Joe B is building what he hopes will be a Passivhaus-certified home in Port Washington, New York, a town on the north shore of Long Island in Climate Zone 4. The house is well underway, but Joe worries about the potential for trouble in a very complex roof design.
In a post at GBA’s Q&A forum, Joe runs down the basics of his architect’s plans. Roughly 70% of the roof is pitched at 5-in-12, with the remaining roof area all but flat — with just enough of a slope to drain water. Three pitched sections form a U-shaped perimeter to the low-slope section, with with water draining toward the eave where it will run past a chimney (see Image #2, below).
Pitched roof sections are being framed with 11 7/8-inch laminated veneer lumber (LVL) rafters and covered with Zip sheathing.
“We are planning to spray 3 inches of closed-cell foam between the rafters and under the Zip sheathing and the balance (about 9 inches) of the rafter cavity will be filled with open-cell foam (to save on cost),” Joe writes. As required by the manufacturer, he adds, the sheathing will get a band of peel-and-stick waterproofing membrane at the perimeter of the roof, in the valleys, and at the overhangs. That will be followed with a vapor-permeable roof underlayment called Delta-Foxx, and, finally, Vermont slate shingles.
The 1,000-square-foot low-slope area consists of 3/4-inch AdvanTech sheathing over 9 1/2-inch LVLs. Beneath the roof deck will be 3 inches of closed-cell spray foam and another 6 inches of open-cell foam. The entire surface will be covered with Grace Ice & Water Shield. That will be followed by a 4-inch layer of rigid foam insulation, and another layer of tapered foam insulation that will maintain the roof pitch at 1/4 inch per foot for drainage.
The low-slope roof will…