The opening visible from outside is not the same as the unobstructed ventilation area published for a product. Ask for model-specific net free area, intake totals, exhaust totals and the attic zones each vent serves.
The product number belongs in the calculation
A large decorative grille can create a misleading impression when proposals compare vents by exterior dimensions. Ask each bidder to identify the manufacturer, model and published net free ventilation area. Keep intake and exhaust on separate lines. If a bid names only a length of ridge vent, request the product sheet and the installed length being counted; do the same for the soffit product. This makes the quantities traceable instead of leaving the owner to compare two unrelated measurements.
Check what the calculation assumes
Have the roofer mark the attic floor area and boundaries on a simple sketch. Include enclosed additions, inaccessible sections and any vents proposed for removal. A product rating describes the product, not proof that the installed opening behind it is clear. Ask which openings will be examined and whether existing screening or framing creates an unresolved restriction. Record a limitation as a limitation, rather than silently treating inaccessible vents as fully functional.
Compare a complete installation scope
For an owner reviewing bids, a useful comparison sheet has columns for location, model, quantity, published rating and planned access. Ask the contractor to show the applicable design basis instead of choosing the largest total. If the proposal changes during installation, retain the revised calculation alongside the final product list. That record helps the next contractor understand the intended arrangement without guessing from a photograph taken at ground level.
Consider a bid that names continuous ridge ventilation but lists intake only as existing soffits. Before comparing its price with another bid, ask whether those soffits were identified and examined. The unanswered item is not necessarily a defect; it is an assumption that should be visible. A completed worksheet should make it possible to trace each quantity back to a product and location.
For related project preparation, see roof ventilation changes during replacement and roof replacement sequencing with attic insulation work. Bring the relevant photographs and existing proposal to Perkins when discussing the roof-specific scope.
Technical references: GAF: attic ventilation products; GAF: proper attic ventilation. These references provide background; the selected assembly and applicable project requirements determine the installation detail.