Single Ply Roofing vs Liquid and Felt Roofs

What single ply roofing is: TPO, PVC and EPDM

Single ply is a thin, factory-made membrane rolled out in wide sheets. The sheets are fixed to the deck and joined at the seams, so a big roof closes up quickly.

There are three main types:

Each can be mechanically fixed through the insulation into the deck, fully bonded, or held down with ballast such as paving or gravel. The wind load, the deck and the roof's height set which one the designer uses.

Single ply is light, which helps on a steel-framed shed with little spare capacity. It is quick on large, open roofs. Its weak points are the details: corners, upstands, outlets and penetrations need trained hands and the maker's own parts.

Liquid roofing: where it beats a membrane

Liquid roofing is applied wet, with a reinforcing fleece, and cures into a membrane with no joints. It shines where a sheet struggles: roofs crowded with plant, pipes, upstands and rooflights.

The Liquid Roofing and Waterproofing Association lists its uses well beyond plain roofs: balconies, walkways, green roof terraces and podium decks that people walk on. It also lists overlay systems, where a liquid goes over a sound existing roof instead of stripping it.

That overlay route can save a strip-out and keep the building dry throughout. It only works if a survey shows the old roof and insulation are sound underneath.

Liquids need a dry, clean, primed surface and the right thickness, so weather and supervision matter. Most cold-applied liquids need no naked flame, which suits occupied buildings. Many roofs combine the two: single ply on the open field, liquid at the details.

Built-up felt and when it still fits

Built-up felt is two or three layers of reinforced bitumen sheet, often torched on. It is heavier and slower to lay than single ply, but it is tough.

It still makes sense in three cases:

1. Roofs people walk on. Plant rooms and service routes where engineers cross the roof every week.
2. Patch repairs. Matching an existing felt roof that is mostly sound.
3. Mixed roofs. Small areas between other systems where felt detailing is simplest.

The catch is hot works. Torching on an occupied building needs a hot works permit, a fire watch and often a cold-applied alternative near combustible details. Cold-applied and self-adhesive felts now avoid the flame where that matters.

Lifespan, warranty and fire rating

How long a flat roof lasts depends less on the label than on the product, the installation and the upkeep. Three documents tell you what you are really buying.

1. The third-party certificate. An Agrément certificate, such as from the BBA, assesses a system's fitness for purpose, durability and fire performance. The LRWA requires its manufacturer members to hold one.
2. The guarantee. Read who backs it, what it covers and what inspection it requires. A guarantee tied to an approved installer is worth more than a supply-only one.
3. The fire classification. Roof coverings are classified for external fire performance under BS EN 13501-5. Approved Document B sets what a roof needs, mainly by its distance from the boundary.

Ask for the classification of the whole build-up as tested, not just the membrane. The flat roofing trade associations, LRWA, NFRC and SPRA, published joint guidance on fire safety for flat roofs and balconies after the 2022 changes. The code of practice for flat roofs, BS 6229, was revised in 2025.

Over a live warehouse or office: installing without closing

Most commercial roofs are replaced over a building that keeps working. The method decides whether the business notices.

1. Work in sections. Strip only what can be made watertight the same day.
2. Choose cold methods over people. Cold-applied liquids and bonded or fixed membranes avoid flame above occupied floors.
3. Protect what is below. Temporary covers over stock, machines and desks, and a plan for sudden rain.
4. Keep access apart. Hoists, skips and roof access away from loading bays, staff entrances and escape routes.
5. Treat rooflights as fragile. Edge protection, covers or nets before anyone walks the roof.

A roof survey first records the deck, the insulation, any asbestos and every rooflight. The system choice, the programme and the price all come from it. Our guide to commercial roof replacement cost explains what moves that price. Need a roof surveyed first? Book a roof survey.

Choosing a system for solar and green roofs

A new roof should be chosen for what goes on it next. Plan for the use over the roof's whole life, not just the next winter.

If panels are planned, put them in the roof design and the structural check now. Our commercial solar panels team works from the same survey, so the roof and the array are designed together. For curtain walls and facades on the same building, see our guide to curtain walling.