Exterior Opening
To transpose the center
of the ceiling opening to the roof, locate the center of each ceiling
frame header, mark them and align a plumb bob from the underside of
the roof sheathing down to each mark. Mark the opposing points on the
sheathing and draw a line connecting them.
Use this center line as reference to transpose the recommended roof
window opening dimensions.
NOTE: Some
roof windows set in the frame and attach with brackets, while better
designed windows have a flange that allows them to set on the roof over
the frame.
With the opening marked, reinforce the rafters like the ceiling joist,
nailing same-sized lumber to the peak. Poke nails through the roof at
the corners of the drawing. Up on the roof, snap a chalkline using the
nails as guides.
Using a carbide-tip blade in a circular saw, carefully cut the opening
and finish the corner cuts with a hand or reciprocating saw.
WARNING: Because
the blade will gum up with tar and may kick back, some people prefer
to cut through the shingles and roofing with reciprocating saw. However,
with this method it's more difficult to cut a straight line. You may
opt to do waterproofing before leaving the roof (mention later).
Pry off the cut piece and returning to the attic, cut the exposed rafters
in the opening, again, allowing for the headers on each side (usually
3"). Now, install the headers and trimmer studs for the correct height
and width of the roof opening.