Flat roof – green and blue roofs

Flat roof – green and blue roofs

Green roofs are a popular option for reasons of aesthetics, sustainability and drainage performance. The growing medium takes the place of the traditional gravel or paving slab ballast typically found on inverted warm flat roofs.

Not only do green roofs provide much-needed greenery and habitat diversity in urban areas, the growing medium also acts as a buffer during periods of heavy rain. Rather than the water flowing straight off the roof and potentially overwhelming the drainage system, it is ‘stored’ on the roof temporarily, draining gradually to release the storm water at a more manageable rate.

What is a blue roof?

For a more engineered rainwater attenuation solution, a blue roof is essentially a green roof designed specifically to hold a certain capacity of water and discharge it over a prolonged period. Such systems help designers to meet requirements for sustainable urban drainage systems (SUDS).

The flat roofing industry is still refining guidance on the use of, and best practice associated with, green and blue roofs. Blue roofs in particular remain a relatively new approach. Advice is available from industry bodies about the correct design and specification for either solution, and that advice is reviewed and updated on an ongoing basis.

The extra elements associated with green and blue roof constructions typically sit above the standard inverted roof construction, and extruded polystyrene (XPS) therefore remains the preferred insulation option for either scenario.

What Polyfoam XPS products are suitable for a green or blue 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.

Whether a relatively simple green roof, or a roof garden designed for pedestrian access, Polyfoam Roofboard Extra can be specified as part of a durable, high performance build-up. The Polyfoam roof system will continue to provide performance even if subject to increased water volumes as part of a blue roof design. 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.

Recommended Polyfoam XPS products for green and blue roof applications are: