Materials and systems
Felt, EPDM or GRP? Comparing a complete flat roof proposal
Homeowner advice from Big Ben Roofing Ltd
Choosing a flat roof by the name of its waterproofing alone leaves important questions unanswered. The existing deck, rainwater route, insulation and junctions all need to work with the proposed system. Start with the roof you have and the result you need, then compare suitable specifications.
Felt, EPDM and GRP are different approaches rather than a simple league table. This guide sets out the language you may see in a quotation and the questions that help you assess a complete proposal.
What the material names mean
Reinforced bituminous felt is commonly specified as a layered waterproofing system. EPDM is a synthetic rubber membrane supplied in sheets, with system details for its edges and penetrations. GRP uses glass reinforcement with resin to form the waterproofing. These are broad descriptions: the manufacturer, product range and installation specification still matter.
The NFRC homeowner guide describes these systems alongside other flat roof options. Use it to understand terminology, then ask which exact system is proposed for your property. A familiar material name is not, on its own, evidence of suitability or of the installer’s training.
Ask what sits underneath the waterproofing
A new surface is only part of a renewal proposal. Ask whether the existing deck has been assessed, whether it is to be retained and what the quote assumes about concealed condition. If an overlay is offered, ask how the contractor will determine whether the existing build-up is suitable to remain.
Insulation and moisture control should be part of the design discussion where relevant. Ask for a clear description of the proposed roof build-up and who is responsible for its design. Do not assume that adding insulation at any convenient level produces a suitable assembly.
Compare the edges and water route
Request a proposal that names the outlets, perimeter edges, wall upstands, rooflight junctions and any penetrations. Ask what will be renewed at each location and what interfaces with neighbouring work. If standing water or overflow is part of the existing complaint, ask how the proposal addresses the cause rather than only replacing the surface.
- How will rainwater reach the agreed outlet?
- Which adjoining details are included in the system specification?
- Is access needed to neighbouring land or a shared rainwater route?
Read the installation and handover terms
Ask what installation competence the contractor can demonstrate for the specified product. Discuss access, weather constraints and protection while the roof is open. If a guarantee is offered, ask who issues it, what it covers, the exclusions and any maintenance requirements. Product cover and workmanship cover may have different terms.
For a substantial renewal, establish Building Regulations requirements before agreeing the work. Planning Portal explains that roof changes can involve thermal upgrades. At handover, retain the final specification, photographs, relevant approvals and the written maintenance information for the installed system.
A useful flat roof comparison sheet
- Exact waterproofing manufacturer and system, with the proposed roof build-up.
- Deck inspection, retained layers and assumptions about concealed condition.
- Drainage, edges, upstands and penetrations included in the quotation.
- Design responsibility, installation competence and written handover terms.
Further reading
These independent sources provide background. Requirements and suitable work depend on the particular building and proposal.
- NFRC: homeowner guide to flat roof types
Introduces common flat roof systems and questions about installer competence and guarantees.
- Planning Portal: insulation and thermal elements
Explains why substantial changes to a roof can involve thermal upgrade requirements.
Continue reading
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Discuss the concern at your property
Send your postcode, the affected roof area and what you have noticed. Include photographs you already have from a safe position so the team can review the next step.
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