Field guide
Roofer Roof Ventilation Pages That Do the Intake Math Out Loud
Ventilation pages say the attic needs to breathe and stop there, as if the roof had lungs and good intentions.
Air moves for reasons: intake low, exhaust high, and enough open area at both ends. A page that shows the arithmetic gets the call from the owner whose shingles are aging twice as fast on the south slope.
Balanced means both ends, in numbers
Exhaust gets the marketing: ridge vents, turbines, solar fans. Intake does the work and gets ignored. The working rule is roughly half the net free area low at the soffits and half high at the ridge, and a ridge vent on a house with painted shut or never cut soffits is decoration.
Give owners the attic floor math in a worked line: a 1,500 square foot attic at the 1 per 300 rate needs 5 square feet total, split between intake and exhaust, then adjusted for the actual vent product ratings, because a foot of ridge vent and a soffit vent do not move equal air.
Baffles, the two dollar part that decides it
Insulation blown or rolled into the eave blocks the intake slot it was meant to protect. Baffles, simple channels stapled between rafters at the eave, hold an air path open above the insulation. Without them, adding insulation, the right move for energy, quietly kills the ventilation installed years earlier.
A roofer page that asks about the last insulation job is diagnosing, not upselling. Half the ventilation complaints in older suburbs start with an attic that was insulated enthusiastically and baffled never.
Mixing exhaust types makes it worse
A powered attic fan on a roof that also has ridge vent often pulls its air from the ridge, the nearest opening, instead of the soffits, short circuiting the whole design and sometimes pulling conditioned air up through ceiling leaks. One exhaust system, sized and balanced, beats two systems fighting.
Same for gable vents left open beside a new ridge vent. The page should say you close or account for them, because short circuits at the top starve the low intake the system depends on.
What bad ventilation actually costs
Heat is the obvious one: a superheated attic ages shingles from below and loads the air conditioning. Moisture is the expensive one: winter condensation on cold roof decking, frost on nails, then mold and deck rot that show up as a stain blamed on a leak that does not exist.
End with the inspection offer that matters: soffits checked from outside and inside, baffle presence, exhaust type, and the net free area math written down. An owner who gets numbers can compare roofers. An owner who gets the word breathe cannot.
Related reading
Related reading: Roofer Soffit and Fascia Pages That Connect Rot to the Gutter Line, Roofer Moss and Algae Pages That Treat the Roof, Not Just the Stain, and Roofer Skylight Pages That Separate Condensation From a Real Leak.
Frequently asked questions
How much attic ventilation does a roof need?
A common planning rule is 1 square foot of net free vent area per 150 square feet of attic floor, or 1 per 300 where a vapor retarder is present and intake and exhaust are balanced. Half should be low intake at the soffits and half high exhaust.
Why is a ridge vent not enough by itself?
Ridge vent is exhaust only. Without open soffit intake to feed it, air movement stalls and the vent does little. Blocked or never cut soffits are the most common reason a vented ridge still leaves an attic overheating.
Can too much ventilation hurt a roof?
Mixed systems can. A powered fan combined with ridge vent often draws air from the ridge instead of the soffits and can pull conditioned air from the house. The problem is imbalance and short circuiting, not a balanced system sized to the attic.
What are attic baffles and do I need them?
Baffles are channels installed at the eaves that keep insulation from blocking soffit intake air. If your attic insulation reaches the eaves, which it should for energy, baffles are what keep that insulation from quietly disabling your ventilation.