Roof trusses are made up of rafters and ceiling joists. Rafters are cut in two pieces that are joined at an angle that creates the peak of the roof. Joists are the flat horizontal framing members that support the drywall and other ceiling elements. Joists also tie the bottom ends of the rafters together forming the base of the truss triangle. The fascia board is the vertical face that is attached to the ends of the rafters directly under the roof line. Fascia is typically made from 1-inch thick lumber.
Choose 1-inch thick pine that is slightly wider than your joists are tall. For 2-by-4 joists use 1-by-6 fascia, for 2-by-6 joists use 1-by-8 lumber. Select clear lumber with no splits, cracks, checking or open knots. Since joists will be spaced on 16 or 24-inch centers, 8 foot pieces of fascia work well.
Hold your first piece in place starting flush at one corner with the top edge of the board pushed tight against the bottom edge of the roof deck, or flush with the tops of the joists. Mark the board to length so that it will end in the center of the joist that is closest to the end of the fascia board. Use a carpenter's square to mark the board to length even with the mark you made. Cut it to length with a miter saw on a ninety degree angle.
Nail the first fascia board in place with 2-inch finish nails and a hammer. Drive at least two nails through the fascia into the end of each joist. Butt the next fascia piece to the open end of the first so that it it aligned with the center of the joist where the first board ends. Mark and cut it to length using the same technique.
Continue adding fascia boards to the end of the run. Cut the last piece flush with the end of the last truss. Use a nail punch to set the nails slightly below the surface. Fill the seams between the ends of the fascia boards with painter's caulk. Fill the nail head holes with painter's caulk. Smooth the caulk with a dampened finger.