which creates a better joint than the adhesive bond joints of a correctly installed EPDM. TPO is also more resistant to oils, fats and UV degradation than EPDM. This is less of a concern on a domestic roof, but is worth considering.

The less extensive history of domestic use of TPO compared to GRP or EPDM is to some extent expected, as TPO is a newer material. However, it is still concerning that contractors reference data from a completely different industry. While it is fair to be cautious of TPOs domestic use, TPO as a material is quite dependable, so it is reasonable to be cautious of the warranties offered.

At £75 – £110 per square meter, the installed cost is fairly similar to GRP.

What BS EN 13956 actually tests — and what it leaves out

BS EN 13956 lays out specific ways to test flexible waterproofing sheets for things like tensile strength, how much the material stretches before it breaks, how much it resists tearing, how heat causes dimensional changes, and how it bends at low temperatures. However, what is lacking in those guidelines is testing for long-term durability of the material installed in the real world, the materials resistance to foot traffic, or how good the material is at joining and detailing. So an EN 13956 compliant membrane may perform adequately as a material, but it still may not guarantee that the roof is watertight, especially considering that the installation itself may be poor.

Additionally, the standard includes some coverage for polymer-modified bitumen membranes, traditional felt systems, which, while they have a long history of use, are also not included in the GRP/EPDM/TPO comparisons.

What a properly specified warranty covers

Most contractors will provide a 10-20 year warranty on their flat roof membranes. However, before you price their warranty on face value alone, find out if their warranty covers materials only, materials and labour, whether the warranty is contingent on periodic inspections by the contractor, and whether the warranty is backed by the contractor or the manufacturer. A contractor’s warranty is only good for as long as their business lasts. A manufacturer’s warranty, which several GRP and EPDM system manufacturers provide, protect you even if the contractor goes out of business.

Make sure you get the warranty document before the work starts, not as an afterthought after the work is done.