Inverted flat roof

A flat roof in which the insulation is laid above the waterproofing membrane, not below: the «inverted roof». The membrane, shielded from sun, frost and impact, lasts far longer; the water-resistant XPS insulation is held down by gravel or pavers. A robust, inspectable build-up, ideal for walkable roofs and green roofs.

The inverted roof reverses the usual order of the flat roof: the thermal insulation is laid above the waterproofing membrane instead of below it. The consequence is a radical protection of the most delicate and costly element to replace — the membrane — which, shielded by the insulation, works at a stable temperature, sheltered from UV, thermal shock and mechanical damage.

In a traditional (warm) roof the waterproofing membrane sits at the surface, exposed to thermal swings that on a dark roof can exceed 80 °C in summer and drop below zero in winter: the main driver of ageing and cracking. In the inverted roof the membrane is buried under the insulation and the ballast: it works within a narrow temperature range, sees no sun and takes no impact. Its service life lengthens markedly, and with it the reliability of the whole roof.

Laying the insulation above the waterproofing means exposing it to the rainwater that passes through the ballast. For this reason the inverted roof admits almost exclusively extruded polystyrene (XPS): closed-cell, it does not absorb water and keeps its conductivity over time even when wet. Above the insulation a filter fleece holds back the particulate and lets the water drain; a small amount of heat is nonetheless lost to the water running over the membrane, and must be accounted for in the thermal calculation (rain-cooling correction).

The floating insulation must be ballasted against wind and buoyancy: washed round gravel, concrete pavers on supports, or the build-up of a green roof. Below the membrane, a screed to falls (≥ 1.5-2%) guides the water towards the outlets, which must be oversized and fitted with safety overflows. The care of the singular points — edges, parapets, outlets, penetrations — is where watertightness is really decided: most leaks start there, not in the open field.