Roof Ventilation Calculator
Attic floor area in, net free area out, split into the intake and the exhaust halves. The number almost every quote leaves out.
Roof pitchRoofing squaresRoof costRoof lifeDeck costGutter costRoof ventilation
4.67 sq ft of net free area in total. Check the NFA printed on the vents, not their outside size.
Get the attic checked, freeThe rule underneath it
The residential code sets a minimum net free ventilating area of 1/150 of the space being vented. That drops to 1/300 where the ventilation is balanced and a Class I or II vapor retarder is in place, which is the normal case on a modern roof. GAF's technical bulletin R-120 sets out the same math, and the FHA guidance of one square foot per 300 square feet split evenly agrees with it.
Half the area goes low at the soffit as intake and half goes high as exhaust. That split is the whole point. A ridge vent running the length of a roof does nothing if the soffits are painted shut, which in older Triangle houses they often are.
Why this matters more here than the brochure suggests
Asphalt ages by losing granules, and it loses them faster on a hot deck. A roof that runs hot all summer gives you the bottom of the published 15 to 30 year range rather than the top. Ventilation is the cheapest line on a roofing quote and the one most often trimmed to win the job.
Working out lifespan? The roof lifespan guide covers what else shortens it here.
Attic ventilation questions
How much attic ventilation do I need?
The code baseline is one square foot of net free area for every 150 square feet of attic floor. That halves to 1/300 when the ventilation is balanced between intake and exhaust and a vapor retarder is in place, which is how most modern roofs are built.
What is net free area?
The actual open area of a vent once you subtract the louvers, the mesh and the frame. A vent's physical size is not its net free area, and the difference is large. Every manufacturer prints the NFA in square inches on the product.
What does balanced ventilation mean?
Equal intake and exhaust, so half your net free area low at the soffits and half high at the ridge. Air has to get in before it can get out. This is the part most often wrong.
What happens if the attic is under-ventilated?
The deck runs hot, which ages the shingles from underneath and takes you to the bottom of the published lifespan instead of the top. In winter it traps moisture, which rusts nails and dampens insulation.
Can you have too much exhaust?
Yes, and it is a real problem. Exhaust without matching intake pulls air from the easiest opening it can find, which is often another roof vent or the house itself. Two different exhaust types on one roof can short-circuit each other the same way.
Does ventilation affect my shingle warranty?
It can. Manufacturers write ventilation requirements into their warranties, so an attic that does not meet the minimum can void cover on a claim that had nothing to do with it.
Get your attic ventilation checked, free
We get on the roof, photograph what is actually there, and hand you a written report with a price. No charge, and no obligation to book the work with us. If the roof has years left in it, that is what the report will say.
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