Even on flat roofs, wind uplift may present serious challenges for retention of substrate and plants. Wind damage can be dangerous to people and property and costly to repair. Retention systems may be required to ensure that materials cannot be blown off the roof.
Wind uplift pressure is lowest on the centre of the roof and highest at the edges, around the perimeter and at corners. The higher the building, the greater is the risk. On pitched roofs, the roof peak is also subject to uplift. Minimising untethered components greatly reduces the risk of damage to the green roof by uplift.
Where possible, waterproofing should be fully adhered to the roof, or mechanically fixed. For waterproofing layers that are not bonded to the roof, the green roof assembly must provide adequate ballast to prevent uplift. Edge treatments are the most critical: the un-vegetated zone around the perimeter of a green roof (see here) may require heavy concrete paving slabs rather than loose gravel ballast. The materials used must conform to the design wind load calculated for the specific green roof location.
On some green roofs, perimeter balustrades or parapet walls will provide some protection against wind flow. Other treatments, such as jute erosion control netting (see here) or coated wire retaining systems, may be useful.
Plant selection may also be used to mitigate the impact of winds. If the tolerance of different species to wind exposure is understood, plantings can be planned so that the lowest-growing, most robust species are planted in the most exposed areas. Taller, less wind tolerant species are installed behind them. The resulting gentle gradation of vegetation heights interrupts, and slows, wind flow over the planting.