What happens on the roof
The vent priced here is the shingle-over kind: a low, continuous strip, made as rigid plastic sections or as a roll of fibrous mat, laid over the peak and covered with ordinary cap shingles so the ridge looks unchanged from the street. Better ones have a raised baffle along each edge that turns wind up and over the vent, helping keep driven rain out.
The crew strips the old cap, then runs a circular saw along each side of the peak with the blade set to the deck thickness, so it cuts sheathing and not rafters. The slot width comes from the vent maker, and it is wider on a stick-framed roof with a ridge board, because the board blocks the middle of the opening. The cut stops short of the gable ends and of any chimney or hip junction, so both ends of the vent sit on solid deck.
The vent is nailed over the slot and new cap shingles go on top, fastened with nails long enough to pass through cap and vent into the deck. Old cap shingles rarely survive being pried up, which is why the estimate prices fresh ones at $63 to $126, and why a new ridge can stand out against weathered field shingles. The sealant covers the exposed nail heads at each end.
Not in the figure: intake vents, closing off old exhaust vents, and deck repair. An attic poorly ventilated for years often has soft sheathing at the peak, and finding it turns part of the job into roof decking repair.
Without intake, a ridge vent works backwards
A ridge vent is an exit. Warm air leaves through it only while cooler air comes in through the soffits to replace it. If the soffits are solid, or buried under insulation on the attic side, the ridge vent still draws, but from the rooms below: through gaps around ceiling lights, the attic hatch and holes drilled for pipes and wires. That air is warm and humid, and in winter it condenses on the cold underside of the deck the vent was meant to dry.
The crew priced here looks at the soffits as its last step, but looking is not fixing. If the eaves are closed, add soffit vents and baffles to the job, aiming for at least as much intake area as exhaust.
One kind of exhaust per attic
Box vents, a turbine or gable louvers left beside a new ridge vent compete with it. Air takes the shortest path, so the ridge vent draws through the nearest box vent a few feet below rather than from the soffits, and the lower attic stops moving air.
When a ridge vent goes in, the other exhaust openings on that attic should normally be closed. A box vent is removed, the hole patched with plywood and shingled over; a gable vent is usually sealed from inside. Neither is priced here, so ask for removal to be quoted per vent.
When the ridge is too short
A ridge vent can only exhaust as much as its length allows. Shingle-over vents are commonly rated at around 18 sq in of net free area per foot, though the product data gives the real figure. IRC R806.2 asks for 1 sq ft of net free area per 150 sq ft of attic floor, reducible to 1 per 300 under certain conditions, with roughly half of it as exhaust. On a 50 by 30 ft house at the 1 per 150 ratio, that is 1,440 sq in in total and about 720 sq in of exhaust: some 40 ft of ridge vent.
Built as a gable, that house has a 50 ft ridge and room to spare. Built as a hip roof with the same pitch on all four sides, its ridge is only 20 ft, the length minus the width, and the slot has to stop short of the hips. A ridge vent alone falls well short. Some roofers top up the short ridge with box vents set as close to the ridge line as the shingling allows; others, keeping to one exhaust type, switch the whole attic to box vents. Ask which a quote proposes and how the numbers were reached; the roof ventilation calculator gives the exhaust area your attic needs.
Now, or with the next roof
The cheapest time to add a ridge vent is a re-roof. The old cap is coming off anyway and the cap shingles are already in the order, so the vent adds little beyond its own price. This estimate prices the retrofit: a crew out for the ridge alone.
Retrofit now if the roof has years left and the attic shows the symptoms: frost or dark staining on the underside of the deck in winter, or an attic far hotter than the outdoors on a summer afternoon. If the roof is due within a few years, wait and specify the ridge vent with it.
On a written quote, look for the vent length, the product and its net free area per foot, whether the new cap is matched to the roof, what happens to existing exhaust vents, and whether anyone checked the soffits. Ask about the last two before you sign.