Single ply roofing means a factory made sheet, laid in one layer and joined by hot air welding or by adhesive, rather than the several bonded layers of a traditional built up felt roof. It is the standard covering on commercial flat roofs and it appears on larger domestic extensions and flat roofed houses. The name describes the build up, not the material, and the materials behave differently.
The materials behind the name
The common families are PVC, TPO and EPDM. PVC and TPO are usually hot air welded, which makes a joint as strong as the sheet and lets a roofer cover a large or complex roof in panels. EPDM is a rubber more often bonded and taped, and on small domestic roofs it can go down as a single piece with no joints at all. Asking which of the three is being quoted tells you more than the phrase single ply does.
How it is held down
Three methods, and the choice is engineering rather than preference. Mechanically fastened sheets are screwed through into the deck along the laps. Adhered sheets are glued to the insulation across the whole area. Ballasted roofs are held by gravel or paving. Wind uplift decides which is appropriate, and it varies with the height of the building, its exposure and the position on the roof, so the perimeter and corners are always fixed more heavily than the middle.
Where it suits a house
On a large flat roof, a long dormer, or a roof with several penetrations to weld around, single ply is quick and predictable. On a small garage roof the advantage largely disappears, because a one piece rubber sheet does the same job with fewer joints and less specialist plant. Welding needs the right equipment and a trained operative, so ask who is doing it and what they are trained on.
Detailing, drainage and fire
As with any flat roof the failures cluster at upstands, outlets and penetrations, and the fall to the outlet matters more than the sheet. Where a flat roof is near a boundary the fire performance of the covering comes into it, which Approved Document B deals with. On a refurbishment the thermal requirement in Approved Document L for renovating a thermal element usually applies as well.
Questions people ask about single ply roofing
Is single ply better than felt?
It is a different construction rather than a straight upgrade. A modern high performance built up felt roof is a serious covering. Single ply is usually faster to lay, lighter, and easier to detail around obstructions, which is why it dominates commercial work.
How are single ply joints made?
PVC and TPO are hot air welded so the laps fuse into a continuous sheet. EPDM is more often bonded with tape or adhesive. A welded lap can be probed and tested, which is one reason commercial specifiers like it.
Does a single ply roof need maintenance?
It needs the outlets kept clear and an occasional look at the upstands and laps. Most membrane guarantees assume reasonable maintenance, so keeping gutters and outlets free is not optional if you want to rely on the cover.
Can single ply go over an existing flat roof?
Sometimes, using an overlay board, but the same caution applies as with any overlay: it hides the deck's condition and does nothing for the falls. A strip and re lay tells you what you actually own.
Who should install it?
Someone trained in the specific system, because manufacturer guarantees are usually tied to approved installers. Ask for the training record for the system being quoted rather than a general roofing credential.