Roof ventilation calculator: give your roof room to breathe.

The calculator needs one measurement: the floor area of the attic, which is the ceiling area of the rooms below it, not the larger area of the sloping roof. Multiply length by width; a 60 by 49 foot attic is 2,940 square feet, where the slider starts. Then pick the ratio: 1:300 if the roof has, or will have, exhaust near the ridge balanced by intake at the eaves, and 1:150 if it does not or you are not sure.

The result is the total net free area your attic needs, in square inches. Net free area is the open space a vent lets air through once its screens and louvers are counted, and vent makers print it on the product. The calculator divides the attic floor by the ratio and multiplies by 144 to turn square feet into square inches.

Worked through for the starting attic on the balanced ratio: 2,940 ÷ 300 = 9.8 square feet, and 9.8 × 144 = 1,411 square inches. Now split it. The code wants 40 to 50 percent of that as exhaust high in the attic, so about 565 to 706 square inches near the ridge, and the rest, 706 to 847 square inches, as intake at the eaves. With a ridge vent rated at 18 square inches per foot, a typical published figure, 565 square inches is a little over 31 feet of ridge vent. The products on your estimate carry their own ratings, so an honest estimate names and counts them. On the 1:150 ratio the same attic needs 2,822 square inches, twice as much.

Both ratios are in section R806 of the International Residential Code, which Texas adopted as the residential building code for its cities (Local Government Code section 214.212); cities may adopt later editions and amendments, so check yours. In a vented attic the code requires cross ventilation, with a net free area of at least 1/150 of the area of the vented space. The exception allows 1/300 where at least 40 percent and not more than 50 percent of the required area is provided by vents in the upper part of the attic, no more than 3 feet below the ridge, with the balance at the eaves. A second route to 1/300, a vapor retarder in the ceiling, applies only in climate zones 6, 7 and 8, the coldest parts of the country.

The code also wants at least 1 inch of air space between the insulation and the roof deck at the vents. An attic sealed with spray foam on the underside of the deck is unvented, covered separately in section R806.5, and the calculator does not apply.

An attic breathes in a loop: cool air in through the soffits, warm air out at the ridge, driven by heat and the wind across the peak. The split matters as much as the total. Exhaust without enough intake pulls air from wherever it can, including your rooms through light fixtures and the attic hatch, so you pay to cool air that leaves through the roof. Keep at least as much intake as exhaust.

A dark roof soaks up the sun all afternoon, and our ventilation guide puts a Texas attic at 140°F on a July afternoon. That heat radiates into the ceilings below into the evening, which is why the upstairs stays hot at night. Moisture is the other half: humid air that cannot leave condenses under the deck on cool nights, rusting the nails and, over years, rotting the wood.

Over a hot attic the asphalt dries out, the granules let go early, and a shingle rated for 30 years can curl in twelve; our repair or replace guide puts a Texas shingle roof's life at 15 to 30 years, and the attic decides where it lands. The warranty is the other reason to get the number right: CertainTeed's, for example, cuts any shingle over an inadequately ventilated deck to a reduced 10-year limited warranty. If your attic comes out short, fix it when the roof is replaced.

• Mixing exhaust types. A ridge vent plus gable vents, turbines or a power fan gives the air a shortcut, and the bottom of the attic never moves. Use one type of exhaust.
• Blocked soffits. Insulation pushed into the eaves or soffit vents painted over: intake on paper, none in practice.
• Counting vents instead of area. Two vents of the same size can have very different net free areas. Add up the ratings, not the holes.
• Adding exhaust to fix a hot attic. The usual fault is too little intake, and more exhaust with the same intake makes it worse.

Our attic ventilation service measures what your attic has against what it needs and writes the fix as line items. The full background is in our guide to attic ventilation requirements.