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Guidance - Flat Roof - Green Roofs

Flat Roof - Green Roofs

Green Roofs

Green roofs offer a sustainable and ecologically friendly solution, whereby plants and vegetation can live on the deck of the roof.

The plants on a green roof contribute to improving air quality by producing oxygen and absorbing air pollutants, dust and CO2.

The thermal mass of a green roof provides a good level of thermal performance providing passive heat storage, as well as significantly reduce cooling requirements in the summer.

Green roofs contribute to suds (sustainable urban drainage).

Rainwater is stored in the substrate and vegetation reducing the volume of rainwater runoff from the roof which provides a more natural drainage process for storm water than a traditional flat roof.

There are two main types of green roof:

  • Extensive

    Supplied as a complete system this type of roof can be built using a only 50mm soil base and lightweight construction.

    However vegetation for this type of roof is limited to sedum, grasses, and mosses contained within a sedum blanket.

    This type of green roof requires minimal maintenance is required, although an irrigation system may be required for use in dry weather feeding with a slow release fertilizer a biodiverse roof is the soil seeds naturally and no watering is required.

  • Intensive (roof garden)

    Intensive roofs may contain a variety of plants including shrubs and trees, as well as garden furniture and even small ponds.

    Intensive roofs can only used on roof with a maximum pitch of 20o.

    They require a soil base at least 150mm deep and a substantial steel or concrete deck is needed to support the weight of the soil and plants.

    Regular intensive maintenance is required for this type of roof.


The majority of green roofs both intensive and extensive are build-from the following elements:

  • Vapour control layer - placed above the roof structure.
  • Rigid slab insulation – must be strong enough to withstand the additional loads.

    Where the insulation is above the weatherproofing, only extruded polystryrene (XPS) should be used.

  • Waterproof root barrier - for an intensive roof this should be reinforced bitumen membrane (RBM) or mastic asphalt.

    This will prevent roots penetrating the roof membrane. Suitable materials include, rubber mats, bitumen, slate-surfaced layers or polyethylene.

  • Drainage layer- controls the rate of rainwater runoff.
  • Filter layer - prevents the soils etc from blocking the drainage layer or causing damage – includes foams, mineral wool, plastic sheets and granular drainage.
  • Growing medium (soil) - the depth of the growing layer will depend on the type of roof. For an extensive roof the growing medium could be just a 20mm geotexile mats. However soil 400mm deep is typical for an intensive roof.
  • Vegetation – this is the planted layer which could be sedum, mosses and grasses for an extensive roof or shrubs and trees for an intensive roof.

Green roof systems should be installed by a specialist installer to drainage falls of 1:60 min.

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