How to Calculate Ridge and Soffit Ventilation

Updated February 21, 2017

Proper ventilation of the attic space protects it against mould growth by removing moisture quickly. It's second job is equalising the temperature of the attic to the outdoor temperature. This reduces the likelihood of ice damns forming on your roof surface in the winter. Your local building code determines the ratio of ventilation space to attic area. Consult your local building authority for the exact numbers for your area. Calculate the amount of ventilation once you know the size of your attic.

Measure the attic floor's width and length with a measuring tape.

Multiply those two numbers with the calculator. The result is the area of your attic in square feet.

Divide that number by 300 or the ratio your local building code requires. The required ratio of attic ventilation is typically 1:300. That is, you need 1 square foot of ventilation for every 300 square feet of attic space.

Divide the result by two. The resulting figure is the amount of ridge and soffit venting you need to install. Divide the amount of ventilation in the attic equally between the soffit and ridge vents to have a balanced system.

Things You'll Need

  • Measuring tape
  • Calculator
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About the Author

Bob White began his writing career in 2006. Working in sales, he was a technical writer tasked with responding to requests for proposal. White has a Bachelor of Arts in computer science and a diploma in home inspection. He has also worked in construction, landscaping and the pool industry for more than 15 years.