It happens: snow loads trash them, historic commissions don’t allow them, or architects and clients simply don’t like the way they mess up clean roof lines. What do you do when you can’t use gutters to manage all that water coming off of roof eaves or valleys?
Underground water barrier
An underground roof is a deflecting surface just below grade that slopes away from the building and directs all that roof water away from the foundation. Any impervious sheet material (heavy-duty polyethylene, rubber membrane, rigid insulation) at least 3 feet wide is placed along the foundation about 8 to 12 inches below grade and sloped at anywhere from a 2:12 to a 4:12 pitch. (In a cold climate, using rigid foam insulation has the additional benefit of warming the soil, even if just a bit, beneath the insulation and next to the foundation.) Well-draining soil is placed over the sheet material up to grade, where a large-diameter topping material, such as pea stone or wood mulch, helps to break up the falling water and reduce splashback to the building.
Source: Bill Rose
Although I am not completely sure that Bill Rose, a research architect for the Building Research Council at the University of Illinois Urbana/Champlain, takes full credit for this underground flashing detail, he certainly is the one who popularized it. Rose has used this for years for water management on historic buildings; you can find more information on his detail in both his seminal book, Water in Buildings, and a 1997 Fine Homebuilding article, “Details for a Dry Foundation”.
This approach to handing roof water does affect what types of plantings you can use around the perimeter of any building, but it’s a good idea to keep that organic material (and substantial plantings) away from the structure anyway. How or even if the barrier is fastened or adhered to the foundation wall may not be all that critical; it can depend on how well the soil drains, how far the roof eave extends, and how far the roof load lands away from the foundation. Even if the leading edge of the barrier is pinned by soil material against the foundation wall, all — or certainly most — of the roof water will follow the sloped underground barrier away from the foundation. The underground roof requires little to no maintenance and is cost-competitive with gutters and downspouts as well. As a retrofit detail, it’s mainly the digging that drives the cost.
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