The idea of grass-topped rafters isn't new, as any Swiss miss
can tell you. But the technology's been updated in recent years
to serve specific purposes, like controlling runoff. In
hard-paved cities with little or no absorbent ground cover,
municipal sewer systems can be overwhelmed by excess flow
following a hard rain. A green roof absorbs 50 to 75 percent of
the rainfall that strikes it, significantly diminishing the
amount of water that reaches the gutter.
Roof pitches of up to 12/12 can be green,
although the system's complexity and cost may increase
accordingly. Outside Stuttgart, Germany, a contemporary-style
house with a verdant roof updates the archetypal thatched
cottage (top). A private rooftop garden — also near
Stuttgart — puts to rest any argument that a flat roof is
an ugly roof (above). And in the Seattle area, another lift of
growth medium hits the deck (right).
That's not the only big-city problem that green roofs can
alleviate. Urban centers tend to be hotter than surrounding
areas, partly because of the cumulative acreage of black
asphalt rooftops that soak up solar heat and radiate it outward
into the air and downward into the buildings. A green roof
maintains a lower surface temperature, reducing a structure's
heat gain by as much as 50 percent.
Other benefits — not necessarily proven — include a
30-year typical roof-membrane life; quieter interiors; and
reclaimed outdoor living space.
In the U.S., commercial projects claim the lion's share of
green-roof installations, but Europeans have embraced the
technology on a broader scale. In Germany, for example, 7
percent of all new flat roofs, both commercial and residential,
are said to be green, with a total of 140 million square feet
covered to date.
Heads up — we may be looking at a growth