How to Build a Soffit for a Flat Roof

Measure the underside of your eaves with the measuring tape. Divide the total length by a number that will give you a whole number. For example, if the length is 15 feet, divide that by three to get five. This means you need five soffits that are each 3 feet long. If you can't get a whole number, try to get a number that is easy to calculate. For example, if the length of the eaves is 13 feet, divide it by four to get three and a quarter. Since 1 foot is equal to 12 inches, one quarter of that is 3 inches, so you would have four soffits that are 3 feet and 3 inches long each.

Cut the plywood boards to fit the measurements of your eaves with the rotary saw. These boards are your soffit boards. Cut them into equal sizes for easy installation.

Hold the soffit over the eave. Hammer nails into the studs on the other side of the soffit to attach it to the house. Hammer about four to six nails per stud.

Measure the outside of your eave's length and width for the fascia boards. Cut out the fascia boards with the rotary saw. The fascia boards should be as long as the eaves and 1 to 2 inches wider so they hang over the front of the soffit boards to hide them. You can cut the fascia boards into equally sized segments like you did with the soffit boards for easy installation.

Fit the fascia board over the outside of the soffit, right below the gutter. Hammer the fascia board into the studs and eaves to provide proper coverage for the soffits. The fascia board will hang 1 to 2 inches over the soffits.