Roof Ventilation Explained: Intake, Exhaust and the 1/150 Rule

Roof ventilation, more precisely attic ventilation, is outside air flowing through the space just under the roof deck. Air comes in through intake vents low on the roof, usually in the soffits under the eaves, and leaves through exhaust vents near the ridge. ARMA, the asphalt shingle makers' trade group, explains that it works because warm air rises: hot, moist attic air escapes at the top and cooler outside air is drawn in at the bottom ².

It matters for the roof itself. ARMA says proper ventilation moves heat and household moisture out of the attic, helps the roofing materials reach their full service life and may reduce ice dams in snowy climates ¹. Without it, ARMA lists premature failure of the roof system, buckled shingles from deck movement, rotting wood, moisture in the deck and insulation, and ice dams ¹.

Intake and exhaust have to balance

An exhaust vent with no intake, or intake with no exhaust, moves very little air. ARMA's guidance ²:

• Put intake vents at the eaves or the lower part of the roof, and exhaust vents at the ridge or upper part.
• Balance them: intake should provide 50 to 60% of the open vent area and exhaust 40 to 50%, with intake always at least equal to exhaust ².
• Keep the path open from eave to ridge in every rafter bay; incorrectly installed insulation is one blockage ARMA names.
• Do not mix different types of exhaust vents over the same attic space, which can short-circuit the airflow.

NRCA, the roofing contractors' association, likewise suggests balancing ventilation between the eaves and the ridge, with about 50% of the vent area at or near the ridge ⁴. It gives a common example of a short circuit: adding a ridge vent while leaving existing vents at or near the ridge in place ⁷.

Ridge vents, box vents and the rest

• Ridge vents run along the peak. ARMA says eave and ridge vents, where used, should be continuous along the full length of the eave and ridge ².
• Box or static vents are individual openings near the ridge. ARMA says they should be equally spaced and close enough together to ventilate the whole attic ².
• Soffit and eave vents are the intake. ARMA notes they are often on the underside of enclosed eaves ⁵.
• Gable vents sit in the end walls. ARMA warns that gable vents combined with ridge vents, without enough soffit intake, may pull moisture into the home in heavy rain or snow ⁶.

Which mix suits a house depends on its roof shape, so a roofer should explain why the vents quoted fit your attic.

What the building code asks for

The ventilation rules are in section R806 of the International Residential Code. As quoted in East Point, Georgia's guide to the 2018 IRC ³:

• R806.1: enclosed attics and enclosed rafter spaces need cross ventilation, with openings protected against rain and snow and covered with corrosion-resistant mesh with openings of 1/16 to 1/4 inch.
• R806.2: the total net vent area must be at least 1/150 of the area of the space ventilated. It may drop to 1/300 if 40 to 50 percent of the required area is in the upper part of the space, at least 3 feet below the ridge, with the rest at the eaves, or if a vapor barrier rated at 1 perm or less is installed on the warm-in-winter side of the ceiling.
• R806.3: insulation must not block the airflow at eave vents, with at least a 1-inch space between the insulation and the roof sheathing at the vent.

In practice, 1/150 means about 1 square foot of net vent area for every 150 square feet of attic floor. ARMA calls 1/150 the long-standing standard recommendation, and says the codes generally allow 1/300 when features such as balanced ventilation with vapor retarders are present ¹. ARMA also notes that in climate zones 6, 7 and 8 the 1/300 reduction depends on an appropriately installed vapor barrier ⁵. Your building department enforces its own edition, so check its version.

Why it matters in winter and summer

ARMA notes that an average family of four generates about 2 to 4 gallons of water vapor a day from breathing, showering, cooking and dishwashing ⁶. In a cold attic that vapor can condense on the underside of the deck, damaging the wood and wetting insulation ⁶. Ice dams form when heat leaking into the attic melts snow over the warmer part of the roof and it refreezes at the cold eave; ventilation reduces that heat ⁵.

NRCA is more measured about summer. It says the strongest case for ventilating asphalt shingle roofs is in cold climates where snow sits on roofs for long periods, that ventilation lowers average attic air temperature in summer, but that research has not verified a significant effect on the roof's surface temperature ⁴. NRCA also notes that shingle makers' warranties may require minimum ventilation ⁴.

Reroofing is the time to fix it

ARMA recommends replacing the vents in the field of the roof, such as static and ridge vents, when reroofing, and says soffit and gable vents can stay if they still work ². It also notes that some building codes require ventilation to be brought up to code levels during a reroof ². Costs vary with the house: Fixr reports that hot climates may need additional roof vents or insulation at $100 to $2,000 ⁸, and lists vent repair at $75 to $250 ⁹.

Questions to ask the roofer

• How much intake and exhaust does the attic have now, and how much does the quote add?
• Are the vents balanced, with at least as much intake as exhaust?
• Are any old vents being left in place that could short-circuit a new ridge vent?
• Are the soffits actually open behind the vent covers?
• Does the shingle maker's warranty set ventilation requirements?

Ventilation works with the underlayment and flashing below the shingles; types of roofing materials covers the covering above. When you are ready, you can compare quotes from roofing companies and ask each to show its ventilation plan.

About these figures

• ¹ Reported: ARMA on why ventilation matters, problems from poor ventilation and the 1/150 and 1/300 ratios, checked November 11, 2026.
• ² Reported: ARMA attic ventilation best practices, including the 50 to 60% intake and 40 to 50% exhaust balance and replacing field vents when reroofing, checked November 11, 2026.
• ³ Official record: City of East Point guide quoting 2018 IRC sections R806.1 to R806.3 (1/16 to 1/4 inch mesh, 1/150 and 1/300 ratios, 40 to 50 percent upper vents at least 3 feet below the ridge, 1-perm vapor barrier, 1-inch insulation clearance), checked November 11, 2026.
• ⁴ Reported: NRCA consumer answers on balanced ventilation with about 50% at the ridge, cold-climate benefits, summer research findings and warranty conditions, checked November 11, 2026.
• ⁵ Reported: ARMA on attic ventilation system selection, the climate zone 6 to 8 vapor barrier condition and how ice dams form, checked November 11, 2026.
• ⁶ Reported: ARMA ventilation fast facts, 2 to 4 gallons of water vapor a day for a family of four, condensation and the gable-and-ridge vent caution, checked November 11, 2026.
• ⁷ Reported: NRCA's Professional Roofing article on reroofing, including the ridge-vent short-circuit example, checked November 11, 2026.
• ⁸ Reported: Fixr roof replacement cost guide, additional vents or insulation in hot climates at $100 to $2,000, updated February 4, 2026, checked November 11, 2026.
• ⁹ Reported: Fixr roof repair cost guide, vent repair at $75 to $250, updated January 21, 2026, checked November 11, 2026.