Q:
The exterior of our house is insulated with 2 in. of rigid foam topped with 3⁄4-in.-thick furring strips and clapboard siding. What’s the best way to install a ledger board so that it sticks out beyond the foam without being structurally problematic?
Karen via GreenBuildingAdvisor.com, None
A:
Project House editor Justin Fink replies: A freestanding deck—that is, a deck that is fully supported by piers rather than relying on a ledger connection— is my first suggestion. This eliminates the thermal and structural worries of attaching a ledger over exterior foam. That said, a modified ledger design is also an option.
The drawing—which shows a simplified version of a detail from the Cold Climate Housing Research Center’s free publication Remote, A Manual—demonstrates a solution that relies on blocks rather than on a traditional ledger board. The benefit here is that you’re limiting the amount of exposed wood, thereby reducing the amount of thermal bridging from the exterior to the interior.
As in all cases of structural load requirements, an engineer should be consulted to determine the block size, spacing, and specific fasteners to fit your design. Also, any fasteners that extend from the exterior through the wall, and that are exposed inside the stud bays, should be covered with foam insulation on the interior side to avoid condensation or icing problems.
I would not block just to the end joist blocking only. Sometimes are not properly nailed. I would install your blocking as low as that least the Mud sill. That will prevent rotation. Pitching the block is a good idea however we would always use a Z bar or drip edge. I would also recommend, strongly, a gap between The Ledger and other framing members to prevent water from collecting and creating dry rot situations. Unistrut Square washers work nicely galvanized.