Flat roof covering

Frequently asked questions

What actually holds a flat roof down?

One of three things, and which one decides most of the rest of the design. A FULLY ADHERED system is glued to the substrate over its whole area, which gives the best wind performance and the least tolerance for a damp or dusty deck. A MECHANICALLY FASTENED system is screwed through into the deck along the laps, which is fast and cheap and puts thousands of penetrations through the insulation — each one a small thermal bridge, and each one a potential leak if the deck below is not what the fixing schedule assumed. A BALLASTED system is held down by weight: gravel, pavers or a green roof build-up, which imposes a substantial dead load the structure has to have been designed for. The design question is wind UPLIFT, which is highest at the corners, then the edges, then the field — so a specification that quotes one fixing rate for the whole roof has usually been taken from the field figure and under-fixed the corners, which is exactly where roofs lift.

What is an inverted or protected membrane roof, and why turn the build-up upside down?

The waterproofing goes UNDER the insulation instead of over it, and the insulation is then held down by ballast. The point is that the membrane — the expensive, vulnerable, difficult-to-repair layer — spends its life at a nearly constant temperature, protected from ultraviolet light, from foot traffic and from the daily thermal cycling that ages it, instead of being the hottest and coldest surface on the building. The cost is that the insulation is now outside the waterproofing and standing in water whenever it rains, so it has to be a closed-cell board that does not absorb it — in practice extruded polystyrene — and its performance has to be derated for the water that flows over it. It also means the roof cannot be inspected by looking at it. Where freeze-thaw is severe, or where a roof will be walked on or planted, the trade is usually worth making; on a small domestic extension it rarely is.

Why does a flat roof fail at the details rather than in the middle?

Because the middle is a single sheet of waterproofing and the details are junctions, and every junction is a change of plane, a change of material or a penetration. The recurring four are worth naming. UPSTANDS: the membrane has to turn up at every abutment by enough that water driven up the roof cannot get over it, and be mechanically held at the top rather than relying on adhesion. OUTLETS: an outlet set above the finished surface guarantees ponding around it, and one with no overflow turns a blockage into a structural load. PENETRATIONS: pipes, vents and fixings each need a collar, and a mastic fillet is a repair rather than a detail. And the PERIMETER, where uplift is highest and where the edge trim is the thing holding the membrane against it. A roof that leaks in its field usually has damage; a roof that leaks at a detail was usually built that way, which is why the detail drawings are the deliverable that matters on a flat roof.