Most ventilation "calculators" ask for your squares and multiply. That is wrong on every pitched roof, and the error grows with the pitch.
Attic floor area is not roof area
Ventilation is sized against the attic footprint — the horizontal area of the space being ventilated. Squares measure the sloped surface of the roof, which is always larger.
The gap is the slope factor:
- At 6/12, sloped area is about 12% larger than the footprint
- At 9/12, about 25% larger
- At 12/12, about 41% larger
Size from squares and you over-ventilate by that margin on every job. It looks conservative. It is not — it costs the homeowner money and can pull conditioned air out of the house.
If you have a measured attic square footage, use it. If you do not, derive it from roof area and pitch rather than from squares.
The required number
The rule most codes accept is 1/150 — one square foot of net free area for every 150 square feet of attic floor. Where a vapour retarder is present or the ventilation is balanced, 1/300 is commonly permitted.
Working in square inches, because that is how vents are rated:
Required NFA (in²) = attic square feet ÷ ratio × 144
A 3,000 sq ft attic at 1/300 needs 1,440 in² of net free area.
Split it, then check the split
That total is divided between intake (low, at the soffit) and exhaust (high, at the ridge). A 50/50 split is the common guidance: 720 in² each in the example above.
Here is the part that matters more than the arithmetic:
Exhaust must never exceed intake.
An exhaust-heavy attic cannot draw enough air from the soffits, so it pulls make-up air from the conditioned space below — through can lights, hatches and top plates. That is how you create the exact moisture problem ventilation is meant to prevent, while the roof looks correctly vented from the driveway.
If the intake is short, the answer is to add intake. It is not to add ridge.
Count what is already there
Existing intake is usually easier to count than to measure. Nobody in an attic knows their net free area in square inches — but anyone can count holes.
- 2″ round bird blocks — count them
- Rectangular soffit vents — width × height × quantity
Then derate for the insect screen. A louvre's gross opening is roughly twice its net free area, so halving the counted figure is the honest number. Twenty bird blocks and four 16″ × 8″ rectangular vents comes to about 287 in² of real intake, not the 574 in² the raw geometry suggests.
Never mix exhaust styles on one plane
Ridge vent plus box vents or a turbine on the same roof plane short-circuits the airflow: the ridge pulls from the nearest opening rather than from the soffits. Both GAF and Owens Corning say the same thing. If the roof already has a style, extend that one.
What good output looks like
A homeowner cannot act on "you are 433 square inches short". They can act on "you need 49 lineal feet of intake vent and 10 ridge pieces". Convert the deficit into the product you would actually order, using that product's rated NFA per unit — and if a product has no price attached, show the quantity and say the pricing is on the estimate rather than quietly omitting a line.
This is a summary of how sizing works, not an engineering certification. Under-venting voids shingle warranties and over-exhausting drives moisture into the house — a licensed contractor should confirm any specification.