Is roof venting necessary, and why are roof vents necessary?
Short answer. Roof vents are necessary on any roof over a vented attic because they are the exit for hot, moist air: intake vents at the eaves let fresh air in, and roof vents near the peak let it out. Without that exhaust, heat and condensation build up under the deck, shorten shingle life and can rot the plywood.
What the vents protect
- The decking. OSB and plywood swell and lose strength when they absorb moisture over and over. Dark, delaminating sheathing near the top of the attic is a classic sign of weak exhaust.
- The shingles. Asphalt shingles age faster when they bake from below as well as above. Excess heat dries out the asphalt and can contribute to blistering and early granule loss.
- The fasteners. Condensation forms on cold nail tips poking through the deck. Rusty nails and drip marks on the attic floor after cold nights are moisture, not a roof leak.
- The warranty. Most shingle manufacturers state ventilation requirements in their warranty terms, and poor ventilation can reduce what they will cover.
Why the vent goes in the roof itself
Warm air rises. The highest point of the attic is where air is hottest and wettest, so exhaust has to be at or near the ridge to work by natural convection. Gable wall vents help a little, but they mostly move air across the top of the attic instead of pulling it up from the eaves. A ridge vent or a set of static vents near the peak, paired with clear soffit intake, sets up a steady flow that needs no electricity.
When the answer is no
Low-slope roofs with no attic, cathedral ceilings built with rigid foam above the deck, and spray foam encapsulated attics are designed without roof vents. Cutting vents into those assemblies can cause problems rather than fix them. See roof ventilation basics, and OSB vs plywood decking for how moisture affects the deck.