Summary

Felt and GRP fibreglass are the two most commonly quoted flat roof coverings for UK domestic extensions, garages, dormers, and porches, and the choice between them is frequently made on price alone. That's a mistake that shows up five to ten years later as a callback, because the two systems fail differently, last differently, and suit different substrates and uses. A tradesperson who defaults to "whatever's cheapest" on every quote is leaving money on the table on suitable jobs and setting up premature failures on unsuitable ones.

The fundamental difference is manufacturing versus fabrication. Felt (modern torch-on modified bitumen) is a manufactured sheet product with defined, factory-controlled properties — the installer's job is to bond and lap it correctly. GRP is fabricated on the roof from liquid resin, catalyst, and glass matting — its final properties depend entirely on what the installer does that day: mixing ratios, temperature, timing, and technique. This has real consequences for the two systems' failure modes, weather sensitivity during installation, and how forgiving they are of substrate imperfections.

This guide compares the two head-to-head on cost, lifespan, performance, and suitability, so the recommendation can be matched to the actual project rather than defaulting to habit or the cheapest number on the page. For a full three-way (or four-way, including EPDM and single-ply) comparison, see flat roofing.

Key Facts

Quick Reference Table

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Factor Felt (torch-on) GRP Fibreglass
Installed cost (£/m²) 40-110 90-150
Lifespan (years) 12-30 (2-layer to 3-layer) 25-40
Manufactured vs fabricated Manufactured sheet Fabricated on-site from liquid resin
Weather sensitivity during install Low-moderate High (5-25°C, must be fully dry)
Deck flex tolerance Forgiving Unforgiving — cracks if deck moves
Walkable / trafficked surface Not designed for regular foot traffic Yes, genuinely walkable
Joint/seam failure risk Yes — lap joints are the weak point None — seamless once cured
Repair skill required Low-moderate Moderate-high
Installation skill dependency Moderate High — outcome depends on installer technique
Best for Budget jobs, roofs the client won't keep long, non-trafficked garages/porches Balconies, roof terraces, long-term ownership, premium finish

Detailed Guidance

Cost — the headline number hides the real comparison

Felt looks like the clear budget winner, and on a straight per-m² basis it is — roughly half the cost of GRP. But the comparison needs adjusting for lifespan. Over a 30-year horizon, a 3-layer felt roof at £85/m² (mid-range, 25-year life) needs replacing once more within that period, while a GRP roof at £120/m² (mid-range, 30-year life) needs replacing zero or one times depending on exact lifespan achieved. Run the numbers for the specific job rather than quoting from memory — on a small garage roof the labour setup cost dominates and re-doing it in 15 years is a real, quantifiable cost the customer should see.

The other cost factor that catches installers out: GRP pricing depends heavily on deck condition, because GRP requires a rigid, gap-free 18mm+ ply or OSB/3 deck. If the existing deck is felt-era softwood boarding in poor condition (common on older garages and extensions), the deck replacement cost can exceed the roofing cost itself. Always inspect and price the deck separately — see flat roof inspection survey.

Lifespan and failure modes

Felt fails predictably: UV degradation embrittles the cap sheet over time, laps open up at the edges (particularly on poorly torched joints), and ponding water accelerates blistering and eventual splitting. Because felt is a manufactured, layered sheet, failure is usually visible and progressive — a competent roofer can often identify a felt roof approaching end of life during a routine inspection, giving the client warning before a leak occurs.

GRP fails differently and less predictably. When correctly installed on a stable deck, it is extremely durable — genuinely comparable to EPDM in service life. But its failures are almost entirely installation-driven: under-catalysed resin that never fully cures, incorrect key-coat timing causing delamination, or deck movement cracking the rigid laminate. A badly-installed GRP roof can fail within a few years, while a well-installed one comfortably reaches 30-40 years. This makes installer competence a much bigger variable for GRP than for felt — see grp fibreglass flat roofing for the specific quality-control points (catalyst ratio, key coat timing, cure temperature).

Substrate and deck requirements

This is the most important technical differentiator and the one most often overlooked at quoting stage.

Felt tolerates a reasonably imperfect deck — the flexible membrane will bridge small gaps and minor movement without immediate failure, though a poor deck will still shorten life. GRP has effectively zero tolerance for deck flex: the rigid laminate cracks wherever the deck beneath it moves, whether from board gaps, inadequate fixing centres, seasonal movement, or foot traffic deflection on an under-specified deck. A GRP quote on an existing or borderline deck should always include a deck inspection and, where in doubt, deck replacement or overboarding — under-quoting this is the single most common cause of comeback GRP jobs.

Trafficked surfaces — the deciding factor for balconies and terraces

Where the flat roof will see regular foot traffic — a roof terrace, balcony, or walkway — GRP is close to the only sensible recommendation between these two options. Felt's mineral cap sheet is not designed as a walking surface: repeated foot traffic accelerates wear, and standard cap sheet finishes lack slip resistance. GRP's hard gelcoat topcoat, particularly with a non-slip aggregate additive, is purpose-suited to this use and is the standard specification for domestic roof terraces.

For a roof that will never see foot traffic beyond occasional maintenance access (a typical garage, dormer cheek, or single-storey extension roof), this factor doesn't apply and the decision comes down to budget and expected lifespan.

Installation conditions and programme risk

Felt's torch-on application is more forgiving of UK weather — it can proceed in cool, even slightly damp conditions provided the deck itself is dry, because the torch actively dries the surface immediately ahead of the roll. This makes felt easier to programme reliably, particularly in shoulder seasons.

GRP requires genuinely dry conditions throughout application and for the initial cure period (minimum 5°C, ideally not above 25°C, no rain or heavy dew). A GRP job scheduled into unpredictable UK weather carries real programme risk — a rained-off lay-up mid-laminate is a costly abort. Factor weather risk into GRP programming, particularly for autumn/winter/early spring installs, and build weather contingency into the quote or programme rather than absorbing the cost of a failed attempt.

Fire and safety during installation

Torch-on felt involves an open flame reaching over 1,000°C at the torch head, working over or near a timber deck and often close to combustible fascias, rafter ends, and roof voids. This requires a hot-works permit-to-work system, a dedicated fire watcher during application, and a minimum one-hour fire watch after work stops — see torch on felt systems for the full protocol. This is a genuine and recurring cause of roofing fires in the UK and insurers increasingly scrutinise hot-works compliance.

GRP carries no fire risk during installation but does involve handling catalysed resin (skin and eye irritant), MEKP catalyst (corrosive, requires careful handling and storage separate from resin), and styrene VOC fumes requiring ventilation and, in enclosed spaces, respiratory protection. COSHH assessment is required for GRP resin and catalyst handling.

Frequently Asked Questions

Can GRP be applied over an existing felt roof?

Not directly and not recommended. GRP needs a rigid, stable, correctly-fixed timber deck — an existing felt roof (even if stripped back to the deck) needs full inspection, and any deteriorated boarding must be replaced before GRP is applied. GRP should never be laminated directly over old felt, as trapped moisture beneath the felt will cause blistering and delamination of the new GRP.

Which is cheaper over the long term, felt or GRP?

It depends on the specific lifespan achieved and how long the client keeps the property. As a rule of thumb, if the roof will need replacing again within the client's ownership horizon (say, under 15 years), felt's lower upfront cost usually wins. If the client is staying long-term, GRP's roughly double lifespan for roughly double the cost is close to cost-neutral and delivers a better roof with less disruption over time.

Is felt or GRP better for a garage roof?

For a standard non-trafficked garage roof on a tight budget, felt is a perfectly reasonable and widely-used specification — see garage roof replacement pricing guide. GRP is worth the premium where the client wants the longest realistic life, plans to use the garage roof space (occasional access, a green roof conversion, or a future extension), or where the deck is being renewed anyway and the marginal cost difference is smaller relative to the whole job.

Does GRP really last twice as long as felt?

In well-documented cases, yes — manufacturer warranties of 20-25 years and demonstrated service lives of 40+ years are achievable for correctly installed GRP, roughly double the 12-20 year range typical of 2-layer felt. The comparison is less stark against 3-layer felt (20-30 years), where the lifespan gap narrows considerably. The bigger determining factor for GRP is installation quality — a poorly installed GRP roof can fail faster than a well-installed 3-layer felt roof.

Do both systems need the same insulation?

Yes, in principle — both sit within a warm-roof or cold-roof build-up and must meet the same Part L U-value targets (0.18 W/m²K new build, 0.25-0.30 W/m²K renovation), typically using PIR insulation boards beneath the covering. The choice of felt or GRP as the outer covering does not change the insulation specification, though GRP's rigid deck requirement means the insulation and deck build-up must be specified together for structural compatibility.

Regulations & Standards