Green roofs: Considerations for installing onto curved and pitched roofs

Green roof installations onto sloping roof pitches

Green roof applications are particularly well suited to flat roofs and in those cases fewer installation challenges can occur. However, even relatively steep roof pitches can be used to accommodate an extensive or biodiverse green roof design when the correct components are taken into consideration.

Geogrid & geocellular components

The primary challenge of installing green roofs onto steeper slopes comes from retaining the growing media and vegetation without shear forces causing them to slide once placed onto the roof. To prevent this scenario, a roof design that specifies a combination of reinforcing geogrids and geocellular retaining systems is required.

The geogrid layer adds friction and provides additional interlock between the roof surface and the upper green roof layers. The geocell soil containment layer is anchored to the geogrid and is typically a 100-300mm deep panel that is expanded into position to create a retention web across the roof. The geocellular pockets retain the soil and help to maintain optimum moisture levels, with drainage holes included to prevent the growing media from becoming overly saturated during heavy rainfalls.

Drainage & reservoir layers

The containment and reinforcement layers are installed onto a Roofdrain drainage reservoir board with a cuspated core providing a store of water for plants during periods of dry weather, with holes in between for drainage. The drainage board includes a geotextile filter bonded to the top of the cuspates to prevent soil intrusion.

Roof access & safety systems

Additional roof access measures including specialist harness equipment will be required to install the geosynthetic layers safely and correctly onto sloping sections and suitable edge protection and fall arrest systems are required as with all roofing work and project involving working at height.

Wind uplift resistance

Hard roof surface finishes such as pavers and waterproofing layers are more susceptible to wind-suction effect and require a mechanical or adhesive fixing. Green roofs are less exposed to wind uplift risk owing to the permeability of the surfaces within the vegetation and growing media layers affording an open surface structure with irregularities in form and height that help to distribute and disrupt wind flow and prevent negative wind pressure from gaining a strong grip. Where pitched roof elevations are exposed to strong winds, a choice of low-growing and drought tolerant sedum planting is recommended. Sedums are adapted to living in dry conditions where the soil is thin or poor and so they make a great choice for roof plants.

Irrigation system

On a sunny aspect, bear in mind that the temperature on the roof is likely to be at least a couple of degrees higher than the temperature on the ground and it will also be more exposed to drying winds. So the plants will need to be very drought tolerant and the maintenance team needs means to water the roof in dry conditions, in the form of permanent irrigation or, for smaller roofs, a simple water point at roof level. In warmer climates and with the advent of climatic changes and hotter summers, the watering systems can be supported by deeper Roofdrain reservoir drainage board layers.