The Lungs of Your Attic
Attic ventilation is critical to the lifespan of a house. In the summer, solar radiation bakes an unventilated attic to 150°F, forcing your air conditioner to run continuously and literally cooking your asphalt shingles from underneath. In the winter, warm, moist air from your bathrooms rises into the freezing attic, condenses into liquid water on the cold roof deck, and causes massive mold growth and wood rot.
A modern roof must "breathe."
The most efficient exhaust system ever invented for residential roofing is the Ridge Vent. It is a continuous plastic or mesh vent installed along the absolute highest peak (the ridge) of the roof. Because heat naturally rises, the hottest air in the attic effortlessly escapes out the top of the ridge, pulling cool, fresh air in through the soffit vents at the bottom.
Calculating Ridge Vent Sections
Unlike asphalt shingles that come in bundles, ridge vents are physical plastic units that are nailed directly over a slot cut into the roof decking.
They are almost universally sold in standard 4-foot sections.
Calculating how many sections you need is the easiest math in roofing, but understanding where to install them requires specific rules.
The Formula
- Measure the total linear length of the highest horizontal peak (the Ridge) of your roof in feet.
- Determine the length of the vent product you are buying (usually 4 feet).
- Divide the Ridge Length by the Vent Length.
- Round up to the nearest whole piece.
Total Ridge Vent Pieces = Roundup(Ridge Length ÷ Vent Length)
Example Calculation
You have a simple gable roof. The ridge running across the top of the house is 38 feet long. You are purchasing standard 4-foot rigid plastic ridge vents.
- Divide:
38 ÷ 4 = 9.5 pieces - Round up: 10 pieces
You will purchase exactly 10 pieces. During installation, you will install 9 full pieces, and cut the 10th piece down to 2 feet to finish the run.
The Edge Gap Rule
While you calculate based on the total ridge length, roofing best practices dictate that you do not run the ridge vent all the way to the absolute edge of the house.
You should leave the last 12 to 24 inches of the ridge unvented on both ends. This prevents driving, horizontal rain from blowing directly into the end of the vent during severe storms.
Therefore, on a 38-foot ridge, you might only cut a 34-foot slot in the plywood. You still buy 10 pieces of vent to cover the whole 38 feet aesthetically, but only the middle 34 feet are actively ventilating.