Your Garden Variety Roof

Green roofs   February 22, 2008

Hello. This is Greg Zimmerman, executive editor of Building Operating Management magazine.

Today’s topic is green roofs, specifically what to be aware of if you’re considering a green roof for your next project.

By green roofs, I mean REAL green roofs – the ones with plants, foliage and other greenery, as opposed to cool roofs, which are sometimes labeled green in the sense of being environmentally responsible because they can reduce energy use.

With that distinction made, here’s another one: There are two varieties of green roofs. Intensive green roofs are the fancy kind, most often recognized as roof gardens that allow people to walk around and relax. They often include flowers, bushes, and even small trees. Extensive green roofs are usually less involved, with a thinner planting medium, and usually include smaller plants with shallower roots, like grass and flowers.

Two important considerations when selecting a green roof are waterproofing and weight. Green roofs are much heavier than traditional roofs, so selecting a green roof means making sure the facility has sufficient load-bearing capacity. Because of their much thicker planting medium, intensive green roofs can weigh upwards of 120 pounds per square foot.

The waterproofing system for a green roof can be either a sheet system, a built-up system or a fluid-applied system. Many experts recommend a waterproofing system without seams to reduce possible water-entry points. Waterproofing is certainly the one area where you don’t want to value-engineer.

If a green roof is waterproofed and installed correctly, it can last many years longer than a traditional single-ply roof. Other potential benefits of green roofs include better aesthetics, reduction of the urban heat island effect, energy savings, and reduction in stormwater runoff.


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