How to Calculate the Roof Ridge Ventilation Area of a Residential House

Measure the dimensions of the roof you need to vent if you not already know the exact number. Use a tape measure to measure one of the long sides of the roof to determine the length, and then lay the tape measure across the roof to measure the width. Multiply the two resulting numbers together to find the area of the roof in square footage. For example, a roof that’s 60 feet long and 30 feet wide would provide 1,800 total feet.

Calculate to determine the amount of ventilation a roof of your size requires. Divide the resulting number from Step 1 by 300, the minimum number required by nearly all building codes for adequate ventilation in a 1:300 ratio. Using the previously mentioned example, a roof of 1,800 feet leaves you with 6, or rather 6 total feet of ventilation area, most of which should be situated on the ridge of the roof itself and not so much in the soffit.

Calculate how many square inches of venting you need. Standard sized roof venting shingles are 18 inches in length; so, multiply 18 by the length of the roof. Using the example, 1,080 square inches of ventilation space is needed. When divided by 12 inches (1 foot), that means you need 7.5 inches according to the example numbers.

Purchase your roofing supplies according to the numbers determined by your calculation. Keep in mind all shingles – venting shingles included – are sold in bundles that number 21 to 27 in count, depending on what style of shingle you choose for your home. Use this number to determine how many bundles you need, but always have extra vents on hand should something happen to the others.