Inverted roof

A roof construction in which the thermal insulation is placed above the structural deck, keeping the deck and supporting structure at close to internal temperature. A vapour control layer is normally installed on the warm side of the insulation to limit the risk of interstitial condensation.

Loose material, typically rounded gravel or paving slabs, laid over a roof membrane or over inverted roof insulation to hold the system down against wind uplift and to protect it from ultraviolet light and mechanical damage. Ballasted systems avoid fasteners and adhesives but add significant dead load and make leak tracing more difficult.

A roof with a very low pitch, in UK practice generally taken as no more than 10 degrees to the horizontal. Flat roofs rely on a continuous waterproof covering rather than overlapping units, and must be laid to falls so that rainwater drains to outlets.