A.Tim Uhler, a lead framer for Pioneer Builders in Port Orchard, Wash., responds: According to the IRC, rafters and ceiling joists need to have at least 1 1/2 inches of bearing when supported by wood or metal (R802.6, 2006 IRC). While this clearly establishes the horizontal seat cut's minimum length, its maximum length varies according to the size of the rafter and the pitch of the roof.
For example, the IRC allows the ends of rafters to be notched as much as one-fourth their depth (R802.7.1), but on a shallow-pitched roof this can result in a seat cut that's wider than the top plate. When that happens, roof loads are carried by the toe rather than the heel of the rafter, reducing the rafter's bearing capacity and increasing...
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