A 30-year shingle on a poorly ventilated roof is a 15-year shingle. Manufacturer warranties are explicit about this — improper ventilation voids most of them.
How it works
Cool air enters through soffit vents at the eaves, rises through the attic, and exits through ridge or static vents at the top. The temperature differential creates a chimney effect that drives constant airflow even without wind.
The 1:300 rule
Code requires 1 sq ft of net free ventilation area per 300 sq ft of attic floor, balanced 50/50 between intake and exhaust. Most older homes are under-vented on intake — soffits get blocked by insulation or paint over the years.
What bad ventilation does
- Attic temps over 140°F that cook shingle backs from below.
- Trapped moisture in winter that condenses on the deck and rots sheathing.
- Premature granule loss and curling — looks like UV damage but it's heat.
- Voided warranty if you file a manufacturer claim.
The fix
On a re-roof, we always verify intake/exhaust balance and add ridge venting where the deck allows. On existing roofs, we can usually add intake by retrofitting smart vents at the eaves without tearing the roof off.