Flat roof – conventional warm roof

Flat roof – conventional warm roof

Most warm flat roof constructions consist of a structural deck with a vapour control layer, thermal insulation layer and waterproofing installed over. It is often described as a ‘conventional’ warm roof because it is a common solution, suitable for projects of all sizes – from small domestic to large commercial or industrial.

There is a huge range of waterproofing options, from traditional and modern types of built-up felt roofing, through the many polymeric single ply membrane materials (with different attachment methods), to liquid-applied systems. All require slightly different things of the insulation board to which they are applied.

What should I consider when specifying XPS insulation in a warm roof?

In a conventional warm roof, the protection afforded to the insulation by the waterproofing layer means a wide range of insulation materials can be used. The performance characteristics that make extruded polystyrene (XPS) the preferred choice for inverted flat roofs, green roofs and car park decks are less significant.

Nevertheless, XPS boards are specified in warm roof constructions. Usually the intended waterproofing is a single ply membrane, but specifiers should consult with the manufacturers of the insulation and the intended waterproofing for further guidance, as there are significant differences in performance requirements compared to an inverted roof.

For example, the ballast on an inverted roof is usually specified because it is known to resist fire spread. The fire performance of a conventional warm roof depends on the waterproofing and the deck type, so available test data should be consulted to confirm the suitability of a proposed specification.

What Polyfoam XPS products are suitable for a conventional warm flat roof?

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.

For a conventional warm roof, Polyfoam Roofboard Extra can be specified as part of a durable, high performance warm flat roof. Product benefits include:

  • 15mm lap joint.
  • Excellent thermal performance.
  • High compressive strength.
  • Lightweight and easy to install.
  • Tough and durable, not easily damaged.
  • Dimensionally stable.
  • 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.

Recommended Polyfoam XPS products for conventional warm roof applications are: