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 and the cap.
Assuming shingle or clapboard siding, install a 1x2 furring
strip around the perimeter of the box flush with the top
Fasten the box to the roof framing, preferably with screws
or lag bolts. Install blocking between the roof trusses or
rafters if necessary, so the screws bite into something. Check
to be sure the box is plumb and square at the top. If
necessary, frame in a cricket on the uproof side.
Install step flashing and roof shingles as usual. Install
the siding, and then cover the 1x2 furring at the top with a
1x3 or 1x4 piece of finish trim all around the box. Measure the
top of the trimmed box, taking dimensions carefully. Make a
sketch locating the chimney within the perimeter. Note the
actual chimney diameter; the chimney hole diameter should be
1/4 inch larger than the pipe.
Make or have made a galvanized sheet metal cap with a
separate 4- to 6-inch tall collar that slips over the chimney.
I prefer to use 16-gauge material because it’s more rigid
than 20 gauge. Diagonally crease the metal cap in both
directions before installing the collar. The creases add
rigidity to the cap and give a hip effect to shed water. The
edges of the cap should fold over the trim about 2 inches, with
a 1/4-inch drip kick along the bottom edge.
Snip and bend tabs from either the top or the neck for a
Although some builders secure the tabs with pop-rivets and
seal the seams with high-temperature silicone, I am doubtful
that a caulked joint will be durable. It’s best to braze
or solder the joints in the galvanized metal.