Question

What type of waterproofing system is used under these green roofs, and what is the expected lifespan of the actual roofing material?

Asked by Fred Oompalumpa
Saint Peters, MO

Green roofs: what keeps the water out and for how long?

Answer

Polly Osborne, FAIA, LEED AP

Answered by Polly Osborne, FAIA, LEED AP

Los Angeles, CA

Polly Osborne Architects

August 26, 2010

Dear Fred,

By green roofs, I am assuming you mean vegetated roofs as opposed to reflective roofs. The term is used for both, so I will talk a little about both.

Reflective roofs

Reflective or “cool” roofs are materials and coatings used to help keep a building cooler by reflecting solar energy away from the building. Reflective roofs and roof coatings are measured by the percent of solar reflectance (SR) the roof has; that is, the portion of solar energy that is reflected away from the roof, and it is expressed in a fraction of a percent from 0.0 (no energy reflected) to 1.0 (100% energy reflected).

Thermal emittance is another measurement taken here, as the roof also emits solar heat back into the atmosphere. The measurement index most commonly used today, the Solar Reflectance Index (SRI), combines both these measurements. A good reflective roof can make a huge difference in summer cooling bills. Cool roof technology goes beyond reflectivity, however, because other factors aid in reducing the heat build-up on a roof.

Vegetated roofs

Plants cool a roof through transpiration (the evaporation of water into the air), and earth, serving as a solar mass, absorbs heat as the sun shines on it during the day and releases it in the cooler temperatures of the night.

The environment around a building, the shape of the building and the prevailing winds can, some places, be manipulated to effectively cool a roof through convection (wind movement) as well.

Vegetated roofs also have other benefits, such as reducing stormwater pollutants by filtering the water running off them and creating an additional layer of insulation for winter heating (although the “insulation” factor is far less than the “solar mass” factor). The density of the covering also protects the roof waterproofing from ultra-violet light. And of course, living roofs are beautiful, restore habitat, and make places for recreation.

But your question was “what waterproofing system” is used, so I’m thinking you are asking about what waterproofing is used under the growing plants and you may be wondering how it could last with all those little roots trying to find their way through it. You are right to ask “what system,” because a single waterproofing layer doesn’t work.

Here are the building blocks of a living, vegetated, or green roof, starting from the inside out:

  • Insulation: Insulation is different from solar mass. Insulation can go above or below the structure. Insulation resists temperature changes, thereby holding in whatever temperature you have inside and keeping out the outer temperatures.
  • Solar mass, on the other hand, collects heat when it is hot and releases it when things cool down. Dirt is a better solar mass than insulator, so insulating your green roof building is usually a good idea.
  • Structure: Growing medium is heavy. Water is heavy. You need the building structured to carry it. There are growing mediums lighter than good old-fashioned dirt, but they are still heavier than nothing at all. The deeper the growing medium is, the more varieties of plant you can grow, and the heavier it will be.
  • Waterproofing substrate: Here’s your waterproofing. It can be made of several things and every company has their formula. It is usually a flexible elastomeric membrane that is self-healing. Bitumen, a viscous, gooey, tar-like petroleum product, is a common waterproofing component. Another is bentonite clay, which expands when it gets wet to form the waterproofing. The first green roof I ever worked on, around 1980, was waterproofed with bentonite clay sandwiched between pieces of cardboard. This system is still available today. Hydrotech, one of the biggest green roof companies around, uses a formula of refined asphalts and synthetic rubbers.
  • Drainage layer: This is a layer that gathers the water and sends it away before waterproofing actually has to go to work. It often looks like a tray made of tiny egg-crate type pockets. It can also be used to hold water to be absorbed back up through the filter fabric.
  • Filter Fabric: The filter fabric stops roots and soil from going into the drainage layer.
  • Growing medium and/or soil: This is the layer for the gardener scientists to constantly be playing with, perfecting, and modifying for different kinds of plants.
  • Plants: The easiest plants to grow on a roof are from the succulent family, sedums. They take the least amount of growing medium and water. But if you have the depth, many other plants will happily take root.


Warranties on the waterproofing are going to depend on how it was applied, by whom, and with what other components. I know some of Hydrotech’s roofs that use their waterproofing are up to forty-five years old now and doing fine, but I don’t know the specific warranties they carry.

Many companies require you use their installers, which can be very pricey. Some go so far as to require you use their plant mix. So after a very long introduction, I can’t really answer your question. You need to take it up with the roofing company of your choice.


For more information:

Read Barbara Collins's Q&A "NYC has a 'cool roof' program. Is there a white paint or coating to make my roof cooler?"

Tagged In: green roof

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