GRP fibreglass suits rectangular flat roofs up to about 40 square metres, giving a seamless, hard-wearing finish. EPDM rubber suits larger, irregular or heavily penetrated roofs and tolerates deck movement better. Both last 25 to 40 years in Hull's climate and both far outperform the felt they usually replace.
If your flat roof needs replacing in Hull or Beverley, the conversation will arrive at a choice between GRP fibreglass and EPDM rubber. Both are significantly better than traditional felt in the East Yorkshire climate — but they're not the same system, and the right choice depends on your roof's size, shape and specific conditions.
Why Not Just Felt Again?
Traditional felt flat roofing has a realistic design life of 10–15 years in East Yorkshire. The persistent Hull Valley humidity, Humber-influenced damp air, and the pronounced freeze-thaw cycling of East Yorkshire winters are hard on felt. Many Hull properties built in the 1970s and 80s are now on their second or third felt flat roof on single-storey extensions — and still dealing with the same repeat failure.
GRP Fibreglass: The Case For
BS 6229 sets a minimum finished fall of 1:80 for flat roofs, with design to 1:40 recommended so construction tolerance does not eliminate the fall.
Source: BS 6229:2018, Flat roofs with continuously supported flexible waterproof coverings
GRP is a fibreglass composite applied wet to the deck and allowed to cure in place. Its defining advantage is that it's completely seamless — no joins, no laps, no seam adhesive. Water cannot penetrate a correctly installed GRP roof at any point other than a drainage outlet. On small and medium extension flat roofs — the most common flat roof call in Hull and Beverley — this makes GRP the preferred specification.
GRP also produces a hard, rigid surface that is easy to walk on safely, is UV-resistant, and takes surface treatments if appearance matters. On garage roofs that double as accessed terraces, GRP is the obvious choice.
EPDM Rubber: When It's the Better Choice
EPDM is available in very large sheets — up to 15m wide. This makes it practical for larger flat areas where covering the roof in a single or very small number of pieces is achievable. On a standard garage roof of 25–40m², a single EPDM sheet can often cover the entire area with no field joins at all. EPDM is also more flexible than GRP, making it better suited to irregular shapes, multiple penetrations, and surfaces with slight movement.
Installation Method and Site Conditions
The two systems are installed in fundamentally different ways, and this drives most of the practical differences between them. GRP is a wet-applied system: a catalysed polyester resin is rolled onto a prepared deck, chopped strand mat is laid into it and consolidated, and a pigmented topcoat is applied. It cures chemically over a few hours into a single seamless shell.
EPDM is a factory-cured synthetic rubber sheet, supplied in large rolls, bonded to the deck with contact adhesive and dressed into trims at the perimeter. Nothing cures on site. The consequence is that GRP is weather-critical — it needs a dry deck, ambient temperature reliably above about 5°C, and no rain during cure — while EPDM is more tolerant, needing only a dry deck and temperatures above the adhesive's minimum. In an East Yorkshire winter that difference frequently determines which system can actually be installed.
Detailing, Trims and Complex Shapes
GRP's advantage is that it is formed in place, so upstands, drip edges, outlets and penetrations are laminated as part of the same continuous shell with no joints. On a rectangular extension roof with a single outlet this produces an exceptionally reliable result. Its disadvantage is rigidity: cured GRP is stiff, and it tolerates deck movement poorly. On a deck that flexes — older joists, wide spans, or timber that will move seasonally — GRP can crack, usually along a board joint.
EPDM is flexible and accommodates movement readily, which makes it the better choice on older or lightly built structures and on roofs with complex geometry, multiple penetrations or awkward junctions. The trade-off is that every corner, upstand and penetration is a detail formed from separate pieces bonded together, so the quality of the result depends more heavily on the installer's care at those points.
Foot Traffic, Repairs and Longevity
GRP has a hard, abrasion-resistant surface and tolerates occasional foot traffic well, which matters if the roof carries a boiler flue, a satellite dish or an access route to a window. EPDM's rubber surface is softer and more easily damaged by dropped tools or sharp objects, though it is far from fragile in normal use.
Repairability favours EPDM. A puncture is patched with a bonded repair piece in minutes and the repair is as durable as the original sheet. GRP repairs require abrading back, re-laminating and re-coating, and a repair is visible. Both systems, correctly installed, give 25–40 years. In practice, failures in either system are far more often installation defects at the upstand or a deck problem than material failure.
Appearance, Colour and Overlooked Roofs
GRP topcoats are available in a range of colours and produce a smooth, uniform, slightly glossy finish that can be specified to complement the building. EPDM is black, and while it can be coated, the standard finish is a matt black sheet with visible seams at the trims.
This matters where a flat roof is overlooked — a rear extension seen from upstairs windows, or a garage roof visible from a neighbouring property. On many Hull and Beverley extensions nobody ever sees the roof and the point is irrelevant. Where it is visible, GRP generally looks better and is easier to specify sympathetically, and it is worth weighing alongside the technical considerations.
Which System for Which Hull Property
Our default across Hull and the East Riding is GRP for rectangular single-storey extension roofs up to about 40m² on a sound modern deck, installed between April and October. It gives a seamless result, a hard-wearing surface and a good appearance, and on that kind of roof the rigidity is not a limitation.
We specify EPDM where the roof is larger, where the shape is irregular or heavily penetrated, where the structure is older and likely to move, or where the work has to proceed in conditions that GRP cannot be laid in. Both are substantially better specifications than the felt they usually replace, and on the twenty-to-forty-year-old extension roofs across Hull the choice between them matters far less than the decision to move away from felt.
Free survey, no call-out fee. Call 0148 290 8437 or book online.
Last reviewed: 22 July 2026 by Beverley Roofing Experts.
Frequently Asked Questions
Both are far superior to felt in Hull's Humber-influenced climate. GRP is the preferred choice for most extensions and smaller flat roofs — seamless, no joins, longest design life. EPDM suits larger flat areas (garages over 30m², irregular shapes) because it comes in larger sheets and is more flexible around complex penetrations.
A correctly installed GRP roof has a design life of 25–40 years in the Hull climate. EPDM runs 25–35 years. Both significantly outperform traditional felt (10–15 years in East Yorkshire conditions) and represent far better whole-life value.
Sometimes. If the existing deck (plywood or chipboard) is sound, a new membrane can often go over the existing deck. If the deck has moisture damage, it needs replacing before the new membrane. Installing over a compromised deck dramatically shortens the new roof's lifespan — we always assess the deck before quoting.
Traditional felt has a realistic design life of 10–15 years in Hull's climate. Many Hull properties built in the 1970s–80s are on their second or third felt flat roof — and still dealing with the same repeat failure. GRP and EPDM were developed specifically to address felt's limitations and have design lives more than double that of felt.
A standard extension or garage flat roof in Hull typically costs £800–£2,500 in GRP or EPDM depending on area. Larger flat roofs with deck repairs required will cost more. Free survey and itemised quote — call 0148 290 8437.
Yes, and in some cases it's the most cost-effective long-term solution for a persistently problematic flat roof. A low-pitched tiled addition eliminates the flat roof maintenance cycle entirely. Planning permission may be required. Call 0148 290 8437 to discuss your specific situation.
Very little. An annual check of drainage outlets — clearing debris from the outlet and confirming the surrounding membrane hasn't lifted — takes ten minutes. A visual inspection of the surface and upstand edges every two or three years is good practice. Neither system requires painting, coating or chemical treatment during its service life.