Things You'll Need
1/4-inch thick luan plywood
Roof edge trim on most residential homes has two main parts. The fascia board runs horizontally along the bottom edge of the roof, attached to the ends of the joists. Directly behind this trim board is a flat panel extending horizontally from the back edge of the fascia toward the house, where it meets the wall. This panel is known as soffit. Soffit is typically made of luan plywood, 1/4-inch thick. The best way to install it is to nail it directly to the bottom edges of the joists extending out at the top of the walls.
Prepping the Soffit Panels
Use a tape measure to measure the space from the face of the wall to the back face of the fascia. Use a chalk line to mark enough 1/4-inch luan plywood 4-by-8 panels to width to cover the entire length of the eaves, or underside of the joists, with soffit.
Cut along the chalk lines on each panel to cut them to width, using a circular saw. Set the depth of the saw to 1/2 inch and use a fine-tooth blade for best results. Sand the edges to remove splinters with 100-grit sandpaper.
Roll on two coats of latex paint with a medium nap roller to cover each panel. Work in a W shaped, overlapping panel for adequate coverage. Allow the paint to dry for one hour between coats. Allow the paint to dry to the touch before installing.
Installing Soffit Panels
Fit the first panel of soffit, painted face down, with one narrow end against the inside face of the front gable with its long edges against the back of the fascia and the face of the wall. Nail up through the soffit into the bottom edge of the joists above with 2 inch pin nails, one every 8 inches.
Align the next sheet of soffit with its narrow end butted up to the first, painted face down. Nail it in place in the same way. Continue adding sheets of soffit along the eaves as far as you can go with full sheets. Measure the distance from the end of the last piece to the inside face of the back gable.
Cut a sheet to fit the remaining space with the circular saw and nail it in place. Caulk the seams and along all edges of the soffit sheets with latex painter's caulk. Smooth the caulking with a dampened finger for a smooth finish. Allow the caulk to dry for one hour. Apply latex paint to the caulked seams to seal them and blend them in.