Choosing between a felt roof and a GRP (fibreglass) roof is one of the most common decisions facing London homeowners, landlords, and property buyers. Both materials are widely used on flat roofs across the capital, yet they differ significantly in cost, lifespan, maintenance, and long-term performance.
This guide provides a balanced comparison of felt and GRP flat roofs. Whether you’re replacing an ageing roof on a Victorian terrace extension, installing a new garage roof, or weighing up survey recommendations before a property purchase, you’ll find clear answers to help you make an informed choice.
Why London Homeowners Compare Felt vs GRP Flat Roofs
Flat roofs on London extensions, garages, and bay roofs
Flat roofs are a defining feature of London’s housing stock. From single-storey kitchen extensions and bay roofs to garages, porches, and dormer conversions, flat roofing solutions are essential across residential properties throughout the capital.
Many London properties built between the 1960s and 1990s feature flat-roofed extensions constructed with traditional torch-on felt. These roofs were cost-effective at the time, but decades of exposure to London’s weather have taken their toll. Consequently, homeowners often face a decision: repair the existing felt roof or upgrade to a more durable alternative, such as GRP.
Common issues with older felt roofs in London properties.
Older felt roofs are prone to leaks, blistering, and ponding water. Surveyors regularly flag these problems in homebuyer reports, prompting questions about repair costs and remaining lifespan. Additionally, felt roofs on exposed sites can suffer from cracking, lifted edges, and flashing failures, especially where maintenance has been neglected.
Meanwhile, modern GRP roofs are increasingly common on new extensions and roof replacements. However, they are not without potential issues. Poor installation can lead to cracking, pinholes, and premature failure, underscoring that material choice is only part of the equation.
Why cost, lifespan, and maintenance drive the comparison
Ultimately, the decision between felt and GRP comes down to three factors: upfront cost, expected lifespan, and ongoing maintenance. Felt roofs typically cost less to install but require more frequent repairs and replacement. In contrast, GRP roofs have a higher initial cost but offer longer life and minimal maintenance. For London homeowners balancing tight budgets against long-term property plans, understanding these trade-offs is essential.
What Is a Felt Flat Roof?
Felt flat roofs use layers of bitumen-impregnated material to create a waterproof membrane. Modern bitumen felt has evolved considerably, offering improved flexibility and weather resistance.
How modern bitumen felt flat roofs are constructed
A typical felt roof consists of a timber deck (usually OSB or plywood boarding), followed by a vapour barrier and insulation layer to meet Part L energy efficiency requirements. The waterproof membrane is then applied in overlapping layers.
Most modern installations use torch-on felt, where heat is applied to bond the felt layers together. This method creates strong seams and reduces the risk of water ingress. The roof surface is finished with a mineral felt top layer, which provides UV protection and fire resistance. Proper installation requires adequate overlaps, careful sealing around upstands, and correct formation of drips and flashings.
Typical lifespan of felt roofs in London (10–15 years)
When professionally installed and maintained, a felt flat roof typically lasts between 10 and 15 years. However, lifespan varies depending on installation quality, maintenance routines, and exposure to weathering.
London’s climate presents specific challenges. Frequent rainfall, temperature fluctuations, and occasional frost cycles accelerate the degradation of bitumen. Over time, UV exposure can make the felt brittle, while thermal expansion and contraction can cause cracking. Furthermore, inadequate drainage and ponding water significantly shorten a felt roof’s life.
Common felt roof problems in London homes.
Leaks, blistering, and ponding water
Leaking is the most frequent complaint with felt roofs. Water can penetrate through cracked seams, damaged flashing, or areas where the felt has lifted. Once water enters the roof structure, it can cause damp stains on ceilings, damage insulation, and even lead to timber rot.
Blistering occurs when air or moisture becomes trapped beneath the felt surface, creating raised bubbles. If left untreated, blisters can burst, allowing water to enter. Similarly, ponding water—rainwater that pools on the roof surface—adds weight, stresses the membrane, and accelerates deterioration.
Cracking, lifted edges, and flashing failures
As bitumen ages, it loses flexibility and becomes prone to cracking. Temperature swings cause the roof to expand and contract, and over time, this movement creates splits in the felt. Edges and upstands are particularly vulnerable, especially where detailing is inadequate.
Flashing failures around walls and chimneys are another common problem. Lead or felt flashings can crack, lift, or pull away from the wall, creating entry points for water. At Smart Roofing London, we often see flashing issues during inspections—proper chimney repairs and leadwork are crucial for preventing leaks on older London properties.
What Is a GRP (Fibreglass) Flat Roof?
GRP stands for glass-reinforced plastic, commonly known as fibreglass. GRP flat roofs use layers of polyester resin and glass fibre matting to form a seamless, waterproof surface.
How GRP flat roofs are installed (decking, laminate & topcoat)
GRP installation begins with a clean, dry timber deck, typically 18mm OSB boards. The deck must be structurally sound and properly secured, as movement or flexing can cause the rigid GRP to crack.
First, a trim is fixed around the roof perimeter and upstands. Then, the installer applies a base coat of polyester resin to the deck. Fibreglass matting is laid into the wet resin and saturated to ensure no air pockets remain. This process may be repeated to build up multiple laminate layers for added strength.
Once the laminate has cured, a pigmented topcoat is applied. The topcoat provides UV protection, colour, and a smooth finish. Proper installation requires dry conditions, as moisture prevents the resin from curing correctly. Additionally, the installer must create expansion gaps and use the correct boarding to accommodate thermal movement.
Typical lifespan of GRP roofs in the UK (25–30 years)
GRP flat roofs typically last between 25 and 30 years when professionally installed and properly maintained. Some manufacturers advertise GRP as a “50-year roof” or even a “lifetime” system, but these claims are generally overstated. Realistic expectations should be based on 25–30 years of reliable performance.
The seamless nature of GRP eliminates the weak points found in felt roofs, such as seams and joints. Furthermore, GRP is highly resistant to water ingress, UV degradation, and biological growth. However, lifespan depends on correct installation, adequate expansion provision, and periodic topcoat maintenance.
Common GRP roof problems to be aware of
Cracking from movement
Despite its durability, GRP is a rigid material. If the roof deck moves, flexes, or expands beyond the GRP’s tolerance, cracking can occur. Common causes include inadequate expansion gaps, incorrect boarding thickness, or structural movement in the building.
Pinholes and poor curing
Pinholes are tiny holes in the GRP surface caused by trapped air during installation or insufficient resin coverage. Although small, pinholes can allow water to penetrate the laminate, leading to leaks and delamination over time.
Poor curing is another issue. If the resin does not cure properly—due to cold temperatures, damp conditions, or incorrect mixing—the GRP surface may remain tacky, soft, or prone to failure. For this reason, GRP installation should be carried out in dry, mild weather by experienced installers.
Issues caused by incorrect boarding
Using the wrong decking material or insufficient board thickness is a common installation mistake. Standard chipboard is unsuitable for GRP roofs because it swells when damp. OSB boards are the preferred choice, but they must be the correct grade and thickness (usually 18mm minimum). If boards are too thin or poorly fixed, the deck will flex under load, causing the GRP to crack.
Felt vs GRP Flat Roof Costs in London
Cost is often the deciding factor when choosing between felt and GRP roofing. While felt is cheaper upfront, GRP offers better long-term value due to its extended lifespan and lower maintenance costs.
Felt roof cost per m² in London
Material and labour ranges
In London, felt roofing typically costs between £30 and £50 per square metre, including materials and labour. This range accounts for variations in felt quality, roof complexity, and installer rates.
Basic mineral felt systems with torch-on installation tend to fall at the lower end (£30–£40 per m²). Higher-quality felt membranes with improved warranties may cost £40–£50 per m². Labour costs in London are higher than in other UK regions. Roofers in the capital typically charge between £200 and £300 per day, reflecting the higher cost of living and operating a business in London.
GRP roof cost per m² in London
Material and labour ranges
GRP roofing costs between £65 and £95 per square metre in London, including materials and professional installation. Standard GRP installations on straightforward flat roofs typically cost £65–£75 per m². More complex roofs with multiple upstands, penetrations, or awkward access may reach £80–£95 per m².
For a typical garage or small extension (10–15 m²), a GRP roof might cost between £800 and £1,400. Our flat roofing services cover both felt and GRP installations across London, with transparent pricing and no hidden costs.
Why flat roofing costs are higher in London & the South-East
London and the South-East consistently have the highest construction and labour costs in the UK. Higher labour rates, material delivery costs, compliance challenges, and demand pressures all contribute. On average, roofing costs in London are 20–30% above the UK average. Therefore, homeowners should budget accordingly and obtain multiple quotes from reputable, local contractors.
Lifespan, Maintenance & Repair Comparison
Beyond initial cost, the ongoing maintenance and eventual replacement costs significantly affect the total cost of ownership.
Felt roof maintenance and repair frequency
Felt roofs require regular maintenance to maximise their lifespan. Typical tasks include annual inspections, gutter and drain clearance, blister repairs, seam and edge resealing, and flashing repairs. Even with good maintenance, felt roofs often need repairs every 3–5 years, and full replacement is typically required after 10–15 years.
Guttering, fascias, and soffits must be kept clear to prevent ponding and water damage—this is critical for both felt and GRP roofs.
GRP roof maintenance requirements
GRP roofs are largely maintenance-free. The seamless surface eliminates the seams and joints that cause problems on felt roofs. Recommended care includes annual inspections, gutter and drain clearance, and topcoat renewal every 15–20 years (typically costing £15–£25 per m²). Overall, GRP roofs require minimal intervention during their 25–30 year lifespan.
Long-term cost comparison: felt vs GRP over 25–30 years
Consider a 15 m² flat roof over 30 years:
Felt roof: Initial installation £600 + repairs (£300 × 3) + replacement after 15 years £600 = £2,100 total
GRP roof: Initial installation £1,050 + topcoat renewal after 20 years £300 = £1,350 total
In this example, GRP offers better long-term value despite the higher upfront cost. Additionally, fewer repairs mean less disruption and hassle.
Performance in London’s Climate & Urban Conditions
Ponding water and drainage requirements (BS 6229 minimum fall 1:40)
British Standard BS 6229:2018 specifies that flat roofs should have a minimum fall of 1:40 to promote water drainage. Ponding water is a major cause of premature failure for both felt and GRP roofs.
Felt roofs are particularly vulnerable to ponding. Accumulated water adds weight, stresses the felt membrane, and accelerates degradation. GRP roofs tolerate ponding better due to their seamless, waterproof surface. However, prolonged ponding can still cause problems. Therefore, proper drainage design is essential for all flat roofs, regardless of material.
Impact of heavy rain, frost, and temperature fluctuations
London experiences frequent rainfall, occasional frost, and significant temperature fluctuations throughout the year. Felt roofs expand and contract with temperature changes. Over time, this thermal movement causes the bitumen to crack and lose flexibility.
GRP roofs are more stable but still vulnerable to movement-related cracking if not properly installed. The topcoat provides UV protection, but prolonged exposure to sunlight can eventually cause chalking and colour fading. Overall, both materials perform adequately in London’s climate when installed and maintained correctly.
Pollution, debris, and blocked gutters in London
London’s urban environment introduces additional challenges. Air pollution can settle on flat roofs, accelerating weathering and discolouration. Furthermore, trees, leaves, and debris are common in residential areas. Blocked gutters and downpipes prevent water from draining, leading to ponding and increased risk of leaks. Regular gutter clearance is essential for both felt and GRP roofs.
Which Flat Roof Is Better for Your Property?
Best option for tight budgets and short-term ownership
If upfront cost is the primary concern—for example, if you plan to sell within 5–10 years or need a temporary solution—felt roofing may be the better choice. The lower installation cost (£30–£50 per m²) makes felt accessible for homeowners on tight budgets. Additionally, felt roofs can be installed quickly, making them suitable for urgent replacements. For small structures like sheds or outbuildings, felt offers adequate performance at an affordable price.
Best option for long-term homes and investment properties
For homeowners planning to stay in their property for 15+ years—or for landlords and investors managing rental portfolios—GRP roofing is generally the smarter choice. The 25–30 year lifespan and minimal maintenance requirements deliver superior long-term value.
Moreover, GRP roofs enhance property appeal. Buyers and tenants appreciate modern, low-maintenance roofing systems, and surveyors are less likely to flag concerns during property inspections. Landlords also benefit from reduced callouts and repair costs.
Felt vs GRP for garages, extensions, and dormers
For detached garages and outbuildings, felt is a cost-effective option. However, if the garage is integral to the house or frequently used, GRP offers better durability. Kitchen and living room extensions benefit from GRP’s seamless waterproofing and long lifespan. Dormer roofs are often more complex, with multiple upstands and flashings. GRP’s ability to create seamless, watertight details makes it well-suited for these applications.
Considerations for Victorian and older London properties
Many Victorian terraces and period homes in London have flat-roofed rear extensions or bay roofs. When replacing these roofs, several factors come into play. Some conservation areas have restrictions on materials and appearance—check with your local planning authority before proceeding. Older properties may have weaker roof joists or inadequate decking. GRP requires a stable, properly constructed deck, so structural upgrades may be necessary. Both materials can be used on older properties, but a professional assessment of the existing structure is essential.
Landlords, Compliance & Building Regulations
Part L energy efficiency considerations
Building Regulations Part L sets minimum thermal performance standards for roofs. Both felt, and GRP roofs must incorporate adequate insulation to meet current U-value requirements (typically 0.18 W/m²K or better for domestic properties). The choice between felt and GRP does not significantly affect energy efficiency, as insulation is determined by the overall roof build-up rather than the membrane itself.
Part C: moisture resistance and drainage
Part C of the Building Regulations addresses moisture resistance and drainage. Flat roofs must be designed and constructed to prevent water ingress and ensure effective rainwater disposal. Key requirements include a minimum fall of 1:40 for drainage (BS 6229:2018), adequate vapour control layers, proper integration of flashings, and provision of gutters and downpipes. Both felt and GRP systems can comply with Part C when correctly designed and installed.
Insurance, fire safety, and warranty considerations
Insurance companies and mortgage lenders may have specific requirements for flat roofing materials. GRP roofs often benefit from longer warranties (typically 20–25 years from reputable installers), which can provide reassurance to lenders and buyers. Both felt and GRP can achieve acceptable fire ratings when combined with appropriate decking and insulation materials. Landlords should retain copies of installation certificates, guarantees, and compliance documentation.
Installation Quality Matters More Than Material
No matter which material you choose, the quality of installation is the single most important factor determining roof performance and longevity.
Common installation mistakes with felt roofs
Poor installation is the leading cause of premature felt roof failure. Common mistakes include insufficient overlap, inadequate heating when using torch-on felt, poor upstand detailing, ignoring drainage falls, and using substandard materials. These errors lead to weak bonds, lifted seams, and early leaks.
Common installation mistakes with GRP roofs
GRP installation requires specialist skills and careful attention to detail. Typical errors include incorrect decking, no expansion provision, wet or damp installation, insufficient resin coverage, and poor edge detailing. These mistakes cause cracking, pinholes, and water ingress.
Why using NFRC or SPRA-approved roofers matters
To minimise installation risks, choose roofers who are members of recognised trade bodies such as the National Federation of Roofing Contractors (NFRC) or Single Ply Roofing Association (SPRA). These organisations provide training, quality assurance, and dispute resolution. Working with approved contractors also ensures access to insurance-backed warranties.
Always request references, check reviews, and obtain multiple quotes before committing to any roofing contractor. In London’s competitive market, rogue traders and poorly qualified installers are a genuine risk.
Felt vs GRP Flat Roof – Pros & Cons Comparison
Felt roofing advantages and disadvantages
Advantages: Lower upfront cost (£30–£50 per m²), quick installation, suitable for small or temporary structures, established material with many experienced contractors, and repairable.
Disadvantages: Shorter lifespan (10–15 years), higher maintenance, vulnerable to ponding, seam weaknesses, and weathering/UV degradation, causing cracking.
GRP roofing advantages and disadvantages
Advantages: Longer lifespan (25–30 years), seamless waterproofing, low maintenance, superior water resistance, aesthetic versatility, and strong warranties.
Disadvantages: Higher upfront cost (£65–£95 per m²), risk of cracking due to poor installation, weather-dependent installation, requires specialist skills, and topcoat renewal is needed after 15–20 years.
Quick Verdict—Felt or GRP for London Flat Roofs?
When felt roofing makes sense
Felt roofing is the right choice if your budget is tight, you plan short-term ownership (5–10 years), the roof is on a small or low-priority structure, or you need urgent replacement. Felt can deliver reliable performance when professionally installed and maintained. However, be prepared for ongoing maintenance costs and eventual replacement within 10–15 years.
When GRP roofing is the better long-term choice
GRP roofing is the smarter investment if you plan long-term ownership (15+ years), low maintenance is a priority, the roof covers high-value or habitable spaces, you’re a landlord seeking durability and compliance, or resale value matters. GRP’s higher upfront cost is offset by lower maintenance expenses and a longer lifespan, delivering better value over the roof’s lifetime.
Frequently Asked Questions
Q: Which is cheaper, a felt roof or a GRP roof in London?
A: Felt roofing is cheaper upfront, costing £30–£50 per m² in London, compared to £65–£95 per m² for GRP. However, felt’s shorter lifespan (10–15 years) and higher maintenance costs mean GRP often delivers better long-term value, especially for homeowners planning to stay in their property for 15+ years.
Q: Which lasts longer, felt or GRP flat roofs in the UK?
A: GRP flat roofs last significantly longer than felt roofs. A professionally installed GRP roof typically lasts 25–30 years, while a felt roof lasts 10–15 years. This difference means you may need to replace a felt roof two or even three times during the lifespan of a single GRP roof.
Q: Is GRP roofing worth the extra cost?
A: For long-term homeowners, landlords, and investment properties, GRP roofing is generally worth the extra cost. The extended lifespan, minimal maintenance, and superior waterproofing reduce the total cost of ownership. However, if you’re on a tight budget or plan to sell within a few years, felt may be a more practical choice.
Q: Are felt roofs outdated in the UK?
A: Felt roofs are not outdated. Modern bitumen felt has improved significantly and remains a popular choice for budget-conscious homeowners and small flat-roof structures. However, GRP and other advanced materials are increasingly preferred for their durability and low maintenance, particularly on larger or more important roofs.
Q: Which flat roof is best for a garage or small extension in London?
A: For a detached garage or small outbuilding, felt roofing offers a cost-effective solution. However, if the garage is attached to your home or frequently used, GRP provides better long-term performance and weather protection. For small kitchen or living room extensions, GRP is usually the better choice due to the importance of the space.
Q: Do GRP roofs crack or leak over time?
A: GRP roofs can crack if poorly installed or if the roof deck flexes or moves. Common causes include inadequate expansion provision, incorrect boarding, or structural movement. Pinholes caused by insufficient resin coverage can also lead to leaks. However, when professionally installed with proper substrate preparation and expansion gaps, GRP roofs are highly durable and leak-resistant.
Q: How often does a felt roof need maintenance compared to a GRP roof?
A: Felt roofs require annual inspections and regular maintenance, including gutter clearance, blister repairs, and flashing checks. Minor repairs may be needed every 3–5 years. In contrast, GRP roofs are largely maintenance-free, requiring only periodic gutter clearing and a topcoat renewal every 15–20 years.
Q: Does London’s weather affect GRP and felt roofs differently?
A: Yes. London’s frequent rainfall, temperature fluctuations, and occasional frost can accelerate the degradation of felt roofs. Bitumen becomes brittle with age and UV exposure, leading to cracking and splitting. GRP roofs tolerate wet conditions better due to their seamless, waterproof surface, but they are vulnerable to cracking from thermal movement if not properly installed.
Final Thoughts – Choosing the Right Flat Roof in London
Matching material to budget, property type, and long-term plans
Selecting the right flat roof material is a balance of cost, performance, and future plans. Felt roofing suits tight budgets and short-term needs, while GRP offers superior durability and value for long-term homeowners and landlords. Consider your ownership timeline, the importance of the roofed space, and your tolerance for ongoing maintenance when making your decision.
Importance of professional inspection and correct installation
Regardless of material, professional installation is critical. Poor workmanship causes premature failure, leaks, and costly repairs. Always choose experienced, reputable roofers who are members of recognised trade bodies like the NFRC or SPRA. Request references, check reviews, and obtain detailed written quotes before committing.
When to seek expert advice before replacing a flat roof
If you’re unsure which material suits your property, or if a surveyor has flagged concerns about your existing roof, seek professional advice. At Smart Roofing London, we provide honest guidance and free inspections to help you make the right choice. Whether you need a new roof installation or advice on flat roofing options, we’re here to help London homeowners and landlords make informed decisions about their properties.
Need Expert Help Choosing Between Felt and GRP Roofing?
Choosing the right flat roof material for your London property doesn’t have to be complicated. At Smart Roofing London, we’ve helped hundreds of homeowners, landlords and property buyers across the capital make informed decisions about felt and GRP roofing.
Whether you’re dealing with a leaking extension roof, planning a new garage installation, or need honest advice after receiving a surveyor’s report, our experienced team is here to help.
We offer:
- Free, no-obligation roof inspections across London
- Transparent pricing with detailed written quotes
- Expert installation of both felt and GRP flat roofs
- Honest recommendations based on your budget and property needs
- Fully insured workmanship with comprehensive guarantees
Contact us for immediate advice or to book your free inspection.
