Felt Roof vs Fibreglass: Which Roof Is Best?

A leaking garage roof after a week of Manchester rain is not the time to discover that two flat roofing materials perform very differently. In the felt roof vs fibreglass decision, the right answer depends on the roof’s condition, its size and detailing, how it will be used, and whether you are prioritising lower upfront cost or a longer-lasting finish.

Both systems can protect a home well when they are specified correctly and installed by experienced roofers. The problems usually begin when a material is chosen simply because it appears cheaper, or when the existing deck, drainage and edge details are overlooked.

Felt roof vs fibreglass: the main difference

Modern felt roofing is usually a multi-layer bituminous membrane system. It is commonly installed using a torch-applied method, with layers bonded together and finished with a mineral cap sheet. This is a considerable step up from the older, thinner felt roofs many homeowners remember. A professionally fitted modern felt roof can be a reliable, cost-effective option for garages, extensions, porches and other flat roof areas.

Fibreglass roofing, often called GRP roofing, is formed from resin and glass-fibre matting applied over a suitable roof deck. Once cured, it creates a hard, continuous and joint-free surface. It is normally finished with a coloured topcoat, giving the roof a clean appearance and strong resistance to standing water when the roof has been designed and detailed properly.

The most visible difference is the finish. Felt has seams and typically a granular surface, while fibreglass is smooth and seamless. More importantly, each material reacts differently to movement, damage, maintenance and installation conditions.

Lifespan and weather resistance

For many property owners, lifespan is the deciding factor. A good-quality felt roof can often last around 15 to 25 years, depending on the product used, the number of layers, the roof’s exposure and the standard of installation. Higher-specification systems may last longer when maintained properly.

A well-installed fibreglass roof can commonly offer a lifespan of 25 to 30 years or more. Its seamless surface means there are no lap joints across the main roof area that can gradually weaken. It is also highly resistant to ultraviolet light and can perform well through the changing weather experienced across the North West.

That does not mean fibreglass is automatically the better choice for every roof. GRP is a rigid material once cured. If the roof deck moves, is poorly prepared or has existing structural issues, cracks can develop. A felt system has more flexibility, which can be helpful on roofs subject to minor movement. The roof structure should always be assessed before deciding which covering is suitable.

Water management matters just as much as the membrane. A flat roof should not be truly flat. It needs a suitable fall towards outlets, gutters or drainage points. Neither felt nor fibreglass will compensate for poor drainage, damaged fascias or water repeatedly backing up at an edge.

Cost: upfront price versus value over time

Felt is usually the more affordable option at the outset. It can be an excellent choice when a sound, professionally installed roof is needed within a tighter budget. For a straightforward garage or small extension, a modern felt system may provide very good value without compromising on weather protection.

Fibreglass generally costs more because the installation involves careful deck preparation, resin work, curing times and specialised trims. However, the higher initial investment can be worthwhile if you expect to stay in the property for many years, want a smart finished appearance or prefer the potential for a longer service life.

A quotation should never be judged on the headline figure alone. Ask what is included: removal of the existing covering, replacement of rotten decking, insulation upgrades where required, new edge trims, gutter work, waste disposal and the workmanship guarantee. A lower quote can become poor value if essential preparation has been left out.

For landlords, the calculation may be slightly different. A dependable roof with fewer likely call-backs can protect rental income and reduce disruption for tenants. For homeowners planning a future sale, a recently renewed flat roof completed to a clear specification can also provide useful reassurance to prospective buyers.

Installation requirements and disruption

Both roofing systems demand careful preparation. The existing roof may look tired on the surface while the timber deck beneath is still sound, or it may conceal damp, rot and poor previous repairs. A proper inspection is the only reliable way to establish the scope of work.

Torch-on felt must be installed with appropriate safety controls because it uses a flame. This is routine work for competent roofing professionals, but it is not a material suited to inexperienced DIY fitting. The installer must manage fire risk, ensure the layers are properly bonded and finish every upstand, corner and outlet correctly.

Fibreglass is usually installed over new OSB decking or another suitable prepared substrate. The deck needs to be clean, dry and stable. Weather conditions matter because moisture and low temperatures can affect resin curing. A reputable contractor will not rush a GRP installation in unsuitable conditions simply to meet a diary date.

In either case, the details make the roof. Flashings where the roof meets a wall, drips at the perimeter, rooflight surrounds and outlet connections are frequent failure points when work is rushed. The membrane is only one part of a watertight system.

Repairs and ongoing maintenance

Felt roofs are often relatively straightforward to repair when damage is localised. A split, lifted seam or damaged patch may be repaired without replacing the whole roof, provided the surrounding layers remain sound. Regular checks can identify small issues before water reaches the roof deck.

Fibreglass is tough and low-maintenance, but it is not maintenance-free. It should be inspected for impact damage, cracks around edges and degradation of the topcoat over time. Repairs are possible, although matching the finish and ensuring a sound bond requires the correct materials and preparation.

Keep gutters, outlets and valleys clear regardless of the system you choose. Leaves, moss and debris can hold water against the roof, obstruct drainage and make defects harder to spot. Avoid dragging furniture, ladders or heavy objects across a fibreglass roof, and do not treat either roof as a regular walking surface unless it has been designed for that use.

Which roof is better for garages, extensions and balconies?

A modern felt roof is often a sensible fit for garages, sheds, utility extensions and larger straightforward flat roofs where budget is an important consideration. It is also useful where a flexible multi-layer system is preferred and the roof design includes numerous details that can be efficiently dressed in felt.

Fibreglass is particularly popular for smaller domestic flat roofs, including bay windows, porches and extensions. Its smooth finish can look especially neat on a roof visible from upstairs windows. It is also a strong option where the aim is to avoid joints over a compact roof area.

Balconies, roof terraces and regularly trafficked areas need a more specific conversation. Standard felt or GRP roofing may not be enough on its own if people will walk on the surface frequently. The roof needs a trafficable finish, suitable protection layers and appropriate drainage. This is a situation where design and specification matter more than a simple material comparison.

Questions to ask before choosing

Before authorising replacement work, establish whether the roof deck is dry and structurally sound, whether the roof has enough fall for drainage, and whether insulation needs to be improved to meet current requirements. It is also worth asking how the roof will be finished at walls, gutters, rooflights and door thresholds.

Request a clear written scope of work rather than relying on phrases such as “recover the roof”. Overlaying an old roof can occasionally be appropriate, but it can also hide trapped moisture, rotten timber and previous defects. Stripping back to the deck may cost more initially, yet it gives the installer the opportunity to address the cause of the problem rather than covering it over.

At A1 Bespoke Ltd, we believe property owners should be able to understand what is being proposed, why it is suitable and what standard of finish they can expect. A clear survey and transparent quotation provide far more confidence than choosing a roofing material from a photograph alone.

The best choice is the one that suits your roof rather than the one that wins a general comparison. If water is getting in, the surface is blistering or cracking, or the roof is approaching the end of its life, arrange an inspection while the issue is still manageable. A well-planned replacement can protect the rooms beneath it for many years and remove one more uncertainty from looking after your property.