Building codes and roof shingle warranties require that sloped roofs be ventilated for the following reasons:
It lowers summer attic temperatures, extending shingle life and cutting cooling bills.
It rids the attic of excess moisture.
It reduces the chance of ice dams by keeping the roof surface cold in winter.
Most codes specify a net-free vent area (NFVA) of 1/150 of the attic floor area if the ceiling has no vapor retarder. (Or, in In Climate Zones 6, 7 and 8 you can have 1/300 as long as a vapor retarder is included. Ventilation is more effective where there's an airtight ceiling plane to prevent warm, moist interior air from getting into the attic.)
In a home with a complex roof, it's a good idea to exceed these minimums.
Typical Net-Free Vent Areas (NFVA)
Vent Area Multiplier
1/4-inch mesh hardware cloth
1/8-inch mesh screen
#16 mesh screen, with or without plain metal louvers
Wood louvers and 1/4-inch mesh hardware cloth
Wood louvers and 1/8-inch mesh screen
Wood louvers and #16 screen
For site-built or manufactured but unlabeled vents, this multiplier tells how many square feet of vent area is needed to give 1 sq.ft. of net-free vent area.
NFVA is the combined area of all unobstructed vent openings. Most manufactured venting products are labeled for NFVA. (Soffit vent strips and ridge vents usually express this as square inches per linear foot; fixed-sized units tend to list total NFVA for the entire unit.) For site-built, or for manufactured but unlabeled vents, start with the required NFVA then use the multipliers in the table to enlarge it to compensate for screens and louvers.
The building code doesn’t address—or ask roofing professionals to factor in—the actual volume of space under the roof. The volume for a 1,000-square-foot attic under a 12:12 pitch roof is not the same as the volume under a 5:12 pitch. Recommend practice calls for increasing the ventilation by 20% for roofs with a pitch from 7:12 to 10:12. For roofs steeper than that, we recommend increasing ventilation by 30%.