How to Estimate Ridge Vent
Ridge vent runs along the peak of the roof to exhaust hot, moist attic air. Paired with continuous soffit vents, it creates a balanced passive ventilation system that prevents ice dams in winter and heat buildup in summer.
Ridge Vent Pieces Formula
Pieces=Ridge LengthPiece Length
Most ridge vents come in 4 ft pieces. Run vent the full ridge length, leaving 6 inches uncovered at each gable end so air does not flow into the gable structure.
Net Free Area Provided
Exhaust NFA=Ridge Length×NFA / LF
Shingle-Over
18 sq in/ft
NFA per foot
Aluminum
16 sq in/ft
NFA per foot
Rolled Mesh
13 sq in/ft
NFA per foot
Balanced Ventilation
Ridge vent NFA must equal or exceed the soffit intake NFA. Per IRC R806, the total minimum ventilation is 1 sq ft of NFA per 150 sq ft of attic area (or 1:300 with a vapor barrier), split 50/50 between intake and exhaust. Without enough soffit intake, the ridge vent pulls warm air from the home through ceiling cracks.
Do Not Mix Exhaust Vents
Combining ridge vents with gable vents or powered fans creates short-circuiting where one vent pulls air from the other instead of from the soffits. Pick one exhaust system and pair it with adequate intake.Cut the Ridge Slot Right
When installing ridge vent on new construction, cut a 1 to 1-1/2 inch slot on each side of the ridge board, stopping 6 inches from each gable end. Existing roofs need this slot cut into the deck before the vent is installed and capped with ridge cap shingles.