Flat roof – green and blue roofs

Flat roof – green and blue roofs

Traditionally, inverted flat roofs are finished with ballast such as gravel or paving slabs. Increasingly, however, green roofs are a popular option for reasons of aesthetics, sustainability and drainage performance.

Not only do they provide much-needed greenery and habitat diversity in urban areas, but the layer of growing medium that supports the choice of planting acts as a buffer during periods of heavy rain.

Rather than the water flowing straight off the roof into the drainage system, potentially overwhelming a system that is already at capacity, it is ‘stored’ on the roof temporarily. The gradual draining of the growing medium releases the storm water into the system at a more manageable rate.

For a more engineered rainwater attenuation solution, a blue roof is a flat roof that includes a system specifically designed 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) and can be used with ballasted roofs, landscaped and terrace roofs, and green roofs.

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 technology. 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.

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 green and blue roof applications are: