Soffit area = eave length × soffit depth. Eaves of 80 ft in total and 12 in deep make 80 sq ft of soffit, which takes 8 vinyl panels of 12 in × 12 ft with 10% waste, plus 13 pieces of J or F-channel and 7 pieces of fascia. If the soffit carries the attic's intake vents, a perforated panel gives 14 sq in of vent area per sq ft.
How the soffit is calculated
The soffit is the underside of the eave, from the house wall out to the fascia. Its area is the length of eave times its depth, measured level. Each piece of vinyl or aluminum soffit runs from the wall to the fascia, so pieces stand side by side along the eave, one panel width each.
Area = eave length × depth · Pieces = eave length ÷ panel width × (1 + waste) · Panels = pieces ÷ (panel length ÷ cut length)
Worked example: 80 ft of eave at 12 in deep is 80 sq ft, 0.8 square. With 12 in wide panels that is 80 pieces, and 88 with 10% waste. Installers cut each piece ½ in short of the wall-to-fascia distance for expansion (Polymeric Exteriors Council), so a 12 ft panel gives 12 pieces of 11½ in. 88 ÷ 12 = 7.33, so you buy 8 panels. Channel runs along both the wall and the fascia: 2 × 80 = 160 ft, and at 12½ ft per piece that is 12.8, or 13 pieces. Fascia is 80 ft ÷ 12 ft, or 7 pieces. Check the lengths your supplier sells: the piece lengths are inputs.
Soffit, fascia and J-channel
The receiving channel holds the soffit at each end. J-channel or F-channel goes on the wall and on the fascia. Nail channels every 8 to 12 in, with the nail centered in the slot. If the eave is deeper than 16 in, the panels need a nailing strip, at least 4 in wide, across the middle. Adding rake soffit at the gable ends adds to the run and, if you turn on the fascia toggle, to the fascia length too. Soffit usually rides along with the siding order: work that out in the vinyl siding calculator.
Soffit vent calculator: how much of the soffit should be vented
Attic intake comes through the soffit. The IRC sets the total vent area at 1/150 of the attic floor area (R806.2), and a balanced system puts half of it low at the eaves. For a 1,500 sq ft attic that is 1,500 ÷ 150 = 10 sq ft, or 1,440 sq in, of which 720 sq in is intake. Air Vent lists perforated aluminum soffit at 14 sq in per sq ft, so the 80 sq ft in this example gives 1,120 sq in if every panel is perforated. You need 720 ÷ (80 × 14) = 64% of the soffit vented, and the calculator says so when you enter the attic floor area. Lanced soffit is much lower, quoted at 4 to 7 sq in per sq ft, and a continuous strip vent gives 9 sq in per ft. Product numbers vary: read the box of the vent you buy.
| Intake type | Net free area (Air Vent table) |
|---|---|
| Perforated aluminum soffit | 14 sq in per sq ft |
| Continuous soffit vent strip | 9 sq in per ft |
| Undereave vent 16 × 8 in | 56 sq in each |
| Undereave vent 16 × 4 in | 28 sq in each |
Keep the air path open
A vented soffit only works if air can get past the insulation. Leave a gap between the insulation and the roof sheathing at each vent (the IRC's R806.3 asks for 1 in, as quoted in the Air Vent guide) and fit baffles if you blow in insulation.
What to work out next
Now size the ridge or box vents that pair with these soffits in the attic ventilation calculator. The gutters hang on the same fascia: pick a size with the gutter size calculator and set the fall with the gutter slope calculator. If the attic gets new insulation, the blown-in insulation calculator counts the bags. See all roofing tools in the roofing and siding calculators.