A.Builder Mike Guertin responds: First, check the chimney manufacturer’s printed specs to determine the minimum clearance between the chimney and the framing. In most cases, the minimum clearance will be 1 or 2 inches; if uncertain, err on the conservative side.
Frame a wooden box to match roof pitch. I usually build two side walls from 2x3s or 2x4s, and just use sheathing to tie the walls together at the uproof and downroof sides. Make the overall height of the box 6 inches short of the pipe termination exclusive of the weather cap. This leaves enough of the pipe exposed to install the weather collar...
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