How to Install a Ridge Vent

Two forces move air out of a ridge. Buoyancy: warm, moist attic air rises and leaves through the highest opening, drawing replacement air in at the eaves. And wind: air moving up the slope accelerates across the ridge, and a well-shaped vent turns that into suction.

Both depend on a low opening to pull from. A ridge vent with no working intake is a hole in the roof that vents very little — the reason so many homeowners report theirs “did not do anything.”

1. 1

   Confirm intake before you cut anything

   Look at the soffits from inside the attic. If insulation is packed into the eaves, or the panels are decorative rather than perforated, the vent you are about to install has nothing to draw from. Fix that first or you are installing an ornament.

2. 2

   Strip the ridge and snap your cut lines

   Remove the existing ridge caps and pull the nails. Snap a chalk line down each side of the ridge at the offset the vent manufacturer specifies. Slot width varies by product — the instructions state it, and that number governs, not a rule of thumb from another job.

3. 3

   Set the blade depth to the sheathing thickness

   The step that separates a clean job from structural damage. Set the circular saw to cut just the sheathing — 7/16, 1/2 or 5/8 of an inch plus a hair. Too deep and you are notching the ridge board, the truss top chords or the rafter tips, the length of the roof, invisibly.

4. 4

   Cut the slot, and stop short of each end

   Run both cuts and stop six to twelve inches shy of each end so the end sections land on solid deck. A stick-framed roof has a ridge board in the way — you cannot cut through it, so the slot becomes two openings, one each side, and the open area is the sum. A truss roof usually has no ridge board and the opening runs continuously; watch for the gusset plates at the peaks.

5. 5

   Clear the slot

   Drop the cut sheathing into the attic and pull it out, then clear the opening of nails, felt tags and blocking. A slot half-blocked by a strip of underlayment looks finished and vents at half capacity.

6. 6

   Set the vent, then fasten with nails that reach

   Lay the vent centered on the ridge, joining sections the way the instructions specify. Use nails long enough to pass through the vent, the cap shingle and the sheathing and still get a solid bite — usually two to three inches depending on the profile. Short nails are a quiet failure: everything looks right and the run lifts in the first real wind. Do not over-drive either; crushing the vent closes the opening you just cut.

7. 7

   End plugs and cap shingles

   Close both ends of the run with the manufacturer’s end plugs so rain, snow and insects have no straight shot in. Cap with ridge shingles at the correct exposure, lapping away from the prevailing wind, and finish the last cap with a concealed fastener and a dab of sealant.

• ●Shingle-over rigid vent. Molded plastic sections, usually around four feet, with an internal weather filter and a shaped outer profile. Best combination of open area and weather resistance, and the default on quality work.
• ●Rolled or mesh shingle-over vent. A roll of nylon or polyester matrix. Cheap, fast, and it follows a curved or hipped ridge that rigid sections cannot. Many have no external baffle, and the mesh is easy to compress while nailing.
• ●Metal ridge vent. For metal roofs, sized to the panel profile and used with matching closure strips. See metal roof underlayment for how the rest of that assembly needs to be detailed.

The external baffle

The one design feature worth understanding is the raised lip along the outer edge of the vent. Wind coming up the slope hits that baffle and is deflected up and over the opening, which drops the pressure directly above the vent and pulls air out of the attic. The same deflection throws wind-driven rain and blowing snow over the top instead of into it.

A vent without one is a passive slot: it exhausts less in wind, and in a driving rain the wind can push water straight at the opening. When someone tells you ridge vents leak, they are usually describing a baffle-less vent on an exposed roof. Compare products on the baffle before you compare them on price.

Exhaust does not ventilate an attic. Air moving through it does, and air only moves if it can get in somewhere low. Cut a ridge open on an attic with blocked soffits and it finds make-up air along the path of least resistance — the house below, through recessed light housings, the bath fan, the top plates and the attic hatch. You have installed a device that gently exhausts conditioned indoor air into the roof.

• ●Aim for balance: roughly half the net free area as intake at the eaves, half as exhaust at the ridge.
• ●If you cannot balance it, favour intake. An intake-heavy attic is slightly pressurized, so it pushes rather than pulls on the ceiling below. Exhaust-heavy is the failure mode.
• ●Install baffles at every eave so blown or batted insulation cannot choke the soffit. Blocked soffits are the most common reason a correctly installed ridge vent underperforms.
• ●The 1/150 and 1/300 ratios are the math: one square foot of net free area per 150 square feet of attic floor, reduced to one per 300 where intake and exhaust are properly split or a vapor retarder is present. Which applies depends on your adopted code edition and the assembly.
• ●Compare published net free area, not hole size. Vents are rated in square inches of open area per linear foot, and two products the same size can be quite different.

A ridge vent works because the whole attic feeds it from below. Add a second exhaust opening high in the same space and you give the air a shorter path. Short paths win.

• ●Ridge vent plus open gable vents. The ridge pulls straight from the gable instead of the eaves. Air moves, and the far corners of the attic never turn over.
• ●Ridge vent plus a powered fan. Worse. The fan pulls from the nearest opening, which is the ridge vent, so the vent runs backwards as intake and the attic is barely involved.
• ●Ridge vent plus box vents on the same slope. The upper boxes short-circuit into the ridge. If the ridge is genuinely too short, boxes can supplement — but that is a calculation, not a default.

Ventilation manufacturers generally say to close or baffle gable vents when a ridge vent goes in. What nobody argues is that a powered fan belongs on the same attic as a ridge vent.

Turbines get more scorn than they deserve. A spinning turbine moves real volume, they cost little, and on a shed, a shop or a hip roof with almost no ridge they are a sensible answer. The honest criticisms: they draw from one point rather than along a line, the bearings are a maintenance item on a roof nobody visits, and they get noisy with age.

Powered attic fans are where the professional consensus really has landed. A fan strong enough to matter depressurizes the attic, and if the ceiling below is not airtight — almost none are — it pulls conditioned air up out of the house and humid outdoor air in behind it. In the worst case it backdrafts a combustion appliance. The cooling energy it saves is frequently less than the energy it costs.