Intensive green roofs are generally designed to accept recreational activity and to include the wider range of vegetation from grass and herbaceous plants to shrubs. Depths of soil are typically between 200 and 300 mm, which, together with the necessary minimum 50 mm of water reservoir and drainage systems, generate an additional imposed load of typically 400 kg/m2 on the existing or proposed structural system. Intensive green roofs may incorporate both soft and hard landscaping, and slopes of up to 20° are practicable. To conform to Health and Safety requirements edge protection (e.g.handrails) or a fall-arrest system (e.g. harness attachment points) must be incorporated into the design.
A typical intensive green roof system requires the following construction:
- soil, compost and planting – hard and soft land -scaped areas;
- filter fleece to prevent soil from blocking the drain -age system;
- moisture-retention material and drainage system;
- protection mat (to prevent damage to root barrier and waterproofing layers);
- polyethylene foil (isolating layer);
- root barrier;
- waterproof layer;
- insulation;
- vapour barrier.
Where trees are required, soil depths may need to be increased to 750–1000 mm, with the associated increase in overall weight. Care must be taken to ensure that the roof membrane is not damaged by gardening implements. Intensive green roofs will require careful choice of planting to cope with the exposed conditions and regular long-term maintenance including weeding, trimming and pest control.