"Green roof" means a roof that supports a layer of soil or growing medium, which has plants growing in it. These plants can range from small to large, depending on the depth of the soil and the strength of the underlying roof.
Underhill's walls, beams, and roof deck are designed to support a full 12" of soil - enough to grow small trees, shrubs, and other vegetation native to its woodland site.
Complementing this, the walls of the house sit deep in the hillside, so the profile of the house is very low.
There are a lot of benefits to building this way:
Just a few feet below the ground, the temperature is an almost constant 55 degrees. That's why basements are cool and comfortable on a hot summer day. By setting the house deep in the hillside, and by covering the roof with 12" of earth, we've created a home that uses less energy to cool in the summer and to heat in the winter.
LESS STORM WATER RUNOFF.
When rain falls on a typical roof, it runs off quickly, down pipes and into sewers and streams. This burdens the town's storm drains and sweeps contaminants into streams and rivers. Underhill's rooftop layer of soil and plants absorbs rainwater much like an undisturbed wooded hillside, releasing it slowly into the environment .
REDUCTION IN THE "URBAN HEAT ISLAND EFFECT."
On a hot sunny day, a conventional shingled roof (or, in the case of a commercial building, an asphalt roof) can reach 130 to 140 degrees farenheit. Multiplied by the hundreds of thousands of buildings in a city, it causes cities to be on average several degrees warmer than the surrounding countryside. This is what is known as the "urban heat island effect." Roofs covered in vegetation, on the other hand, don't get nearly so hot. This not only benefits the occupants of the building, it reduces the temperature in the surrounding area, benefitting the whole community.
Underhilll's deep earth cover will support more than grasses and other small plants. It will support small trees, shrubs, and other vegetation native to the house's woodland location. This means a full restoration of wildlife habitat right on top of the house. Some parts of the roof form an unbroken slope with the surrounding hillside. Bear, coyote, bobcat, deer, and wild turkey will consider it part of their forest home. And that's just the way we want it.
LEADERSHIP IN BUILDING.
When I applied for my building permit, the town construction official had never heard of a green roof (fortunately, he was a good guy who did his homework). And banks couldn't make the project meet their lending criteria, because they couldn't find comparable homes on which to base a valuation. But every green-roof home that gets built makes it easier for the next builder. This is especially true of a showplace home like Underhill. Nancy and I are highly motivated to show the place, hold open houses for lenders and other professionals, and spread the word about this kind of architecture.