Soffit Calculator

How Much Soffit Do I Need?

100 ft of eave with a 16 inch overhang is 133 square feet of soffit. In 12 inch vented vinyl panels, each 12 ft panel cuts into 9 pieces 16 inches long, so the run takes 100 pieces from 12 panels, plus 8 pieces of F-channel along the wall: $328.

Vented vinyl soffit, 12 inches wide, for common eave lengths and overhangs:

Measure the eaves on every side that has a soffit. A gable house has soffit along its two long sides, plus the rakes if they are boxed in; a hip roof has it all the way around.

Pieces From a 12 ft Panel

Soffit runs across the overhang, so each 12 ft panel is cut into short pieces, one for every panel width along the eave. The default 16 inch overhang cuts cleanly:

Panels needed for 100 ft of eave by overhang depth:

Offcuts shorter than the overhang go to waste, so a depth that divides 144 inches evenly uses the panels best: 12, 16, 18, 24 and 36 inches leave nothing over. A 20 inch overhang gets 7 pieces from a panel with 4 inches left, and a 30 inch overhang only 4 pieces with 24 inches left.

How to Calculate Soffit

Count the pieces along the eave, then how many pieces each panel gives:

Measure the overhang from the wall to the back of the fascia, where the soffit ends. GAF puts typical overhangs at 12 to 24 inches, measured level from the wall. On new framing, the overhang is set by the rafter tails; work out the rafter length for any overhang and pitch.

Soffit Vent Area

Net free area is the open area the vent slots leave for air, and makers list it per square foot or per lineal foot of soffit:

To compare the two, multiply the figure per lineal foot by 12 and divide by the panel's width in inches: Norandex's Double 5 in covers 10 inches, so 4.9 sq in a lineal foot is 5.9 a square foot. The default 133 square feet of fully vented vinyl soffit gives 787 square inches of intake.

Compare it with the vent area your attic needs. The IRC asks for net free vent area of 1/150 of the attic floor, or 1/300 when 40% to 50% of it is in upper vents within 3 ft of the ridge and the rest in the bottom third of the attic (R806.2).

How Much Vented Soffit an Attic Needs

Work backward from the attic. A 30 × 40 ft attic, 1,200 square feet, at 1/300 needs 576 square inches of net free area, and GAF puts half of it, 288 square inches, at the intake. At 5.9 square inches a square foot, that is 48.8 square feet of fully vented soffit: about 37 ft of eave with a 16 inch overhang. The default 100 ft of eave gives 787 square inches, more than twice that. With individual 16 × 8 in soffit vents of 50 square inches each, the same attic takes 6.

Center-vented soffit at 2 square inches a square foot would need 144 square feet for the same attic, so check which version you are buying. GAF also cautions that exhaust should never exceed intake.

Soffit Types and Sizes

• Vinyl: sold in 12 ft panels; Norandex's Builder Double 5 in covers 10 inches and its Triple 4 in 12 inches, 20 and 16 panels to a carton.
• Aluminum: 16 inch vented panels; for the default eave that is 9 panels instead of 12, $411 with F-channel. Its net free area is printed on the label, not in the data we read, so the calculator leaves it blank.
• Solid soffit: for porches and anywhere the attic gets intake air another way; 16 inch solid vinyl brings the default eave to $321.

Soffit meets the fascia and the gutters at the eave and the siding at the wall. Count the siding panels and trim for the walls, and size the gutters along the same eaves.

Common Mistakes

• Ordering by area alone. With a 30 inch overhang, 100 ft of eave is 250 square feet, 21 panels of 12 inch soffit by area, but each panel gives only 4 pieces, so it takes 25; count pieces, not square feet.
• Measuring to the face of the fascia. The soffit stops at the back of the fascia, so measure to there.
• Forgetting the rakes. Boxed-in gable overhangs take soffit too; add their length to the eaves.
• Covering the vents from inside. Insulation pushed into the eaves blocks the air the soffit lets in; the code keeps a 1 inch air space at the vents.

Keep the Vents Open

Vented soffit only works if air can get past it. The IRC requires at least 1 inch of space between the insulation and the roof sheathing at the vents, and no blocking, bridging or insulation in the way of the air (R806.3). Vent openings must be between 1/16 and 1/4 inch in their least dimension, or covered with screening, cloth or perforated vinyl to that size, to keep out birds, rodents, snakes and similar creatures (R806.1).