How do you install a ridge vent on an existing roof?
Short answer. On an existing roof, the old ridge caps are removed, the framing under the ridge is checked from the attic, a slot is cut with the saw set just to the deck thickness, and the vent and new caps go on. The extra steps on a retrofit are closing off old box vents or fans, confirming soffit intake, and working at temperatures where the existing shingles will not crack or scuff.
Retrofit specific steps
- Check the attic: is there a ridge board or do the rafters meet at a point? The slot width depends on it. Note any wiring or lights near the peak.
- Remove the old caps carefully, without tearing the top course of field shingles.
- Snap lines for the slot to the vent's specified width, stopping short of gable ends and walls.
- Cut with the blade set to the deck thickness only, so it does not cut rafters, the ridge board or wires.
- Clean the slot of debris and old felt.
- Install the vent and caps with nails long enough to penetrate the deck.
The rest of the retrofit
- Old exhaust: box vents, turbines and fans on the same attic should come out, with the holes decked and shingled.
- Gable louvers: usually closed from inside.
- Intake: confirm soffit vents are open and baffled, or the new ridge vent will not move much air.
Working on older shingles
Older shingles get brittle. Work on a mild day, not in the cold when they crack or the afternoon heat when they scuff. New cap shingles may look different from weathered field shingles, and patches where old vents were removed can show.
When to wait
If the roof is close to replacement, the retrofit may not pay off. Read ridge vent vs box vents and signs you need a new roof.