Soffit is the horizontal trim piece that covers the bottom of the rafter tails that protrude from the outside of a wall. Fascia is the vertical board that is attached to the ends of the rafters directly under the lip of a roof's edge. Many shed builders forgo the installation of both of these protective pieces. Without it, the edge of your roof and rafters are open to the weather and will begin to deteriorate in short order. Installing fascia and soffit will prevent the wind and rain from weathering your roof prematurely.
Measure the distance from the outside of the wall of your shed to the ends of the rafters. Cut enough ¼-inch thick Luanne plywood pieces to width to cover the rafters around the entire perimeter of your roof.
Hold a piece of the Luanne you cut to width at one corner, between the wall and the end of the rafters. Mark the Luanne in the centre of the last rafter that it reaches and cut it to that length.
Position the cut piece parallel to the wall and end of the rafters, starting at the same corner of the shed. Nail one-inch roofing nails through the plywood, into the rafters. Drive at least two nails into each rafter.
Butt the next piece up to the first and mark it as before. Cut the piece to length and nail it to the bottom of the rafters as you did with the last piece. Continue cutting pieces to fit and nailing them in place to cover the remaining rafters.
Measure the height of the rafters. Most shed roofs have 2-by-4 rafters. Buy enough 1-inch thick lumber to cover the ends of the rafters. For 2-by-4 rafters, use 1-by-6 inch lumber. For 2-by-6-inch rafters, use 1-by-8 lumber to cover the rafter ends.
Position one piece of the 1-inch thick lumber against the rafter ends, starting in one corner of the shed. Mark it at the centre of the last rafter that the piece reaches and cut it to that length. Nail the piece of fascia to the ends of the rafters with 8d nails and a hammer.
Butt the second piece of fascia to the first and mark it as before. Cut the second piece to length and nail it in place. Continue around the shed cutting and adding pieces of fascia until the entire perimeter of the shed is covered.