Once the porch is framed-out, it's important to protect that lumber from exposure to the elements and connect the trusses to strengthen the structure.
CAUTION: Roof work is always dangerous, so please read over some Important Safety Information we've placed on our site before starting this section.
For a project like a three-season porch it's best to set up scaffolding instead of working off a ladder. You'll be spending a fair amount of time sheathing and shingling, so using scaffolding is not only a safety issue, but a time-saver.
Getting whole sheets of roof sheathing up to the trusses can be a chore, so have a partner help with the lifting. You'll also have to measure and cut some odd pieces to get all the sheathing to fit and having someone cutting them on the ground, then handing them up seems to work best.
Sheathing the Roof
We usually use 4x8 plywood sheets to sheath the roof, but oriented strand board is also commonly used. We usually want the sheathing to hang out past the rafter tails by 1/2", so we snap a chalkline up the rafters at 47 1/2" for a guideline.
You may want to measure and mark the layout of the sheets before bringing them up. Make sure to space the sheet ends at the center of the truss rafters so both sheets have a nailing surface. Working from the gable end, flush the first sheet with the gable truss or overhang and progress toward the house.
Position the next sheets on their marks or butt them together over a rafter and nail them in place.
For strength, stagger the joints of the second course of sheathing. Let the ends run wild past the overhang and fill in the odd-sized pieces near the house roof.
Measure those sections and cut the plywood to fit tight against the porch sheathing and angle it into the house roof. Chalkline the wild ends and cut them off with a circular saw.
NOTE: The second sheathing course may not reach the rafters, so nailing up extra supports between the truss rafters may be needed to provide a surface to nail the sheathing edges down.
Protection Against Water Penetration
It's a good idea to put down a rubber waterproofing membrane at the valleys and along the eaves for extra protection against water penetration.
Roll out and staple 15lb builder's felt over the sheathing and any waterproofing material. Overlap each course of felt by about 4".
Start the first row of felt at the gable end flush with the end of the sheathing and cut the felt at the other end to lie against the valley tin.
For more protection against water, you can put down galvanized valley flashing and drip edges over the felt on the gable and under the felt on the sides.
Shingling the Roof
Lay in a first row of flat starter shakes, overlapping the fascia board the same distance as the house shakes.
Continue to the valley, gapping each shake 3/8" to 5/8" from its neighbors. Mark the last shake against the valley flashing for the angle cut that will make it lie flush.
Lay the first course of hand-split shakes right over the starter row, staggering their gaps at least 1 1/2" away from the starter row gaps.
Match the distance (exposure) between the rows of the house shakes. Snap a chalkline that distance across the first row to guide the second row.
Nail on the first row of shakes as far as you can comfortably reach. Fasten the second row of shakes but stop a few shakes short of the first row's length. Stagger the next rows this way, leaving them short of the row below them and creating a pyramid shaped area. Work your way up the section as far as you can get, then move over and continue the pattern.
Work your way up to the peak. If you're using asphalt shingles, just fold and overlap cut sections (hiding the cut edge) over the peak and nail them on each side. For shakes, pre-stapled 90° ridge shakes cap the roof.
Installing Soffits & Fascia
It's usually most appealing to closely match the house soffits, fascia and trim boards to resemble the house's trim. Measure their dimensions and study how they're put together.
Secure a nailer board to the bottom of the rafter tails flush with their ends. Snap a chalkline guide level and parallel to this nailer running along the top of the walls. Then fasten another nailer board to this mark. Position lookouts every 16" between the nailers.
Fasten the plywood soffit material to the nailers and lookouts. Then nail on the fascia boards to the ends of the tails to cover the soffit edges.
At the overhang, bevel cut the soffit ends to match at the peak. Then nail up the soffit to fit flush inside the fly rafters or cap exposed soffit edges with fascia boards.