Question

What's the difference between a green roof and a white roof?

Asked by Angela Corcoran, Brooklyn, NY

We need a new roof and want to go green with our renovation, but we are new to this and confused. What is a "white roof," and what is the difference between a "white roof" and a "green roof"? I want to know the advantages and disadvantages, and I also need an expert in my area to help me out! We would like to be able to walk on the structure, if possible.

Answer

Susan Wisniewski

Answered by Susan Wisniewski

Beacon, NY

Susan Wisniewski Landscape, LLC

May 2, 2008

The term white roof typically refers to a roof membrane coated with a white, water-based acrylic polymer. These roofs reflect sunlight and can help reduce energy demand during summer months.

Green roofs are planted; they also reflect sunlight but can potentially provide many additional benefits. There is evidence that green roofs can:

  • reduce storm-water runoff,
  • provide habitat,
  • extend the life of a roof membrane,
  • increase insulation and energy efficiency, and
  • provide aesthetic value.

How effective a green roof is at any of these things is dependent on a number of variables and part of a lively debate.

White roof

A white roof would be a good option for you if you cannot add a green roof or if the cost to retrofit your roof is prohibitive.

  • The white surface is a paint that dries like rubber and can be rolled or sprayed on the roof membrane.
  • The process is relatively inexpensive and can significantly reduce summertime energy costs.
  • It can also extend the life of your roof membrane and therefore reduce landfill.
  • There is a slight increase in wintertime heating costs, but that is greatly outweighed by summertime savings. Even for those who don't have or don't choose to use air-conditioning, a white roof can improve the comfort of a home's occupants during the summer.

White roofs can also improve health (especially for the elderly), increase roof life, reduce heat-island effect, and improve indoor air quality.

Green roof

There are basically two categories of green roofs—intensive and extensive.

  • Extensive green roofs have a substrate (planting medium) depth between 0.8 and 6 inches and are most frequently planted with a mixture of sedums. Maintenance is generally minimal, although still required, especially during the establishment period.
  • Intensive green roofs have a substrate that is at least 6 inches deep, allowing you to grow a greater variety and larger-sized plants. These roofs need to be maintained like any garden, and weight-load considerations are a greater concern.

There are many ways to design a green roof and numerous companies that offer their own alternatives. But in general the construction of a green roof includes a waterproof membrane, a root barrier and drainage layer, then the substrate, and finally the plantings. Getting enough water to the plants and providing good drainage are two main concerns.

One of the easiest green roof constructions is available from a company called GreenGrid. They have devised a system of shallow, flat trays that contain all of the components—including plants—needed for a green roof. The trays are delivered preassembled. You just arrange the trays on your roof.

New York City

In New York City there are some great examples of green roofs.

  • Start at Earth Pledge—they are trying to raise awareness about green roofs in the City and have one you can ask to see firsthand.
  • Then check out the Greenroof Directory, a website loaded with information.

Since you live in Brooklyn, chances are your building is at least 50 years old and the roof was not designed for the load of a green roof or pedestrians.

Consult a structural engineer or green roofing contractor to see what your options are.

 

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: roofing, green roof, white roof

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