Wind design for modular vegetated roofing systems

A vegetated roof assembly (VRA) is the combination of roofing system (RS) and vegetated system (VS). The wind uplift resistance of VRA is evaluated using the same test procedure for evaluating the wind uplift resistance of roof membranes (i.e. the Canadian Standards Association [CSA] A123.21, Standard test method for the dynamic wind uplift resistance of membrane-roofing systems). Since there are gaps between VS and the roof membrane surface and around the edges, air can readily enter and equalize the pressures above and below the VS to avoid overturning, so the wind uplift resistance of the modular vegetated roof assembly (MVRA) should be at least that of the RS. Therefore, the VS supplier may choose to report the wind uplift resistance of the RS obtained from CSA A123.21 as the sustained pressure. The designer can use the wind uplift resistance of the RS obtained from the CSA A123.21 and the wind flow resistance of the VS obtained from CSA A123.24-15, Standard Test Method for Wind Resistance of Modular Vegetated Roof Assembly, to proceed with the wind design process for the VRA.

The wind flow resistance is determined by dividing the sustained wind speed, which is defined as the incremented speed level before failure occurs, by a minimum safety factor of 1.5, as per CSA A123.24-15. Since the airflow machine can deliver a wind speed of 200 km/h (124 mph) at maximum capacity (current at time of publication), this limits the maximum sustained wind speed of any VS tested to 200 km/h, which corresponds to a maximum wind flow resistance of 133 km/h (82 mph) after applying the 1.5 safety factor. This can become an issue for projects with design wind speed greater than 133 km/h. Wind engineers will need to review the specifics and offer professional opinions for these projects.

The designers should note CSA A123.24-15 is not applicable to a mechanical attached roofing system, but appropriate for adhesive adhered roofing system and a partially adhered roofing system. Currently, CSA A123.24-15 is applicable to a modular vegetated roofing system only and not built-in-place systems, where young vegetation are planted in growing substrate on the roof. However, the CSA A123 technical committee is in the process of revising the standard to include built-in-place VRA in the next revision of the standard based on new research findings coming out of National Research Council Canada (NRC).

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