Wind design for modular vegetated roofing systems

Vegetated mats contain mature plants growing on a carrier filled with growing medium. A basic carrier is made of a thin mat of coconut coir fibres, which decompose on the roof over time. Advanced polymeric carriers consist of a 3D entangled mesh bonded to a geotextile fabric, which promotes root anchorage and mat integrity. This durable carrier can be easily rolled up for roof membrane repair and replaced after.

The vegetated mats are rolled out on other layers such as growing medium, water retention, drainage, filter, and root barrier to form a vegetated system. Adjacent mats are usually installed on a built-in overlap so the roots grow and connect the mats together naturally over time. A vegetated mat system is most vulnerable to wind uplift during the establishment period (approximately the first four to eight weeks after installation) before the roots have a chance to grow through the layers and stitch them together.

This is what happened to Specimen A at 160 km/h (99 mph) during the Canadian Standards Association (CSA) A123.24-15, Standard Test Method for Wind Resistance of Modular Vegetated Roof Assembly, wind flow testing (Figure  6). Specimen A was a modular vegetated roof assembly (MVRA) consisting of four components—sedum mat, retention fleece, drainage layer, and root barrier. Wind caused increased fluttering of the vegetated mat and finally lifted its corner facing the airflow, making the mat airborne and causing the entire system to fly off.

Specimen B was a MVRA consisting of the same four components as Specimen A except the sedum mat was of a different design, which was also thicker and heavier. Specimen B sustained the maximum capacity of the air flow machine at 200 km/h (124 mph) without failure.

Wind can damage vegetation. Specimens C and D had sedum mat and perennial mat, respectively, and both sustained 200 km/h (124 mph). After the flow test that lasted approximately 45 minutes, the sedum laid flat and pointing away from the wind flow direction, but the low-profile succulent did not suffer much damage. On the other hand, some of the herbaceous plants on the perennial mat were stripped of foliage while others suffered desiccation (Figure 7).

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