Caelum Contracts

020 3468 0301

Green Roof Construction


Many green roofing projects are often built using a layer of aggregate under a layer of filter fabric as discussed in green roof structural design. Whilst this method is effective for drainage purposes, it is labour intensive and heavy, making it impractical for some building structures. For projects where structural load is restrictive, a geotextile is often a better option for drainage in place of aggregates as it’s lighter.


Most green roofs are insulated using a rigid board of polyisocyanurate. This is a material which must be kept dry and is therefore installed beneath the roofing material.

Insulation made of extruded polystyrene is another option. This material is water resistant and can be placed on top of roofing material. In this case, the insulation is held in place by concrete pavers or the green roof itself.

Detailing accessories

The perimeter of a green roof must be aesthetically pleasing without jeopardising it’s main functions. Green roofs that extend to roof edges without a parapet wall must be detailed to stop growth media from exiting the roof. Timber can be used to build up a wall to enclose the growth media. However, timber can often warp and does carry additional maintenance requirements in the future. Products are a available that provided a finish to the roof edge with a sheet material (for example a Wallbarn edging connector). The product contains an integrated filter fabric and perforations which allow water to drain from the roof without losing growth media.

Green roofs are sometimes designed to stop short of the roof edge. In this case, the roof perimeter is often covered in aggregate and are often detailed with Growth Media Retainer, separating growth media from the aggregate area. Growth Media Retainers contain filter fabrics and perforations to allow water to drain from the growth media.

Roof drains on a green rood also require detailing in order to prevent growth media from spilling into the drainage and entering the plumbing system. Products are available on the market that use integrated filter fabric to stop growth media and plants from entering the roof drainage system (Green Roof Blocks have a range of appropriate products)

Roof Slope

Many green roofs are built on flat roofs. However, the majority of roofs will have a slope to some degree. Roof slope is essential for allowing water to exit the roof via drainage rather than collect in pools on the roof, which would accelerate the ageing process.

A roof slope of 21:12 and higher (a surface that rises 21 inches for each 12 inches) is considered to be a steep roof. For green roofing projects, steep roofers require additional design considerations to stop growth media and plants from sliding down the roof. There are several products available on the market to do this, whilst a bracing strategy with treated timber is also effective. Modular green roofs can often be used on slopes with up to 4:12 without additional bracing.