What Is Single-Ply Roofing?

The main types

PVC membranes are made from polyvinyl chloride, with reinforcement and plasticisers to keep them flexible. Their seams are welded with hot air, which fuses the sheets into one, and they are available in many colours. TPO, short for thermoplastic polyolefin, is similar in use, with hot-air welded seams, and is chosen for its reflectivity and recyclability. EPDM is a synthetic rubber, known by its initials from the chemical name. It comes in large sheets with few seams, which are joined by adhesive or tape, and it copes well with movement and cold.

All three are thin compared with felt, often only a millimetre or two, so the protection of the sheet matters. They sit on a layer of insulation, with a vapour control layer beneath, and a separating fleece where the insulation could damage the membrane. The full side-by-side of the membrane families is in the flat roof systems comparison.

How the sheet is held down

There are three ways to keep a single-ply sheet on the roof. Mechanical fixing uses screws and plates through the membrane or its edge into the deck, spaced to resist wind uplift. This is quick, and suits large open roofs. Adhesive bonding sticks the sheet over its whole area to the insulation, which gives a smooth surface and avoids flutter in the wind. Ballasting holds the sheet down with a layer of stone or paving slabs, which adds a protective layer but adds weight and hides the sheet, so the structure must carry the load.

The wind is an enemy of every flat roof, particularly at corners and edges where uplift is greatest. A good design puts more fixings in those zones. An inverted roof, with insulation above the membrane, is another arrangement, used where protection from traffic and temperature swings is wanted.

Insulation and falls under the membrane

A membrane is only as good as what it sits on. The insulation boards must be laid tight, fixed to the pattern the maker requires, and shaped to give a fall so that water leaves the roof. A flat membrane over a flat deck will pond, whatever the sheet is made of. Tapered boards or screed are the usual way to build the slope. Rigid board edges should meet cleanly, since an uneven joint shows through the sheet and can wear it over time.

Seams and details

The membrane in the open field is rarely the problem. Seams, upstands, outlets, corners and penetrations are where leaks start. Hot-air welded seams are very strong when they are made properly: the weld must be continuous, free of creases and checked with a probe. Details such as pipe collars and corner patches are made from pre-formed pieces or hand-welded patches. Poor workmanship at a corner can put a leak where nobody would think to look.

These fiddly parts are where site conditions matter. Welds need a dry, clean surface, and cold or damp weather makes them harder to do well. A programme should allow for that, as explained in our page on how long a commercial re-roof takes.

Strengths, weak spots and repair

Single-ply is light, which helps on structures that cannot take heavy felt or ballast. It installs quickly, with large sheets laid in a day, and it avoids naked flames. It also suits renovation, as it can go over an existing felt roof when the surface is sound and dry. Its weak spots are puncture and damage from foot traffic, since a dropped tool or a sharp edge can cut it. Walkway pads protect the usual routes. Over time, a PVC sheet can lose plasticiser and stiffen, and a poorly bonded EPDM sheet can shrink at its edges.

Maintenance is light but real, covering outlets, walkways and a check of the seams after storms. Repairs are simple when the sheet is sound: clean, prepare and weld or bond a patch. A new patch on old PVC may need a special cleaner, and on EPDM a proper primer and tape. A whole roof that is brittle, with wet insulation, is a replacement case. Compare with built-up bitumen in our page on built-up felt roofing, and see the longest-lasting commercial flat roof for how they age.

To see what is on your roof, we walk it, photograph the seams and details, and tell you plainly whether a patch or a new membrane makes sense. A fixed written quote follows. Wider information on business roofs is gathered at commercial roofing in Basingstoke.