General Design Issues
Structural design criteria:
Wind: The green roof build‐up must have sufficient weight to resist uplift due to wind. Erosion control measures may be required to protect the main roof area, however at the building’s perimeters where the highest loads occur, ballasting aggregates or paving slabs are installed.
Dead loads: The saturated weight of the green roof, snow loads and service loads (e.g. pedestrian access loads) must be included in the structural design calculations.
Shear forces: Slippage of the growing medium must be considered a risk where roof pitch exceeds 20°. Anti‐shear measures should account for the roof slope and the mass of the growing medium.
A range of membranes, liquid‐applied waterproofing and metals can be installed beneath a green roof, however the detailing of the system will be critical to the long‐term integrity of the roof covering, ensuring sufficient upstands and termination details. Where the waterproofing layer is not root‐resistant, an independent root barrier is required.
Design must comply with the requirements of BS EN 12056‐3:2000 gravity drainage systems inside buildings. Roof drainage, layout and calculations. If a coefficient is applied to account for green roof retention, as permitted by the UK’s National Annex to BS EN 12056, the detailing must ensure that water can be attenuated on the roof without adversely affecting the building fabric. Inspection chambers are required to ensure that outlets are kept free of blockages.
The green roof must be designed to eliminate the risk of fire spreading during prolonged drought periods. Resistance is designed by increasing non‐combustible components, reducing the substrate’s organic matter and other combustible components and preventing the system from drying out.
Typically, for an extensive green roof, irrigation to saturation is required for the initial establishment of the green roof. If the roof is installed in the summer months, a temporary irrigation may be required for the first 6-8 weeks, whilst the roots establish. Thereafter no irrigation is required. Intensive, and some semi-intensive, roofs typically require a permanent irrigation system.
Legislation requires, amongst other things, safe working platforms and protection against falls to be provided for roof installation and maintenance works, with the choice of system dependent upon the roof type and usage.
Further details, specific to different green roof types can be viewed below.