The rule of thumb is one square foot of vent for every 300 square feet of attic floor, split evenly between intake and exhaust.
At a glance
- Vent area means net free area, not the size of the hole.
- Intake at the eave matters more than most people think.
- Too much exhaust with no intake pulls air from the house.
- Never mix a ridge vent with powered fans on one attic.
The 1 to 300 rule
Take the attic floor area in square feet and divide by 300. That gives the total net free vent area needed in square feet. Half of it goes low as intake and half goes high as exhaust.
For a 1,800 square foot attic the math gives 6 square feet of vent, which is 864 square inches. Split it and you need 432 square inches low and 432 high.
Net free area
Screens and louvers block airflow, so every vent product publishes a net free area number in square inches. A ridge vent might give 18 square inches per linear foot. A soffit vent might give 9 square inches each.
In the example above, 432 divided by 18 means 24 feet of ridge vent, and 432 divided by 9 means 48 soffit vents. Use the product numbers, never the size of the cut.
Why intake gets skipped
Exhaust is easy to add from the roof. Intake means working at the eave, and on many Los Angeles homes the soffits are stucco with a few small screened holes or none at all.
When exhaust runs without intake, the attic pulls make up air through ceiling gaps and can drag conditioned air out of the house. The fix is more intake, not more fans.
What to do on a tile roof
Tile roofs breathe a little on their own through the gaps under the tile, but that is not a plan. Use vented ridge products made for tile, plus eave vents behind the bird stop.
On low slope sections with no ridge, static vents spread across the upper third do the job.
Common questions
Can a roof have too much ventilation
Unbalanced ventilation causes problems. Total area at the rule of thumb, split evenly, is the target.
Do solar attic fans help
Only with real intake below them. Without it they pull air out of the living space.
How do I know my soffits are blocked
Look up into the eave from a ladder on a bright day. If you cannot see light or screen, they are packed.