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Complete roofing material takeoff: shingles, underlayment, drip edge, nails, and ice shield. Enter roof area and pitch for full replacement material list.
Size gutters and downspouts from roof area and rainfall intensity. Calculate required gutter capacity, downspout count, and K-style or half-round sizing.
Calculate rafter length from span, pitch, and overhang. Get rafter run, rise, diagonal length, and birdsmouth cut dimensions for roof framing.
IRC requires 1 sq ft NFA per 150 sq ft attic floor (1/150 rule). With balanced system and vapor retarder, 1/300 may apply.
NFA (sq in) = Attic Area (sq ft) × 144 ÷ 150 (or ÷ 300 with vapor barrier)Balanced ventilation splits NFA equally between soffit intake and ridge or roof exhaust. Unbalanced systems reduce effectiveness.
Intake NFA = Exhaust NFA = Total NFA ÷ 2Divide required NFA by manufacturer-rated NFA per vent strip, panel, or continuous vent product.
Units = ceil(Required NFA ÷ NFA per Vent)Updated: July 2026
Need 1,440 sq in total NFA — 720 sq in soffit intake and 720 sq in ridge exhaust. Continuous ridge vent plus perforated soffit panels.
576 sq in total NFA at 1/150. Smaller attic still needs proportional vent area — do not skip ventilation on attached garages.
Measure existing soffit NFA from vent area. Match ridge vent NFA to at least 50% of total required, not less than existing intake.
Size soffit intake and ridge exhaust ventilation for code-compliant attic airflow. Enter attic floor area and vent product NFA (net free area) to get required vent counts and verify 50/50 intake-exhaust balance.