Roof Venting Basics

Roof venting 101

Diagram showing the airflow of a roof | Construction Pro Tips

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It’s all about airflow

Roof ventilation is based on the simple fact that warm air rises. In summer, the sun heats air in the attic. In winter, heat from your home warms attic air. In either season, good venting occurs when cool air can enter the attic near the eaves and exit near the peak. Ideally, half of the vent area should be low and half high. The ultimate goal is that the temperature and humidity levels in the attic space match the conditions outdoors.

Diagram showing airflow through an attic and roof vents | Construction Pro Tips

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Paths to good venting

For intake air, soffit vents are best. Air can passively exit through ridge vents or hood vents. Turbine vents harness the wind to suck air out of the attic. Electric-powered vents are the ultimate air movers but aren’t necessary in most situations. Gable vents can help by allowing air in or out, but they don’t usually help the air flow evenly throughout the attic.

Diagram showing a poorly ventilated roof | Construction Pro Tips

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Bad ventilation, big problems

Moisture damage:

Most ceilings aren’t perfectly sealed, so warm, humid air leaks into the attic. Then condensation forms on the roof framing and sheathing. That moisture supports rot, which can cause serious structural damage to your home. The water can also drip onto and through the ceiling, causing further damage.

Ice dams:

A poorly vented attic gets warm enough to melt snow on the roof while the eaves remain cold. Then, when the meltwater reaches the eaves, it freezes and forms a dam. More meltwater puddles behind the dam and can find its way under the shingles.

Blowing foam off of a soffit | Construction Pro Tips

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More roof venting basics

Vents get plugged

Soffit vents often get plugged by debris, cobwebs or stray insulation. You can clear them with a compressor and an air nozzle or a leaf blower. Wear eye protection!

Spy on the neighbors

If your roof is bare while neighboring roofs are still snow-covered, it could be a sign of trouble. While your roof may just be warmer because of its design or its more extensive southern exposure, it could also mean that you have poor ventilation or inadequate insulation.

Cut cooling costs?

Attics get super-heated by sunshine, and some of that heat is radiated to rooms below. So it’s reasonable to think that better ventilation would lessen the radiated heat and reduce the load on your cooling system. But most studies find only minor savings with improved ventilation. Adding attic insulation is usually much more effective.

What about ‘hot roofs’?

A “hot roof” has insulation directly under the roof sheathing and doesn’t require ventilation. With careful design and installation, a hot roof can be trouble-free.

Cooler shingles last longer

Heat slowly degrades asphalt shingles. Ventilation helps to keep them cooler, extending the life of the roof. However, the effect of ventilation on shingle life depends on many factors, especially climate; venting matters most in hot, sunny regions. Most shingle manufacturers void their warranties if roof venting standards aren’t met.

How much is enough?

Building codes generally require 1 sq. ft. of vent area for every 300 sq. ft. of attic floor. That assumes half of the vent area is high on the roof and half is low (in or near the eaves). Otherwise, doubling the vent area is required (1 per 150 sq. ft.) These are minimum requirements; there’s no such thing as too much ventilation.

Common ventilation parts: