A Roof That'll Grow on You

Sunday, November 19, 2006

Icalled it our little Arlington brick box when we moved in four years ago.

After living in North Carolina in a new passive-solar house -- with more windows on the south side; fewer on the east, west and north; wide roof overhangs; and masonry floors -- it was frustrating to live in a regular, not-so-thoughtfully designed house.

Our heating and cooling bills were higher, and uncaptured sunlight on our south side felt like a terrible waste. Our Arlington neighborhood, Westover, is walkable and friendly, but it was obvious right away that to really feel connected to the life on our street, we needed a porch. A porch on our south-facing house would shade the house in summer, and when we eventually add windows, the concrete floor will capture the winter sun and help heat our home.

I was a graduate student in landscape design when my husband and I were planning our porch addition. When I came across the concept of green roofs in a class at George Washington University, I fell in love with the idea. Among the benefits:

· Plants on the roof help to slow, clean and cool the rainwater that eventually makes its way to the Chesapeake Bay.

· Living roofs reduce the ambient summer temperature and insulate the space below.

· A green roof initially costs more than a conventional roof, but, because the waterproof membrane is covered and protected from UV rays, it should last at least twice as long as a conventional roof.

I must admit, though, I mostly wanted it because I thought a garden on the roof would be soft and beautiful and very fun.

Our roof, which was installed last December, is called an extensive green roof, which means the specialized growing medium -- in our case, a mix of expanded shale and mushroom compost -- is only three inches deep. The soil and plants weigh about 15 pounds per square foot -- not difficult to accommodate on an existing roof (but check with an engineer) and certainly possible to include in a renovation.

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