GRP vs Felt vs Single Ply: Which Suits a Commercial Flat Roof?

How is each system laid?

Single ply is a single factory-made sheet laid in wide rolls. The Single Ply Roofing Association (SPRA) groups the common materials into thermoplastics such as PVC and flexible polyolefin, which soften with heat and are welded, and elastomers such as EPDM, which are usually joined with tape and a special primer or with adhesive. Because there is only one layer, SPRA stresses that the quality and durability of the seams formed on site are critical. The sheet is held down by one of three principal methods, mechanical fastening, adhesion or ballast, chosen only after the wind uplift forces have been calculated. The differences between the single ply materials are covered in the EPDM, TPO and PVC comparison.

Reinforced bitumen membranes (RBM), still widely called felt, are built up from layers of reinforced bitumen sheet. BS 8217, the code of practice for reinforced bitumen membrane roofing, covers combining the membranes into a roofing system and also covers mechanically fastened single-layer, torch-applied and self-adhesive membranes. Its recommendations apply to overlaying existing roofs too, subject to confirmation that the existing construction is suitable.

GRP (glass reinforced plastic) is a liquid-applied system. The LRWA explains that these systems, often referred to as GRP, glass fibre or fibreglass, consist of a glass fibre reinforced polyester resin cold applied on site, and that the resins must be specifically formulated for roofing to combine wear resistance, flexibility, elasticity and fire performance. Preformed GRP trims are used to finish edges and details. The LRWA design guide adds that reinforcement such as glass cloth, chopped glass mat or polyester fleece is wetted in between successive coats, and that liquid kits are assessed against the European guideline ETAG 005. Polyurethane and methacrylate (PMMA) systems work on similar principles with different resins. How these are specified is covered on the liquid-applied and GRP roofing page.

Feature	Single ply	Reinforced bitumen (felt)	GRP and other liquids
Layers	One sheet, with seams formed on site	Usually several bonded layers; single-layer systems exist	Continuous coats with reinforcement wetted in
Joints	Hot-air welded, taped or adhered	Torch-applied, self-adhesive or mechanically fastened	No laps once cured
Main guidance	BS 6229 and SPRA guidance	BS 8217 and BS 6229	BS 6229, LRWA guidance and ETAG 005 assessment
Typical fit	Large, simple roofs	Robust roofs with many upstands and penetrations	Small, detailed or trafficked areas; occupied buildings

Which standards and certificates apply?

BS 6229 is the code of practice that sits over all three. BSI describes the 2025 edition as a national code of practice for the design, construction, care and maintenance of flat roofs with continuously supported flexible waterproof coverings, and it supersedes BS 6229:2018. BS 8217:2005 adds the detail for reinforced bitumen membranes.

Standards describe good practice; certificates speak for individual products. The BBA explains that an Agrément certificate relates to a specific product and a specific organisation, certified against a set of criteria for a particular use. Certificates have no fixed expiry date but stay valid only while the holder completes six-monthly factory surveillance visits and a check of the certificate every three years. SPRA requires its membrane manufacturers to have independent certification of all their membrane products, and the LRWA makes ETA certification and/or national certification, such as a BBA or KIWA BDA certificate, a condition of membership for liquid products. Whichever system is proposed, ask for the certificate number and check that the product, the use and the build-up on the certificate match the roof.

How long does each system last?

Durability, service life and guarantee term are three different things. SPRA's guidance for clients says a guarantee bears little relation to durability or service life, and is instead a manufacturer's acceptance of financial risk. With that caveat:

• Single ply: SPRA's design guide says the durability of single ply membranes supplied by its manufacturer members is typically in excess of 30 years.
• GRP and other liquids: the European guideline quoted by the LRWA sets a default estimated working life of 10 years, which can be modified to 5 or 25 years, and the LRWA adds that systems for 15, 20 and possibly more than 30 years may be offered. A working life of only 5 years is assumed for products intended purely as a repair, renovation or maintenance medium, and the guideline states that a working life category cannot be read as a guarantee.
• Reinforced bitumen: durability is stated product by product on the system's agrément certificate, so ask for that certificate rather than relying on a general claim.

Every one of these figures assumes the roof was designed, installed and maintained correctly. A roof that ponds, takes uncontrolled foot traffic or is repaired with the wrong materials will fall short whichever system it uses.

How do they cope with details, foot traffic and plant?

Detailing is where the systems differ most in practice. A liquid membrane follows the substrate and has no laps once cured, so it handles roofs crowded with pipes, plinths, rooflight kerbs and awkward junctions well; the LRWA notes that the seamless nature of liquid systems makes them a popular choice where existing plant arrangements are complex. Single ply relies on details welded or bonded to the field sheet, and multi-layer bitumen dresses each detail in several bonded layers.

Foot traffic needs planning for all three. SPRA says all materials developed for single ply roofs can withstand occasional, light foot traffic for inspection, and that where walkways serve roof-top equipment the obvious direct route should be protected, kept clear of areas prone to temporary ponding and given a slip-resistant surface. It also warns that excessive foot and equipment traffic can compress the insulation beneath beyond its design levels. The LRWA notes that different grades of liquid waterproofing can be specified according to the level of foot traffic expected on different parts of the roof.

What about fire and hot works?

The fire performance of the finished roof is a separate question, covered in the guide to BROOF(t4) fire classification. The installation method matters as well. Torch-applied bitumen uses a gas flame, and the NFRC's Safe2Torch guidance says that where a flammable substrate or risk of fire occurs, or the risk is not known, an alternative should be specified from the outset, which may apply only to the risk area. BS 8217 also covers self-adhesive and mechanically fastened bitumen membranes, thermoplastic single ply seams are welded with hot air rather than a flame, and the LRWA notes that most liquid systems are cold-applied, eliminating fire risk on the roofs of occupied buildings.

What maintenance does each need?

None of the three needs much routine work, but all need looking at. SPRA's design guide says routine maintenance of a single ply membrane is not normally required, but the roof should be inspected twice a year, preferably in early spring and late autumn, partly to check that materials left by other trades have been removed. The LRWA says liquid systems need no routine maintenance in normal use, recommends periodic inspections for accidental impact damage or damage from building alterations, and notes that maintenance terms may be written into the guarantee.

Repairs differ by system. Liquid membranes are repaired by cleaning, priming if necessary and applying more material to restore the original dry film thickness. SPRA warns that single ply membranes should not be painted or sprayed and that repairs should only use compatible materials and the manufacturer's procedures. Bitumen roofs are patched with compatible membrane, which raises the hot works question again if a torch is proposed.

Which system suits which building?

• Distribution sheds, retail warehouses and factories with broad, simple roofs: single ply is usually the most efficient, with the attachment method set by the wind load calculation.
• Schools, offices and public buildings with many upstands, rooflights and services: multi-layer reinforced bitumen offers layered robustness, with self-adhesive or mechanically fastened options where a torch would be a risk.
• Plant decks, walkways, balconies, gutters and small roofs crowded with penetrations: GRP or another liquid system, because it forms details without laps and can be specified in grades for foot traffic.
• Occupied buildings where naked flames are unwelcome: cold-applied liquids, self-adhesive bitumen or welded single ply.
• Re-covering an existing roof: any of the three may work if the existing roof is dry, sound and compatible, which only a survey can confirm. The options are set out under commercial flat roof refurbishment.

The right answer for a specific roof comes from its deck, its condition and how it is used. When proposals arrive, compare them against the points above and ask each contractor to explain why their system suits this roof. You can request quotes from commercial roofing contractors through the site's form, or read more about refurbishment routes first.

Sources

• BSI, BS 6229:2025 Flat roofs with continuously supported flexible waterproof coverings and BS 8217:2005 Reinforced bitumen membranes for roofing. Checked 20 October 2026.
• Single Ply Roofing Association: S01 Design guide 2020; S04 Guarantees; S13 Protection of single ply roofs; S14 Securing seams. Checked 20 October 2026.
• Liquid Roofing and Waterproofing Association, Design guide for specifiers 2020 and Generic types of liquid applied membranes. Checked 20 October 2026.
• British Board of Agrément, Frequently asked questions. Checked 20 October 2026.
• National Federation of Roofing Contractors, Safe2Torch guidance. Checked 20 October 2026.

Common questions

Is GRP better than felt for a commercial flat roof?

Neither is better in every case. GRP forms a seamless, cold-applied membrane that suits small or detailed areas and trafficked surfaces, while multi-layer reinforced bitumen gives a layered covering suited to robust roofs with many details. Compare the specific products' certificates, the detailing and the installation method for the roof in question.

Can single ply be laid over an old felt roof?

Sometimes, if the existing roof is dry, sound and compatible. SPRA notes that PVC membranes are generally not suitable for direct contact with bitumen unless a suitable fleece backing separates the two, while FPO and EPDM membranes are generally compatible with bitumen. A survey should confirm the existing build-up first.

Do these roofing systems need a BBA certificate?

The trade bodies expect third-party certification. SPRA requires independent certification of its manufacturers' membrane products, and the LRWA requires ETA and/or national certification, such as a BBA or KIWA BDA certificate, for its members' liquid products. Ask for the certificate number and check that it covers the intended use and build-up.

Which system is quickest to repair?

Liquid systems are usually the simplest to patch, because more of the same material is applied over the cleaned and primed damage to restore its original thickness. Single ply repairs must use compatible materials and the manufacturer's procedures, and torch-on bitumen repairs need the fire risk assessed first. A quick repair only lasts if the cause, such as ponding or a lifted edge, is dealt with too.