Types of flat roof for commercial buildings
Updated 23 October 2026 · SEO Dons Editorial
What are the main types of commercial flat roof covering?
The government's list of minimum technical competences for roofing separates the work into hard and soft metal roofing, mastic asphalt, reinforced bitumen membranes, single ply and liquid waterproofing systems. Those are the families of covering used on UK commercial roofs, and green and blue roofs are built on top of them. BS 6229:2025 is the code of practice for flat roofs, meaning pitches up to 10 degrees, with continuously supported flexible waterproof coverings, and it covers both heavy supports such as concrete slabs and lightweight metal or timber-based decks. The table compares the families, and the flat roof systems hub has a page on each.
| Covering | How it is laid | Details and foot traffic | Fire and hot works | Read next |
|---|---|---|---|---|
| Single ply (PVC, polyolefin, EPDM) | Factory-made sheets joined on site. SPRA describes polyolefin membranes as heat-weldable, and EPDM is jointed with tape and primer or with adhesive. Held down by mechanical fastening, adhesion or ballast. | SPRA says all single ply materials can take occasional light foot traffic for inspection. A walkway to plant should be protected. | Laps are welded or taped, so there is no torch-on step. Fire class is a separate check on the whole roof construction. | Single ply, EPDM vs TPO vs PVC |
| Reinforced bitumen membrane (felt) | Reinforced bitumen sheet in one or more layers. BS 8217 covers torch-applied, self-adhesive and mechanically fastened single-layer membranes. | BS 8217 also applies to overlaying an existing roof, where the existing construction is suitable. | Torch-on is hot work. NFRC says to specify a torch-free alternative where a flammable substrate exists or the risk is not known. | Built-up felt, torch-on vs cold-applied |
| Mastic asphalt | Applied hot: heated until fluid enough to be spread by hand float or by machine. | BS 8218 covers flat and sloping roofs, including trafficked roofs such as rooftop car parks. | Hot-applied, so ask for the contractor's hot-works method statement. | Mastic asphalt guide |
| Liquid-applied (GRP, PMMA, polyurethane, hot melt) | Mostly cold-applied on site. GRP is glass fibre in polyester resin, PMMA is a two-component product applied by spray or roller, and hot melt is the hot-applied exception: polymer-modified bitumen in two layers. | LRWA describes liquid systems as totally seamless and a popular choice where plant arrangements are complex. | LRWA says most systems are cold-applied, eliminating fire risk on the roofs of occupied buildings. | Liquid-applied and GRP, PMMA vs PU vs GRP |
| Green and blue roofs | Build-ups laid over a waterproofing layer, not a separate family of covering. | SPRA says waterproofing under a green roof should be independently certified and root-resistant. For a blue roof it gives a maximum stored head of 100 mm, draining within 24 hours. | Classified as a complete roof construction, as for any other roof. | Green and blue roofs |
| Metal (profiled, secret-fix, standing seam) | Sheets fixed to purlins, not continuously supported. MCRMA's over-roofing guide calls for secret-fix or standing-seam sheets below 4 degrees. | Standing seam has raised interlocking seams joining adjacent panels. | Through-fixed, or fixed over concealed clips or halters, so there is no torch step. | Over-sheeting, industrial flat roofing |
Standards sit underneath each family. BS EN 13707 sets definitions and characteristics for reinforced bitumen sheets used as top layers, intermediate layers and underlayers, and BS EN 13956 does the same for plastic and rubber sheets for roof waterproofing.
How is a covering different from a build-up and a deck?
Three separate choices are often blurred. The deck is the structure: concrete, metal, timber or something older, covered in the roof deck guide. The build-up is the order of the layers. On a warm roof, SPRA says, the principal insulation sits immediately below the covering, which keeps the deck close to the temperature inside. On an inverted roof it sits above the covering. The warm deck versus cold deck guide covers those options. The covering is the waterproof layer in the table above.
Most coverings can sit on more than one build-up, so a quotation that names only a membrane says little. It should state the deck, the build-up and the covering together.
What is the cheapest option for a flat roof?
No covering is cheapest in every case, and this guide quotes no prices. What moves the price is the roof rather than the product name: the area, the state of the deck, whether the falls need correcting, the number of penetrations and upstands, the access, the controls on hot works and the insulation required. The GRP, felt and single ply comparison sets three of the families side by side.
Which is better, EPDM or TPO?
Neither is better in general, because they belong to different material families. SPRA's design guide treats polyolefin membranes as heat-weldable, while EPDM is jointed with tape bonding using a special primer, or with adhesives. The seam method, the wind-uplift zone, the details and the manufacturer's system decide the choice. The single ply membrane comparison goes through it.
What should decide which covering suits a building?
- Plant and penetrations. Laps are joints to detail and inspect. Liquid systems have none once cured.
- Foot traffic. Occasional inspection traffic is one thing and regular access to plant is another. Trafficked roofs are within BS 8218, and a single ply roof needs a protected walkway.
- Hot works. Building use or an insurer may rule out naked flame, which favours cold-applied or welded systems.
- Structure. Ballast, green and blue build-ups add load, so a structural check comes first.
How can you tell which covering a roof already has?
You cannot reliably tell from the ground. LRWA's specifiers' guide lists common misconceptions, and one is that everything black is asphalt: it can also be bituminous felt or a polymeric single ply membrane. The same guide says core sampling is often the most effective method of establishing the construction, with thermal imaging and moisture mapping used where appropriate. The roof survey guide explains the methods.
It matters because materials do not always mix. SPRA says PVC membranes are generally not suitable for direct contact with bitumen unless a suitable weight of fleece backing separates the two. An overlay or strip-and-recover decision starts with the existing covering; see the overlay versus strip-and-recover guide.
To compare contractors on any of these coverings, send an enquiry through the form on this site, and use the systems hub to read about each option first.
Sources
- Competent person scheme: current schemes and how schemes are authorised, Ministry of Housing, Communities and Local Government, GOV.UK, checked 22 October 2026.
- BS 6229:2025 Flat roofs with continuously supported flexible waterproof coverings. Code of practice, BSI Knowledge, checked 22 October 2026.
- BS 8217:2005 Reinforced bitumen membranes for roofing. Code of practice, BSI Knowledge, checked 22 October 2026.
- BS 8218:1998 Code of practice for mastic asphalt roofing, BSI Knowledge, checked 22 October 2026.
- BS EN 13707:2013 Reinforced bitumen sheets for roof waterproofing, BSI Knowledge, checked 22 October 2026.
- BS EN 13956:2012 Plastic and rubber sheets for roof waterproofing, BSI Knowledge, checked 22 October 2026.
- Single ply roofing: design guide 2020 (S1/2020), Single Ply Roofing Association (SPRA), checked 22 October 2026.
- Guarantees: a guide for clients and designers (S04/17), Single Ply Roofing Association (SPRA), checked 22 October 2026.
- Generic types of liquid applied membranes, Liquid Roofing and Waterproofing Association (LRWA), checked 22 October 2026.
- Design guide for specifiers 2020: liquid applied waterproofing systems for roofs and balconies, Liquid Roofing and Waterproofing Association (LRWA), checked 22 October 2026.
- Safe2Torch guidance for the safe installation of torch-on reinforced bitumen membranes, National Federation of Roofing Contractors (NFRC), checked 22 October 2026.
- GD38 Over-roofing guidance document, Metal Cladding and Roofing Manufacturers Association (MCRMA), checked 22 October 2026.
- Technical Paper 6: Profiled metal roofing design guide (archived, reference only), Metal Cladding and Roofing Manufacturers Association (MCRMA), checked 22 October 2026.
- Glossary, Metal Cladding and Roofing Manufacturers Association (MCRMA), checked 22 October 2026.
- Approved Document B, Volume 2: Buildings other than dwellings (2019 edition, collated with later amendments), Ministry of Housing, Communities and Local Government, GOV.UK, checked 22 October 2026.
Common questions
Is a metal roof a flat roof?
Low-pitch metal roofs are used on industrial buildings, and MCRMA's guide says roofs below 4 degrees need secret-fix or standing-seam sheets. BS 6229 covers continuously supported flexible coverings, whereas a profiled metal sheet is fixed to purlins, so metal roofs are usually treated under MCRMA guidance instead.
Does the covering decide the roof's fire rating?
No. Approved Document B classifies roof constructions, not membranes on their own, as BROOF(t4), CROOF(t4) and so on under BS EN 13501-5. The deck, insulation and fixing method are part of the tested construction. The BROOF(t4) guide explains how to read a classification report.
Can I switch from one covering to another at re-roofing?
Often, but the existing roof has to be understood first. Compatibility between the old and new materials, the deck's condition and the load all matter. A survey with core samples establishes what is there before an overlay or strip is chosen.
Which type of flat roof lasts longest?
Sources checked give no fair ranking across all coverings. SPRA stresses that a manufacturer's promise to compensate bears little relation to durability or service life, so read the certificate for the product and build-up. The service life guide explains why one number is never enough.