Green Your Drainage: Vegetated Roofs

Our Green Your Drainage series has covered a number of ways to divert roof runoff, but what if that rainwater never has to leave the roof in the first place? In this fifth and final post of the series, we're talking about the basics and benefits of green roofs.

There are certain green features that become a focal point of any home and vegetated roofs are one of them. Acting like a garden on top of your roof, green roofs help to capture runoff and keep rainwater from overwhelming the municipal sewer or stormwater system. In addition to infiltration, these types of roofs have other benefits:

  • During heavy rains, green roofs may not be able to infiltrate all of the water they receive, but they do help to slow it down, which prevents flooding.
  • Green roofs can reduce energy costs by acting as an additional insulating layer.
  • Plants help to filter pollutants and improve air quality.
  • Green roofs can provide habitat for birds and beneficial insects.
  • A green roof is attractive and can be a teaching tool for neighbors.

While green roofs might appear simple, they are usually highly-engineered systems that are specifically designed for a particular climate and roof structure. Most systems consist of four layers: a waterproof membrane and drainage layer (which allows excess water a path to drain and prevents leaks), a soil-like growing medium and then plants. Best suited for low slope roofs, green roofs thrive when planted with hardy, drought-resistant plants like succulents, grasses, herbs and wildflowers. Vegetated roofs must be irrigated during the first few years, but should be low-maintenance once established.

Green roofs can add considerable weight to a roof, so if you're considering one for your own home, always work with a qualified engineer to ensure your roof can handle the additional load.

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(Image: Rob Harrison for Harrison Architects)

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