Flat roof – inverted roof

Flat roof – inverted roof

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

Unlike a conventional warm roof build-up, the waterproofing is not installed as the outermost layer. Usually a liquid-applied product like hot melt, the waterproofing is applied to the structural deck. The insulation and a membrane, called a water flow reducing layer (WFRL), are both loose laid before being covered by ballast – typically gravel or paving slabs.

What Polyfoam XPS products are suitable for an inverted flat roof?

Polyfoam XPS Ltd is a leading manufacturer of closed cell, extruded polystyrene (XPS) 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.

For untrafficked and pedestrian access roofs, Polyfoam Roofboard Extra provides all the necessary performance characteristics to achieve a durable, high performance inverted roof construction in any UK location. Product benefits include:

  • 15mm lap joint.
  • Excellent thermal performance.
  • High compressive strength.
  • Highly resistant to water absorption.
  • Able to resist repeated freeze/thaw cycles.
  • Lightweight and easy to install.
  • Tough and durable, not easily damaged.
  • Dimensionally stable.
  • Thermal performance of an inverted roof is improved when used in conjunction with Polyfoam Slimline Zero Membrane.
  • British Board of Agrément Certificate.
  • Environmental Product Declaration.
  • BES 6001: Responsible Sourcing of Construction Products.

An important part of flat roof design is the detailing and construction of suitable upstands around the roof edge, and Polyfoam Upstand Board limits heat loss due to thermal bridging at the junction of the flat roof with the external wall.

What are the advantages of an inverted flat roof construction?

Covering the waterproofing with insulation and ballast protects it from stresses such as ponding water, ultraviolet (UV) radiation, expansion and contraction from natural temperature cycles, and extremes of temperature like frost.

Protecting the waterproofing in this way 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. Thanks to its robustness, an inverted roof featuring an XPS insulation layer can be designed as any of the following:

An inverted roof removes the need for mechanical fixings or adhesives to secure the roof build-up. To make sure ballast performs appropriately, it 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.

Recommended Polyfoam XPS products for inverted flat roof applications are: