How much soffit do I need, and do I need soffit at all?

Measuring for material

1. Measure every eave run, including porches, garages, and bump outs. Include rakes if they are boxed in.
2. Measure the overhang depth from the wall to the back of the fascia.
3. Multiply: for example, 180 feet of eave with a 16 inch (1.33 foot) overhang is about 240 square feet.
4. Add about 10 percent for waste and corners, so about 264 square feet.
5. Check carton coverage for the panel you choose and round up. Add J-channel or F-channel for both edges.

How much should be vented?

Use the attic floor area. A 1,600 square foot attic on the balanced 1/300 path needs about 768 square inches of net free area in total, with about 384 square inches as intake. Vented soffit products list their net free area per panel or per foot. Many homes alternate vented and solid panels or use fully vented panels along the main eaves and solid ones over porches, where there is no path into the attic.

Do you need soffit at all?

Not always. Some homes are built with little or no overhang, and older homes may have open rafter tails. Soffit protects rafter tails from weather and pests and provides the easiest place for attic intake, but a house without it can still vent through intake products installed at the roof edge. If you do have soffit, keep it intact, since gaps invite squirrels, bats, and wasps. See roof ventilation basics and roof anatomy explained.