A.Scott McVicker, S.E., of McVicker Associates, Inc., Structural Engineers in Half Moon Bay, Calif., responds: Let's look at the second condition first (see illustration, right): The vent holes in the diaphragm blocking are positioned at the top of the blocks, so they clear the insulation. As far as strength goes, the 2-inch-diameter holes will reduce the shear transfer ability of the block, but not by so much that typical diaphragm nailing will be affected.
Here's a sample calculation, assuming that the horizontal shear strength of the wood is 63 pounds per square inch, with a one-third increase allowed for dynamic (seismic or wind) loading. If the 2-by rafters are spaced 24 inches on-center, the blocks are 22 1/2 inches long. Assume that for venting you need three 2-inch-diameter holes spaced...
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