Soffits and fascias are architectural elements found along the eave area of buildings. Soffits are located on the span beneath the rafters, which are parallel beams that support the roof. Soffits are installed in the area behind the fascias. The fascia is an exposed horizontal band applied to the ends of rafters. These areas provide a finished look to a building, keep out squirrels and bats and, if vented, allow air to circulate through the attic. Experience in carpentry is suggested for proper placement of each.
Ensure that the ends of rafter tails - the exposed area of the rafters extending over the exterior wall - are even before applying the eaves fascia. Cut your fascia board to length and slide it over the edge of plumb cut rafter tails until the top of the fascia is plumb with the top of the rafter but not flush with the top of the rafter tail.
Fasten the fascia in the middle of the course while nailing the board into each rafter tail. Drive two nails into each rafter if they are 2-by-4. Cut a 45-degree mitre into spliced boards. Leave a 1/8-inch gap behind the top board to fit the second board underneath for a tight fit. Drive a nail straight into the lap and the rafter at the splice.
Gable ends of buildings require a different technique in applying fascia board, which is sometimes referred to as "bargeboard." Place 2-by-4 brackets or cantilevers called "lookouts" even with each frieze truss (vertical members on the rake of a gable that support the truss) from the first truss to approximately 30 inches past the gable wall. Leave the lookout at the gable apex longer than the others to support the fascia while you're nailing.
Loosely nail the bargeboard to the end of the eaves fascia then cut the ridge lookout at the gable apex level with the others and affix the bargeboard. Nail the board into each lookout with two nails. Use the same process for the opposing bargeboard. Join the bargeboards at the ridge with a plumb cut.
Prepare for installation of soffit boards by determining the amount of soffit needed for the project by multiplying the width of the area from the wall to the fascia board by the length of the soffit for each overhang. Divide the total of those overhangs by 100 to get the size of each panel. Allow for 1/4 inch or 1/2 inch in the width of the panels for expansion and contraction.
Snap a chalk line on the wall of the house level with the bottom edge of the fascia board. Snap a second chalk line 7/8 inch above the reference line on the house and another 7/8 inch from the bottom of the fascia board. Nail F-channels every 12 inches along those lines with the stem up and the "F" shape down to receive soffit boards.
Slide the soffit panels into the F-channel slot on the wall then into the channel on the fascia board. Using a back and forth motion may help with installation. Drive nails loosely into the soffit through the fascia board. Finish soffit panels with a 45-degree mitre joint or a 90-degree square joint at corners.
If installing soffits on new construction, apply them before finishing the house cladding. Sub-fascias behind the fascia board will help keep it straight. Cut mitred boards from the back to present a cleaner surface on the front. Provide 1/4-inch expansion room on either side of soffit panels.
It is not a good idea to wrap wooden soffits in vinyl. This holds condensation and can trap water, leading to rot. Use safety goggles. Secure ladders firmly. Don't hammer nails into trim too tightly. Allow for expansion. Do not allow soffit panels to buckle when placing them in channels.