Ventilation is the least glamorous part of a roof and the most common reason a good one fails early. It costs very little to do properly and a great deal to ignore.
What ventilation is actually for
A roof system needs to move air continuously through the attic. That airflow does two jobs.
In summer, it carries away trapped heat. An unventilated attic can reach temperatures far above the outside air, cooking the shingles from below and drying the asphalt until the tabs curl.
In winter, it carries away moisture. Warm, humid air from the house rises into the attic, meets the cold underside of the decking, and condenses. Left alone, that moisture rots the decking and ruins the insulation — a roof leak with no leak.
Balanced means balanced
A ventilation system has two halves, and they must be sized to match.
Intake
Air enters low, almost always through soffit vents under the eaves. Intake is the half people forget.
Exhaust
Air leaves high, through a ridge vent along the peak, or through box or turbine vents near the top of the slope.
Exhaust without intake does not ventilate. It simply pulls conditioned air out of the house through every gap in the ceiling, raising your energy bill while the attic stays stagnant.
The most common mistakes
- Blocked soffits. Insulation pushed into the eaves during an attic upgrade seals the intake. Baffles prevent this and cost very little.
- Mixing exhaust types. A ridge vent combined with box vents can short-circuit: the ridge vent pulls air from the nearest box vent instead of from the soffits, and the far end of the attic never breathes.
- Bathroom fans venting into the attic. They must terminate outside. Every shower otherwise deposits moisture directly onto your decking.
- Too little of everything. A rough rule is one square foot of net free vent area for every 150 square feet of attic floor, split evenly between intake and exhaust.
How to check yours
Stand outside and look for continuous vents under the eaves. Then look at the peak for a ridge vent. If you see one and not the other, the system is unbalanced.
Inside the attic on a bright day, you should see daylight at the soffits and feel moving air. Rusty nail tips protruding through the decking, damp insulation, or a musty smell all point to a moisture problem that no shingle upgrade will fix.
The takeaway
Ventilation will not sell you a roof, and it rarely appears in a sales pitch. It will decide how long the roof you buy actually lasts. Ask any contractor to explain, specifically, what the intake and exhaust plan is for your house.
