Why does attic ventilation matter, and what does “balanced ventilation” mean?

An asphalt shingle roof is rated for the shingle temperature, not the air temperature. A poorly ventilated attic in San Jose or Walnut Creek can push shingle surface temperatures past 160°F in late summer — and every 20°F you save on the underside of the deck adds roughly two years of shingle life.

Balanced ventilation means roughly equal net free area (NFA) of intake at the eaves (soffit vents) and exhaust at the top (ridge vents or static box vents). The code minimum is 1 square foot of NFA per 150 square feet of attic floor, split evenly between intake and exhaust.

What goes wrong on Bay Area homes:

• Boxed-in soffits with no intake vents. Common on 1950s–1970s SF and Peninsula homes — exhaust vents draw air from the attic, not from outside, creating negative pressure that pulls humid air up from the conditioned space.
• Bath/kitchen fans vented into the attic. Code requires venting to the exterior; we re-route these during every re-roof when we find them.
• Mixed exhaust types. Ridge vent + static box + powered fan all on one roof creates short-circuits where air bypasses the attic entirely. We standardize on one exhaust type per roof plane.

Bay Area-specific consequences of bad ventilation:

• Premature shingle granule loss (visible bald patches on south and west exposures).
• Condensation drip-back during winter that gets misdiagnosed as a leak.
• Mold growth on the underside of the deck.
• Manufacturer warranty denial — every shingle warranty requires code-compliant ventilation, and adjusters check it on every claim.

On every full replacement we measure the attic, calculate the NFA, and design intake and exhaust to balance. It is on the written estimate as a line item.