An inverted roof — also called an upside-down roof or protected membrane roof — is a flat roof in which the insulation is laid loose above the waterproofing rather than below it. The waterproofing sits directly on the structural deck and also acts as the vapour control layer. Because the insulation is loose-laid, an inverted roof must always be ballasted.
Build-up from bottom to top
- Structural deck — usually concrete, laid to fall
- Waterproofing — bituminous or single-ply membrane, doubling as the vapour control layer
- Insulation — loose-laid rigid boards, normally rebated at the edges
- Filter layer — a water-permeable fleece that keeps fines out but lets water through
- Ballast — gravel, paving slabs on pedestals, or a green roof build-up
Without ballast the insulation will lift in the wind or float in heavy rain.
Why XPS
In practice only XPS (extruded polystyrene) is suitable for an inverted roof. The insulation sits permanently on the outside of the waterproofing and is in contact with rainwater. XPS has a closed cell structure and absorbs almost no water. Mineral wool, EPS and PIR are not suitable here — they take up water and lose their insulating value.
Where it is used
- Roof terraces — paving on pedestals as ballast
- Car park decks — with a heavy wearing course or concrete units on top
- Green roofs — the substrate build-up provides the ballast
- Upgrading existing flat roofs — a sound existing membrane can stay in place; only insulation and ballast are added
Advantages and pitfalls
Advantages: the waterproofing sits protected from UV, hail and temperature swings, which extends its service life. Interstitial condensation is avoided in principle because the warm side of the construction is sealed by the membrane.
Pitfalls:
- Falls are critical. If water stands between the boards, the membrane is cooled and the thermal performance drops
- A correction is applied to the thermal resistance because rainwater flows over and under the boards — use the manufacturer’s declared value for inverted roof use, not the plain board value
- Tracing a leak is difficult, especially under a screed or paving — the ballast has to come off first
- The structure must be able to carry the added ballast load; when upgrading an existing roof, always have this checked by a structural engineer
Related terms
- Flat roof
- Warm roof
- Cold roof
- Vapour control layer
- Green roof
- Roof terrace
- Ballast layer
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