Fascia board and roof sheathing are always installed before the roofing, but which one you install first is up to you. Some buildings also have a 1 × or 2 × board installed behind the fascia, called subfascia. Made of rough lumber, the subfascia, helps compensate for inconsistency in rafter length, ensuring the fascia will be straight. It also provides a continuous nailing surface for the fascia.

The type of sheathing you use depends on the roof covering. Use CDX plywood (it's exterion grade) for asphalt and cedar shingles. Depending on the building design, the fascia may be installed flush with the top of the sheathing, or the plywood may overlap the fascia. If you install the fascia first, cut spacers from the sheathing stock and use them when measuring and installing the fascia. Both shingle types must be installed over a layer of 15# building paper (also called tar paper or roofing felt), which goes on after the sheathing and fascia. The paper protects the sheathing from moisture and prevents the shingles from bonding to it.

As an alternative to plywood sheathing, you can use decking boards as a shingle underlayment. Typically sold in 5/4 dimension (l 1/16" thick), board sheathing creates an attractive "ceiling" for the inside of a building, and the nails won't show through as they do with plywood sheathing.

For metal roofing, install purlins—evenly spaced, parallel rows of 1 × or 2 × boards nailed perpendicularly to the rafters. Install the fascia over the ends of the purlins, flush with the tops.

TOOLS & MATERIALS
Framing square	6d and 8d galvanized finish nails
Circular saw	CDX plywood roof sheathing
Stapler	8d box nails
Fascia & trim material	15# building paper

HOW TO INSTALL FASCIA BOARD

Note: This procedure includes the steps for installing fascia on a gable roof. The basic steps are the same for a gambrel roof. For a hip roof, which has no gable ends, skip Step A and start your installation by tacking the first fascia board to the rafter ends, then working from there.

To install subfascia, follow the same procedure used for fascia, but don't worry about mitering the ends—just overlap the boards at the corners.

Step A: Cut & Fit the Gable-end Fascia

1. Mark a plumb cut on the top end of the first fascia board: If you didn't mark the fascia boards with the pattern rafter, use a framing square to mark the plumb cut, following the same method used for marking rafters. Make the cut with a circular saw or power miter saw.

2. Hold the cut end of the fascia against the end rafter. If the fascia will be flush with the top of the sheathing, use spacers set on the rafter and position the top edge of the fascia flush with the spacers.

3. Have a helper mark the lower end for length by tracing along the rafter end onto the back side of the fascia. Make the cut with a 45° bevel. If you're using a circular saw, tilt the blade to 45° and follow the traced line; if you have a compound miter saw, rotate the blade to match the cutting line and tilt the blade to 45°.

4. Temporarily tack the fascia in place against the rafter with a couple of 8d galvanized finish nails. Repeat this process to mark, cut, and tack-up the opposing fascia piece, then do the same at the other gable end.

Step B: Install the Fascia Along the Eaves

1. Cut a 45° bevel on the end of another fascia piece and fit it against one of the pieces on the gable end. If the board is long enough to span the building, mark the opposite end to length. If you'll need two pieces to complete the eave, mark the board about 1/4" from the far edge of a rafter; cut that end with a 45° bevel angled so the longer side of the board will be against the rafter. Cut the remaining piece with a 45° bevel angled in the opposite direction. This is known as a scarf joint—nail these with 8d galvanized finish nails and drill pilot holes to prevent splitting.

2. Make sure the corner joints fit well, then tack the fascia to the rafters.

3. Cut and tack-up the fascia along the other eave. Make sure all of the joints fit well, then fasten the fascia permanently with 8d galvanized finish nails: drive three nails into each rafter end and a pair of nails every 16" along the gable ends.

4. Lock-nail each corner joint with three 6d galvanized finish nails. If necessary, drill pilot holes to prevent splitting.

5. Install any additional trim, such as 1 × 2, called for by the plan. Miter the ends for best appearance.

HOW TO INSTALL PLYWOOD SHEATHING & BUILDING PAPER

Step A: Install the Sheathing

1. Lay a full sheet of CDX plywood on top of the rafters at one of the lower corners of the roof. Position the edges of the sheet 1/8" from the fascia (or the outside edges of the rafters) and make sure the inside end of the sheet falls over the center of a rafter; trim the sheet, if necessary.

2. Fasten the sheet to the rafters with 8d box nails spaced every 6" along the edges and every 12" in the field of the sheet.

3. Cut and install the next sheet to complete the first row, leaving a 1/8" gap between the sheet ends.

4. Start the second row with a half-length sheet so the vertical joints will be staggered between rows. Measure from the top of the first row to the center of the ridge board, and rip the sheet to that dimension.

5. Install the first sheet of the second row, then cut and install the remaining sheet to complete the row.

6. Sheath the opposite side of the roof following the same process.

Step B: Install the Building Paper

Note: If you are installing asphalt shingles, add drip edge along the eaves before laying the building paper.

1. Roll out 15# building paper across the roof along the eave edge. If you've installed drip edge, hold the paper flush with the drip edge; if there's no drip edge, overhang the fascia on the eave by 3/8". Overhang the gable ends by 1" to 2". (On hip roofs, overhang the hip ridges by 6".)

2. Secure the paper with staples driven about every 12".

3. Apply the remaining rows, each overlapping the preceding row by at least 2". Overhang the ridge by 6". Overlap any vertical joints by at least 4".

4. Install the paper on the other roof side(s), again overlapping the ridge by 6".

5. Trim the paper flush with the fascia on the gable ends.