Should a ridge vent go all the way across the roof?
The look vs the opening
Ridge vents come in rolls or 4 foot sections that sit under the cap shingles. Running the vent end to end keeps the cap line straight; where the deck has no slot underneath, that stretch of vent is simply inactive. What matters for airflow is the length of open slot, not the length of vent product on top.
How much open ridge you need
Divide the attic's exhaust requirement by the vent's rated net free area per linear foot. If an attic needs about 430 square inches of exhaust and the vent is rated at 18 square inches per foot, you need roughly 24 feet of open slot. Long ranch roofs often have more ridge than they need. Hip roofs with short ridges often have less, which is when a manufacturer's hip vent or a few added exhaust vents come in.
Where the slot should stop
- Several inches short of each gable end, per the vent maker's instructions.
- At chimneys, walls and dormers that interrupt the ridge.
- Over a garage that shares a fire separation wall with the house, and over townhome party walls.
- Over cathedral ceilings with no vent channel, since there is no air space below to ventilate.
More is not always better
If the open ridge outruns the soffit intake by a wide margin, the vent pulls air from wherever it can, including the living space through ceiling gaps. Match exhaust to intake rather than maximizing either. See ridge vent vs box vents and roof ventilation basics.