Flat roof – inverted roof

Flat roof – inverted roof

Occasionally referred to as an ‘upside down’ or ‘protected membrane’ roof, an inverted roof is a type of warm roof construction.

Unlike a conventional warm roof build-up, the waterproofing is not installed as the outermost layer, so it is protected from stresses such as ponding water, ultraviolet (UV) radiation, expansion and contraction from natural temperature cycles, and extremes of temperature like frost.

An inverted roof also removes the need for mechanical fixings or adhesives to secure the roof build-up. The waterproofing, usually a liquid-applied product like hot melt, is applied to the structural deck. The insulation and a membrane, called a water control layer, are both loose laid before being covered by ballast, typically gravel or paving slabs.

The ballast must meet a minimum specification, depending on building height and location, to prevent wind uplift, or flotation of the insulation boards in the event of heavy rain.

While an inverted roof specification helps to protect the waterproofing, it places greater performance demands on the insulation layer. Only a few material types are suitable – including extruded polystyrene (XPS), which is classified as suitable for the widest range of roof applications by European technical guidance:

Polyfoam XPS Ltd is a leading manufacturer of closed cell, extruded polystyrene insulation. The Polyfoam range is lightweight, strong, moisture resistant and easy to cut and shape, providing thermal performance and strength for the lifetime of the building. Recommended Polyfoam XPS products for inverted flat roof applications are: