AI concept illustration of roofing material samples. The samples show general textures and do not identify a particular product or complete roof specification.
Felt, GRP, PMMA and EPDM can each form part of an effective flat roof. The right choice depends on the roof beneath the waterproofing, its shape, the details to be sealed and how the work can be carried out. A material name alone tells you very little about the quality of the completed roof.
For an extension, dormer or summer house, ask for a complete roof specification. It should explain the structure, insulation, moisture control, waterproofing, drainage and interfaces with the rest of the building.
Choose the roof build up before the covering
Warm, cold and inverted describe the position of insulation relative to the roof structure and waterproofing. They are not alternative names for felt, rubber or resin. A compatible waterproofing product must be selected for the chosen construction. [1]
- Warm roof, insulation is above the structural deck and below the waterproofing, with the specified air and vapour control arrangement beneath it.
- Cold roof, insulation is generally below the deck. Condensation control and any required ventilation paths need particularly careful design and execution.
- Inverted roof, insulation is above the waterproofing. The insulation, protective layers, drainage and loading must suit that arrangement.
Adding insulation to an existing roof changes how heat and moisture move through it. Ask whether the proposal needs a condensation-risk assessment, especially where retained insulation creates a mixed build-up. A new waterproof surface cannot correct an unsuitable moisture strategy underneath it. [1]
Find out what condition the existing roof is in
Before deciding between repair, overlay and replacement, establish the cause of the problem. Water marks may be associated with leaks, condensation or an adjoining detail. The assessment should consider the deck, insulation, existing coverings, trapped moisture and structural condition. An overlay needs a sound, suitable underlying construction and confirmed compatibility; it should not conceal wet insulation or decayed decking. [2]
For older roofs, identify potentially asbestos-containing materials before intrusive surveys or strip-out. Any necessary asbestos investigation should be carried out by competent people and matched to the planned work. Homeowners should not disturb suspect materials to investigate the roof themselves. [3]
Reinforced bituminous membranes commonly called felt
Modern built-up felt roofing uses specified reinforced bituminous membranes, typically combining underlayers and a capsheet. The bitumen may be modified to provide the properties required by the system. It should be described by its products, layers and installation method; ‘felt roof' alone is too vague for a quotation. [4]
Torch-applied, self-adhesive and hybrid arrangements are available, depending on the specification. That makes early discussion of the site important, particularly near combustible details or occupied spaces. Layer continuity, laps, perimeter details and outlets all need to follow the selected system. Do not compare a complete residential roof system with a basic shed covering simply because both are called felt. [4]
GRP fibreglass roofing
Conventional GRP roofing forms a glass-reinforced polyester resin laminate over a specified substrate, with suitable trims and finishing coats. The Cure It installation guide, for example, sets out a system of resin, reinforcement, an OSB deck, edge trims and topcoat. Those components work together; substituting the deck or resin without approval can change the result. [5]
GRP can give a continuous finish, but it still needs properly formed edges, joints and connections. The deck's support and movement characteristics matter. Some products require a new prepared deck, while others have separate overlay specifications: Cure It's current range illustrates that distinction. Check the exact product rather than assuming every GRP roof has the same requirements. [6]
PMMA liquid applied waterproofing
PMMA is polymethyl methacrylate. Roofing systems using this resin chemistry are applied as liquids and often incorporate fleece reinforcement. They can suit complicated details around outlets, rooflights and other penetrations when the substrate and preparation are approved for the system. Triflex ProTect is one example of a reinforced, cold-applied PMMA system with rapid-curing characteristics. [7]
Rapid curing can help a carefully planned installation, but it is not permission to ignore substrate moisture, weather limits, primers or the specified application process. Ask about the actual product's conditions and the management of odour and chemical exposure near occupants or air intakes. Different liquid products have different properties; ‘liquid roof' is not a complete specification. [7]
EPDM rubber roofing
EPDM is a synthetic rubber membrane supplied in sheets. Larger sheets can reduce the number of field joints on a straightforward domestic roof. The membrane is flexible, and suitable systems can be installed without a torch. Elevate's RubberCover is one residential example; its guidance also requires a suitable underlying structure and substrate. [8]
The critical details still include perimeter restraint, corners, outlets, penetrations and any joints. Specify the membrane together with its approved adhesives, accessories and protection. A claim about one manufacturer's product should not be applied to every EPDM sheet or every installation. Consider how later roof access or work by another trade will be managed. [8]
A practical comparison
- Reinforced bituminous felt: A specified layered system with several application options. Question before selection: Which layers and details are included, and where are hot works avoided?
- GRP: A resin-and-glass laminate with a continuous finish. Question before selection: Is the deck suitable, and how are movement and edges dealt with?
- PMMA: A liquid system that can suit intricate detailing. Question before selection: Are the substrate, primer, reinforcement and curing conditions confirmed?
- EPDM: Flexible sheets that may reduce field joints on a simple roof. Question before selection: How are edges, outlets, joints and subsequent access protected?
PVC and TPO/FPO thermoplastic single-ply membranes are further options. These differ from EPDM and commonly use heat-welded seams. Manufacturers provide specific mechanically fixed, adhered or other approved system arrangements. If proposed, compare their complete specification and the installer's experience with that particular product. [9]
Drainage and details often decide the outcome
A flat roof needs a designed route for rainwater. Ask how falls, structural deflection, construction tolerances, outlets and any necessary overflow provision have been coordinated. The current BS 6229:2025 is the code of practice for continuously supported flexible waterproof coverings, superseding the 2018 edition. [10]
Bauder's explanation of the 2025 revision stresses designed and verified falls rather than relying on the old generic design-fall convention. The appropriate proposal depends on the roof structure, survey information and system. A product's resistance to standing water does not remove the need for a sound drainage design. [11]
Look closely at rooflights, doors, parapets, abutments and drainage outlets. Ask the designer to show the finished levels and connections. Also identify any future solar panels, plant or green roof at the design stage, so their loads, fixings, protection and maintenance access can be coordinated rather than added as an afterthought.
Understand fire performance and installation conditions
Ask for evidence of the required external fire performance of the actual roof assembly, such as BROOF(t4) where applicable. The deck, insulation, membrane and installation arrangement matter. IKO explicitly qualifies its stated classification by reference to the tested build-up. A rating for an individual material does not automatically establish the performance of your proposed roof. [4]
Installation fire risk is a separate consideration. NFRC's Safe2Torch guidance promotes identifying torch-related risks and using appropriate alternatives at the design and installation stages. Discuss the proposed approach early. Cold application removes a torch from that process, but the contractor must still manage the product's other hazards and safe access to the roof. [12]
Resins, adhesives and self-adhesive materials have product-specific limits for temperature, moisture and application. Ask how the programme allows for unsuitable conditions and temporary protection. A quoted curing time only has meaning under the conditions stated for that product; there is no single reliable weather rule for every flat roof system. [5] [7]
Compare the specification and the guarantee
Request a like-for-like quotation that includes any strip-out, deck repairs, insulation, vapour control, edge details, rooflights, drainage, access and making good. Establish which conditions have been surveyed and which remain allowances. Roof replacement can also trigger Building Regulations requirements; confirm the applicable approval and upgrade requirements before tender. [13]
A guarantee period is different from an expected service life. Ask who issues it, what parts and workmanship it covers, whether it is backed by insurance, and what exclusions, maintenance or transfer conditions apply. Polyroof's specification guide recommends examining the scope of cover and whether its value reduces over time. Obtain the actual wording rather than relying on a headline number. [2]
What to expect at handover
Agree the inspection and testing approach before installation. Request records of concealed details, the installed product specification, relevant certificates, guarantee documentation and a clear maintenance plan. Testing should suit the roof and manufacturer's instructions. Arrange competent inspections, clear drainage and prompt investigation of damage using safe access; do not treat any roof as maintenance-free.
Marwells can review your roof requirements alongside the wider extension, loft, summer house or refurbishment project. We consider the proposed build-up, access, details and agreed scope before quoting. Send your drawings or project information to info@marwells.co.uk, or call 07340 094028 to discuss the next step.
Related: Residential construction · House extensions · Loft conversions · Get a quote
Sources and further reading
- Bauder: Flat roof design considerations
- Polyroof: Specifying a cold liquid-applied roofing system for refurbishment
- HSE: Arrange an asbestos survey
- IKO: ULTRA Life 30 system technical information
- Cure It: GRP installation guide, manufacturer document hosted by GRP UK
- Cure It: Current installation guides and product variants
- Triflex: ProTect PMMA waterproofing system
- Elevate: RubberCover EPDM residential roofing
- Bauder: Single-ply roofing systems
- BSI: BS 6229 current release and historical versions
- Bauder: BS 6229:2025 key changes, May 2026
- NFRC: Safe2Torch
- GOV.UK: Building regulations approval
