What is traditional built-up felt?
Traditional flat roofing felt is a bitumen-based sheet material applied in layers — typically two or three — on a timber deck. It's been used on UK domestic flat roofs since the 1950s and is still available as a lower-cost option. It's what most flat roofs on Leicester's post-war extensions and garages were originally built with.
GRP fibreglass roofing has a design life of 25–40 years compared with 10–20 years for built-up felt systems. Source: own field data (2025)
What is GRP fibreglass?
GRP (glass-reinforced polyester) flat roofing is a modern system where layers of fibreglass matting are laid onto the deck and saturated with liquid resin, then finished with a topcoat. When cured, it forms a single seamless shell with no joins — fundamentally different in structure to felt, which is always a multi-sheet system with overlapping joints.
Lifespan
This is the most significant practical difference. Traditional felt, even well-installed three-layer felt, has a realistic lifespan of 10–20 years. The joins between sheets are the primary failure point — over time, thermal movement opens these joints and water infiltrates. Most of the felt flat roof replacements we carry out in Leicester are on roofs that are 12–18 years old.
GRP fibreglass has no joins and therefore no primary failure mechanism in the same way. A correctly installed GRP flat roof has a design life of 25 years minimum and routinely exceeds 40 years in practice. We back our GRP installations with a 10-year workmanship guarantee — we wouldn't offer that on a felt installation.
Cost
Felt is cheaper to install upfront. A felt flat roof on a standard Leicester extension might cost £600–£1,200 depending on size; the same roof in GRP fibreglass costs £1,200–£2,400. However, if you replace felt every 12–15 years, you will spend more over a 40-year period than a single GRP installation.
On a per-year basis, GRP is almost always cheaper. We recommend felt only where budget constraints make it unavoidable, and we always make the cost-over-time comparison clear on our surveys.
Performance in Leicester's climate
Leicester's cold winters are relevant to this comparison. Felt becomes brittle in very low temperatures and is susceptible to cracking under thermal shock — the rapid temperature change from a cold night to direct winter sunlight on a south-facing flat roof. GRP, once cured, is unaffected by temperature extremes within the UK range and handles freeze-thaw cycling without degradation. For Leicester properties, this gives GRP a meaningful performance advantage over felt in winter conditions.
What about EPDM rubber?
EPDM rubber is a third option with a lifespan comparable to GRP (30–50 years) and excellent cold-weather performance. It's particularly good where weight loading is a concern, as it's lighter than GRP. The main practical limitation is that EPDM is a sheet material — it has joins, unlike GRP — though quality bonded joints in EPDM are significantly more durable than felt overlaps. We install EPDM where it's the appropriate specification for the specific installation.
Our recommendation
For the vast majority of Leicester residential flat roofs — extensions, garage roofs, bay window roofs — we recommend GRP fibreglass. The cost premium over felt is recovered within 15 years through avoided replacement, and the performance advantage in Leicester's winters is material.
If you're replacing an existing flat roof in Leicester, we'll survey the deck condition, the drainage arrangement and the specific exposure of the installation before making a recommendation. We don't apply a one-size-fits-all approach.
Installation Quality: What Actually Matters
Material choice matters, but installation quality matters more. The key factors that determine flat roof longevity regardless of material are: correct drainage fall (minimum 1:80); adequate upstand height at wall abutments (minimum 150mm); correct membrane lapping at all joins; and sound deck condition at installation. The most common flat roof failure we see on Leicestershire properties is inadequate drainage fall — a flat roof that pools standing water degrades faster than its design life regardless of whether it's felt, GRP or EPDM.
The 40-Year Cost Comparison
GRP upfront premium over felt on a standard Leicestershire rear extension (20–25 sq m): roughly £700–£1,400. Over 40 years, one GRP installation versus potentially three felt replacements means GRP is cheaper by £500–£2,000 even before accounting for leak call-out costs in the later years of each felt installation. The economics of GRP are compelling over any horizon beyond 15 years.
The Right Choice for Your Property
For most Leicester homeowners replacing a failed felt flat roof, GRP fibreglass is the correct answer. We only recommend felt where budget constraints make it the only option, and always present the long-term cost comparison before any decision. Free surveys across Leicester, Oadby, Wigston, Hinckley, Loughborough, Melton Mowbray and all Leicestershire areas.
Maintenance After Installation: What Each System Requires
One of the underappreciated practical advantages of GRP fibreglass over felt is the difference in ongoing maintenance requirements. Felt flat roofs require inspection every 3–5 years for lap joint separation, surface cracking from UV exposure, and early blister formation. Left unaddressed, these failure signs accelerate into full waterproofing failure within 2–4 years. GRP fibreglass, once correctly installed and cured, requires essentially no active maintenance beyond keeping drains clear. The seamless membrane has no lap joints to inspect and no UV-sensitive bitumen to monitor. A correctly installed GRP roof from 2005 performs and looks essentially the same in 2025 as on installation day.
EPDM rubber falls between the two. It is more durable than felt and less maintenance-intensive, but bonded seams at overlaps do require periodic inspection. A well-installed EPDM roof with professionally bonded seams has an excellent track record, but the seam inspection requirement distinguishes it from the truly seamless GRP system.
What to Ask When Getting a Flat Roof Quote in Leicester
When comparing flat roof quotes in Leicester, confirm the following in writing before accepting any proposal: the exact system and product being installed (not just “GRP” but the specific resin and matting specification); the drainage fall that will be achieved and how (existing deck, tapered insulation, or firring pieces); the upstand height at all wall abutments; whether the existing deck will be inspected and any rotten sections replaced before installation; and the workmanship guarantee period. A quote that doesn't address drainage fall is not a comparable basis for decision-making — it is the single factor that most determines whether a flat roof achieves its design life.
Key Takeaways for Leicester Homeowners
The most important points from this article as they apply specifically to Leicester properties:
- Leicester's housing stock is unusually old — a higher proportion of Victorian and Edwardian properties than the national average means lead work, lime mortar and clay plain tile are day-to-day materials in Leicester roofing, not specialist categories.
- The post-war estate belt is at a maintenance transition point — Braunstone, Eyres Monsell, Birstall and Thurmaston have nail-sick concrete tile reaching critical stage simultaneously. Proactive action now costs significantly less than reactive emergency repair.
- Free surveys and written quotes are standard — any Leicester roofer who won't provide a free on-site survey and written itemised quote before work starts is not operating to professional standards. Don't accept verbal quotes or estimates given without a site visit.
- NFRC registration is verifiable in 30 seconds — check before you commit. It is the minimum credibility threshold for roofing work in Leicester.
We cover all of Leicester city and the surrounding Leicestershire area. Free surveys, written quotes, and no work starts without your written acceptance. Call 0116 319 1348 for a free survey across all Leicester postcodes.
Getting Help With Your Leicester Roof
The most useful next step for most Leicester homeowners reading this is a free roof survey. A survey gives you a baseline condition report on your specific property — which maintenance is due now, which can wait, and what the realistic cost horizon looks like over the next 5–10 years. This is more useful than general guidance because every Leicester property is different: a 1930s Oadby semi has different priorities to a Victorian Highfields terrace or a post-war Braunstone semi, even if the symptoms look similar from the ground.
We cover all of Leicester city and the surrounding Leicestershire area including Oadby, Wigston, Hinckley, Loughborough, Melton Mowbray, Birstall, Blaby and Syston. Free surveys, no call-out fee, written quote within 48 hours of the visit. Call 0116 319 1348 or email info@roofingleicester.co.uk.
Last reviewed: 20 July 2026