We are replacing our roof. Our current attic ventilation consists of two large gable vents and many soffit vents on a gable roof. The roofing company would like us to add 5 box vents to the roof. Do gable vents work with box vents? I’m concerned the box vents will interfere with our current setup. I would rather not make additional holes in the roof unless box vents are better? Thanks for the help!
John Stout with Go Roof Tune Up™, answered:
Thank you for your question. You are very wise to address this now before the installation of the new roof.
The general rule is that you have 1 square foot of attic ventilation for every 300 square feet of attic floor. The attic ventilation should be divided to 50% low and 50% high. This is an easy method for field calculations. It is very simple and has been used in some form by estimators, engineers, building inspectors, etc. for many years. Lomanco Vents (lomanco.com) has used this basic method of calculation to determine the air flow requirements for residences.
It is very important to have vents at 50% high (gable) and 50% low (soffit). Do a calculation of square feet of soffit vents. Then add the square feet of the gable vents. If there is at least 1 square foot for every 300 square feet of attic floor space, and they are half and half, you are right on the mark. This will give you a calculation to decide what you need to do for proper ventilation. There are variations of this depending on the climate zone you are in. Call the local Building Department for that information. Or visit the Lomanco website, lomanco.com.
Increasing the amount of ventilation at the time of re-roofing is fairly simple. Not so simple when the roof is on.
I hope this information is helpful to you. At least it should inspire conversation with your roofing contractor.
Thank you for contacting us,
Your friends at AskARoofer.