Choosing the right material for a flat roof is one of the most important decisions in the whole project. The system you pick affects how long the roof lasts, how it handles the weather, how it is maintained, and how it performs as part of your building’s thermal envelope.
This guide covers the flat roofing systems available in the UK today – what each one is, how it works, where it performs best, and what to watch out for. We have focused on the systems we install and work with regularly at Cornerstone Roofing, alongside other options you may come across.
GRP (Fibreglass) Roofing
GRP – glass-reinforced polyester – is the system we install most frequently on domestic flat roofs in Mid Sussex, and for good reason.
A GRP roof is built up on site by laying a fibreglass mat over the prepared deck and saturating it with a catalysed polyester resin. The resin cures to form a single, rigid, seamless shell. There are no joints, laps, or seams anywhere on the finished surface – and since most flat roof failures originate at joints, this is a significant advantage.
Where GRP works best
- Extensions, dormers, porches, and balconies
- Garages and outbuildings
- Bay tops and canopy roofs
- Any domestic flat roof where long-term durability matters
What to know
- Lifespan: 25 to 30 years with minimal maintenance
- Finish: Available in a range of colours. The surface is smooth, hard-wearing, and handles occasional foot traffic
- UV resistance: Good. Modern topcoats include UV stabilisers
- Seamless: No joints or laps to fail
- Weather sensitivity during installation: GRP resin will not cure properly in wet or very cold conditions. Installation needs to be scheduled around the forecast
- Rigid: Unlike flexible membranes, GRP does not stretch. On very large roof areas or buildings with significant thermal movement, this can be a consideration – though for the vast majority of domestic roofs it is not an issue
GRP has become the go-to system for domestic flat roofing in the UK for good reason. When installed correctly on a properly prepared deck, it is exceptionally reliable.
EPDM (Rubber Roofing)
EPDM – ethylene propylene diene monomer – is a synthetic rubber membrane that has been used on flat roofs worldwide for over 50 years. It is supplied in large sheets and bonded to the roof deck with adhesive.
Where EPDM works best
- Garages and outbuildings
- Smaller domestic roofs and extensions
- Roofs with simple shapes and few penetrations
- Situations where the roof needs to accommodate some structural movement
What to know
- Lifespan: 25 to 40 years
- Flexibility: EPDM remains flexible across a wide temperature range, making it resistant to cracking in cold weather and tolerant of building movement
- UV resistance: Excellent. The rubber does not degrade under UV exposure the way some materials do
- Colour: Typically black, which absorbs heat. Lighter colours are available but less common
- Seams: Large sheets can cover smaller roofs in one piece. Larger roofs require multiple sheets joined with specialist tape or adhesive – these seams are a potential weak point if not done properly
- Puncture resistance: Moderate. EPDM can be punctured by sharp objects, so care is needed if the roof is walked on or if debris falls on it
EPDM is a proven, reliable system. It is particularly well-suited to simpler roof shapes where the membrane can be laid in minimal pieces.
Liquid Applied Membrane
Liquid roofing involves applying a cold-applied liquid coating directly onto the roof surface. The liquid cures to form a fully bonded, seamless, waterproof layer that conforms to every contour, upstand, and detail on the roof.
Where liquid membrane works best
- Complex roof shapes with multiple penetrations, pipes, vents, or skylights
- Roofs with awkward angles or unusual detailing where sheet membranes would need many cuts and joints
- Overlay applications where the existing roof surface is sound enough to be coated over rather than stripped
- Balconies and walkways (with anti-slip aggregate added)
What to know
- Lifespan: 20 to 30 years depending on the specific product and number of coats
- Seamless: Like GRP, the finished surface has no joints – the liquid flows around every detail and cures as one continuous layer
- Versatility: Handles complex geometry better than any other system
- Application: Applied by roller or spray in multiple coats with a reinforcement fleece at critical points. Each coat needs to cure before the next
- Weather sensitivity: Requires dry conditions during application. Cannot be applied in rain or freezing temperatures
- Recoatable: Most liquid systems can be recoated after 15 to 20 years, extending the roof’s life without a full strip and re-cover
Liquid membrane is an excellent choice where the roof shape makes sheet-based systems difficult, or where an overlay is more practical than a full strip.
Single Ply Membrane (PVC and TPO)
Single ply roofing uses large, flexible, factory-manufactured sheets – typically PVC (polyvinyl chloride) or TPO (thermoplastic polyolefin). The sheets are heat-welded at the seams using a hot-air gun to create a continuous waterproof layer.
Where single ply works best
- Larger residential flat roofs
- Commercial and industrial buildings
- Roofs where speed of installation matters (large sheet sizes mean fast coverage)
- Buildings requiring high chemical or UV resistance
What to know
- Lifespan: 25 to 35 years
- Chemical resistance: PVC in particular handles chemical exposure well, making it common on commercial buildings near industrial activity
- UV resistance: Good. Light-coloured membranes reflect heat and resist UV degradation
- Weight: Very light – useful where structural loading is a concern
- Seams: Heat-welded seams are strong when done correctly, but the quality of welding is critical. Poorly welded seams are the primary failure point
- Installation methods: Can be mechanically fixed (screwed through to the deck), fully adhered (bonded with adhesive), or ballasted (held down with gravel or paving)
Single ply is widely used on larger and commercial roofs. On smaller domestic properties it is less common than GRP or EPDM, though it remains a perfectly viable option.
Built-Up Felt Roofing
The traditional multi-layer bitumen felt system has been used on UK flat roofs for decades. Modern torch-on felt is a considerable improvement over older methods – layers of modified bitumen felt are heated with a gas torch and bonded together to build up a robust, waterproof covering.
Where felt works best
- Budget-conscious projects where upfront cost is the priority
- Smaller roofs where the shorter lifespan is acceptable
- Like-for-like replacements on older properties where felt is already in place
What to know
- Lifespan: 15 to 25 years with regular maintenance. High-performance modified bitumen felts can last longer
- Maintenance: Requires more regular attention than GRP or EPDM. The surface can dry out and crack with UV exposure over time, particularly on south-facing roofs
- Layers: Built from multiple layers, giving some redundancy – a puncture in the top layer does not necessarily mean an immediate leak
- Fire: Torch-on installation involves open flame, which requires appropriate fire precautions on site
Felt roofing is a proven system that still has a place, particularly where budget is the overriding factor. However, for most domestic flat roofs we would typically recommend GRP or EPDM for the significantly longer lifespan and lower ongoing maintenance.
Green Roofs
A green roof – also called a living roof – adds a layer of growing medium and vegetation on top of a waterproof membrane. The most common type for domestic properties is an extensive green roof using sedum, which requires minimal maintenance and relatively little additional structural support.
Where green roofs work best
- Extensions and garden rooms where the roof is visible from upstairs windows or neighbouring properties
- Properties in conservation areas or sensitive locations where planners may favour a green finish
- Eco-builds or projects targeting high environmental performance
- Any flat roof where the visual appearance of the finished roof matters
What to know
- Lifespan: The waterproof membrane underneath has its own lifespan (typically 25+ years). The vegetation layer, if maintained, lasts indefinitely
- Weight: Significantly heavier than other flat roof systems when saturated. The structural deck and joists must be designed or verified to handle the additional load
- Drainage: A proper drainage layer beneath the growing medium is essential. Without it, the roof retains too much water and the membrane is under constant hydrostatic pressure
- Insulation: The vegetation and soil layer add measurable thermal insulation, though not enough to replace dedicated insulation boards beneath
- Biodiversity: Sedum roofs attract pollinators and provide habitat in urban and suburban areas
- Maintenance: Minimal for sedum – an annual check and weed removal. Intensive green roofs (with deeper soil and larger plants) require regular maintenance similar to a garden
Green roofs are increasingly popular, particularly on extensions where the homeowner wants something more attractive than a black membrane when viewed from the bedroom window.
Other Materials You May Encounter
Modified bitumen (APP and SBS): These are enhanced versions of traditional bitumen felt – APP (atactic polypropylene) modified bitumen is torch-applied and performs well in heat, while SBS (styrene-butadiene-styrene) modified bitumen is more flexible in cold temperatures. Both are used more widely in commercial roofing.
Metal standing seam: Zinc, copper, or aluminium panels with raised interlocking seams. Extremely long-lasting (50+ years for zinc, 80+ for copper) but significantly more expensive. Occasionally used on high-end domestic projects or heritage buildings.
Mastic asphalt: A traditional poured-hot system that creates a dense, seamless covering. Once the standard for flat roofs on larger buildings, it has largely been replaced by cold-applied systems but is still used on heritage and conservation projects.
Which Material Is Right for Your Roof?
The right choice depends on your specific situation:
- For most domestic flat roofs (extensions, garages, dormers): GRP or EPDM will give you the best combination of longevity, reliability, and low maintenance
- For complex roof shapes with lots of penetrations and awkward detailing: liquid applied membrane
- For larger or commercial roofs: single ply membrane
- For budget-sensitive projects: built-up felt, accepting the shorter lifespan
- For visible roofs where appearance matters: green roof
Your roofer should recommend a system based on your roof’s specific characteristics – size, shape, access, structural capacity, and how the roof connects to the existing building. If they only offer one system regardless of the situation, that is a sign to get a second opinion.
For a detailed walkthrough of what happens during installation – regardless of which material you choose – see our step-by-step flat roof installation guide.
Whichever material suits your project, our flat roof installation and replacement service covers the full job across Mid Sussex. If your existing flat roof is leaking rather than due for replacement, see our guide to flat roof repair in Sussex.
If you are unsure which system suits your property, get in touch or book a free visit and we will assess your roof and recommend the right approach.