The insulation for an inverted roof must:
- resist water absorption
- be unaffected by freeze/thaw cycles
- withstand surface traffic
- protect the waterproof layer long term
- be ballasted to prevent flotation
- be protected from UV and mechanical damage.
General recommendations on the design of flat roofs are contained in BS 6229.
Construction of the inverted roof
In the inverted roof system insulation is laid over the waterproofing layer and suitably loaded to restrain it against flotation and wind uplift and to protect it against damage.
Inverted roof constructions can be categorised as heavyweight or lightweight by reference to the form of building construction involved. If the structure incorporates a concrete slab it will normally be cost effective to design the slab to support the load of 80-120 kg/m2 imposed by a ballasted inverted roof system (Figures 2 and 3).
The inverted roof concept is ideally suited to green roofs where the roof is covered with a plant bearing layer (Figure 4).
Green roofs may be used to:
- reduce a building's environmental impact
- provide a garden area for projects where space is at a premium
- contribute to a building's appearance
- attenuate the discharge of rain water from the roof.