Flat roof

Upside-down roof

Systems

Loadbearing layer

Heat Vapour

Fig. 12: Schematic drawing of building performance parameters

Protective layer Separating layer Thermal insulation Waterproofing layer

Falls in this layer

Loadbearing layer

Protective layer Separating layer Thermal insulation Waterproofing layer

Falls in this layer

Loadbearing layer

Fig. 13: Section through upside-down roof

Upside-down roof

The upside-down roof is a non-ventilated flat roof system with the obligatory functional layers. However, the sequence of the layers is different from a conventional warm deck.

The layer of thermal i nsulation is placed above the waterproofing and must therefore itself be waterproof (extruded polystyrene). This is a single layer of material and must therefore incorporate rebated joints. As the I n-sulation is laid "in the wet" it must be 20% thicker than is necessary to satisfy the actual thermal insulation requirements.

A separating layer of fleece above the i nsulation prevents the gravel infiltrating the joints in the thermal insulation. The use of a special separating fleece which allows most of the water to drain away enables the 20% extra thickness to be reduced to just 3%.

The seamless waterproofing can consist of bituminous or synthetic roofing felt (number of layers as for a conventional warm deck) and is laid beneath the i nsulation, directly on the I oadbearing layer. This also acts as a vapour barrier, and its position below the thermal i nsulation means that it is adequately protected against any damage.

A protective layer is absolutely essential on an upside-down roof. It prevents damage to the thermal i nsulation and also serves as ballast to prevent the i nsulation lifting off the layers below. As on a conventional warm deck a minimum fall of 1.5% must be incorporated, which can be achieved in the loadbearing layer, in a layer specifically incorporated for this purpose or in the thermal insulation.

Fig. 13: Section through upside-down roof

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