If you're replacing a flat roof, the choice usually comes down to two modern systems: EPDM rubber and GRP fibreglass. Both are widely used on UK homes, and both can produce a reliable, long-lasting flat roof when the roof is properly prepared and the system is installed correctly.
There isn't one material that is automatically best for every property. The size and shape of the roof, the amount of detailing, how the roof will be used, how it will look from upstairs windows and the condition of the existing roof deck all play a part. This guide sets out the differences in plain terms so you can understand the options before you speak to a roofer.
What is EPDM roofing?
EPDM is a synthetic rubber membrane, often simply called a rubber roof. On most domestic roofs it is supplied as a single large sheet, cut to suit the roof and bonded to a prepared roof deck with adhesive. Edges are finished with trims, and upstands are formed where the roof meets walls.
Because the membrane is flexible, it copes well with the normal expansion and contraction a roof goes through between hot and cold weather. On a simple, rectangular roof it can often be laid in one piece, which means few or no joints across the main area. You can read more about how we install it on our EPDM rubber roofing page.
What is GRP roofing?
GRP stands for glass reinforced plastic, more commonly known as fibreglass. Rather than arriving as a finished sheet, a GRP roof is built up on site. A layer of resin is applied to the roof deck, glass fibre matting is laid into it and worked in, and the roof is then finished with a coloured topcoat once the laminate has cured.
The result is a hard, rigid and seamless surface, with preformed trims at the edges. Because the covering is laid as a liquid and sets in place, it can be carried up and around details to give a neat, continuous finish. Our GRP fibreglass roofing page explains the system we use.
EPDM vs GRP: the main differences
Both systems do the same job, but they behave differently. The main points of comparison are:
- Appearance: EPDM has a matt black rubber finish. GRP has a smooth, hard finish and can be topcoated in a range of colours, which some homeowners prefer where the roof is overlooked.
- Flexibility: EPDM is flexible and stretches with movement. GRP is rigid once cured, so the deck beneath it needs to be firm and stable.
- Joints and seams: on simple roofs EPDM can often be laid as one sheet. Larger or awkward roofs may need joined sheets, and those joints need to be made carefully. GRP is seamless across the whole surface.
- Complex roof shapes: GRP can be formed around rooflights, outlets and upstands as one continuous layer. EPDM can handle these details too, but they rely on careful cutting, forming and bonding.
- Durability: both are durable when installed well. EPDM resists weathering and movement; GRP gives a tough, hard-wearing surface. How long either lasts depends heavily on the preparation and workmanship.
- Maintenance: neither is maintenance-free. Both benefit from keeping outlets and gutters clear and having the roof checked periodically, particularly after storms.
- Repairs: EPDM can usually be patched using compatible membrane and adhesive. GRP can be repaired by preparing the damaged area and laminating over it. In both cases a repair needs the right materials and a clean, dry surface.
- Installation: EPDM is less sensitive to weather during fitting once the deck is dry. GRP relies on resin curing properly, so it needs dry conditions and suitable temperatures to be laid.
When we would recommend EPDM
EPDM is often a very good fit for straightforward flat roofs where a practical, flexible covering is the priority. Typical examples include:
- Single-storey extensions with a simple rectangular roof
- Garages and carports
- Garden rooms, sheds and other outbuildings
- Larger, simple flat roof areas where a single sheet or a small number of well-made joints will cover the roof
- Roofs where a little movement in the structure is expected
When we would recommend GRP
GRP tends to suit roofs where a hard, seamless finish and a clean appearance are important. That might be a roof that is clearly visible from upstairs windows, a dormer roof, a bay or porch roof, or a roof with detailing such as rooflights, outlets and upstands that benefit from being formed as one continuous layer.
Because GRP is rigid, it does need a solid, stable roof deck. Where the deck is suitable and the detailing is right, it can give a very tidy, durable finish.
Why preparation matters as much as the covering
The covering is only one part of a flat roof. Even a premium material can fail early if what sits underneath isn't right. A flat roof relies on a sound roof deck, correct falls so water drains away instead of pooling, properly detailed edges and upstands, outlets that are positioned and sealed correctly, and trims that are fixed securely.
That's why, when we assess an existing flat roof, we look at the condition of the roof as a whole rather than simply covering over an existing problem. Soft or damaged decking, poor falls and failed edge details are usually dealt with before any new covering goes on. Our flat roofing page explains how we approach this.
Flat roofs should be assessed safely, from ground level, an upstairs window or by a roofing professional. Please don't climb onto a flat roof to inspect it yourself, as older decking can be weaker than it looks.
Can an existing leaking flat roof be repaired instead?
Not every leak means the whole roof needs replacing. Depending on the cause and the general condition of the roof, a professional repair may be the sensible option, for example where the problem is a single damaged area, a failed edge detail or a blocked outlet.
Where the covering has failed in several places, the deck has gone soft or water has been getting in for some time, replacement is often the more reliable long-term answer. If you're unsure where the water is getting in, our guide to finding where a roof leak is coming from is a useful starting point, and our roof repairs page explains how we deal with leaks.
EPDM or GRP: which would we choose?
Honestly, it depends on the roof. Neither system is better in every situation. EPDM is often a practical choice for simple extensions, garages and outbuildings. GRP often suits roofs where a hard, seamless finish and appearance matter, and where the deck and detailing are suitable for it.
What matters most is choosing the system that suits the individual roof, then preparing and installing it properly. That's the approach we take on flat roofs across Coventry and the surrounding areas.
Not sure whether EPDM or GRP is right for your roof?
The Roofing Lads can inspect your flat roof, talk you through the options and recommend the system that suits your property, rather than automatically pushing one material. Contact us with a few details and photographs of the roof if you have them.
Need Advice on Your Flat Roof?
Tell The Roofing Lads about your flat roof and send photographs where available. We can inspect it, discuss the options and recommend the system that suits your property.