How to install a dormer vent on a comp shingle roof
Drive a large nail from your attic to mark the location of your dormer vent.
Mark your roof using the dormer vent and nail as a guide
Cut out the shingles with a roofing knife using your marks as a guide.
See step 5 (Leave space for water flow)
Cut out the sheathing using a power saw. (Avoid cutting the shingles as this will dull the blade instantly.)
Remove any fasteners to prevent vent from sliding under shingles. Slide in dormer vent
Be sure shingles are cut back 1″ to 1-1/2″ to allow water to run freely from side of dormer vent as indicated below
Install vent when roof is cool to prevent scaring shingles and allow shingles to be separated. Water can be sprayed on roof to cool shingles. Use a flat roofing bar to remove nails.
Use a roofers knife with hook blades or diamond shaped blades to cut shingles.