Can you install roof vents after the shingles are already on?
Short answer. Yes, roof vents can be installed on a roof that is already shingled. The installer cuts the opening through the shingles and deck, then lifts and unnails the shingles above so the top and sides of the vent flange slide underneath while the bottom edge lies on top, which lets water shed over it like any other shingle.
How a retrofit vent goes in
- Confirm the location from the attic, between two rafters and near the top of the slope for exhaust vents. Drive a nail up through the deck to mark it.
- On the roof, trace the opening from the vent's template and cut through shingles and decking.
- Break the seal on the shingles above and to the sides, and pull the nails that would block the flange.
- Slide the vent so the upper flange tucks under at least one course and the lower flange overlaps the shingles below.
- Nail the flange where the instructions say, renail the lifted shingles, and seal exposed nail heads with roofing sealant.
Things that make it harder
- Hot or cold weather. In Georgia summers shingles get soft and scuff easily; on cold mornings they crack when lifted. Mild temperatures are best.
- Old shingles. Brittle shingles near the end of their life can break when the seal strip is released.
- Ridge vents. These can also be added after the fact, but require removing the ridge cap and cutting a continuous slot.
- Mixing vent types. A new box vent on a roof that already has a ridge vent can short circuit the airflow.
Check intake first
Adding exhaust to an attic with blocked or missing soffit vents rarely helps. Make sure air has a way in before adding a way out. See roof ventilation basics and roof flashing explained.