How to Make a Roof on a Screen Porch

Get the porch dimensions, width and length, and choose a roof style and pitch. Use prefabricated trusses for easiest installation. Match these to the basic house roof pitch if attaching to a gable end, but with a lower pitch or slope so the roof connects to the gable siding.

Use a simple king post truss, with two angled rafter chords, a horizontal bottom cord and a single center vertical brace for a porch up to 16 feet wide.

Remove the siding where the roof will attach with a pry bar. Try to preserve it to be re-installed. Locate rim joists, double top wall plates or studs to find wall framing strong enough to support the weight of the porch roof. Leave the wall moisture membrane and sheathing intact. Mark the wall for installation of the truss bottom chord and both rafter chords. Snap chalk lines to mark the outsides of this triangle.

Place the first truss against the house wall, with the bottom chord resting on the porch walls. Nail it in place temporarily with 16d framing nails driven with a hammer through the truss chords into framing members in the house wall.

Mark locations of the studs, wall plates or other framing members on the truss chords. Drill pilot holes with a power drill and a 3/8-inch bit through the truss into the framing. Fasten the truss with 1/2-inch lag bolts, which have a bolt head and a screw end, driven with a ratchet wrench at least 1 1/2 inches into the framing members. Use one lag bolt for each stud, wall plate or other framing. Nail truss bottom chords to the wall plates on either side.

Lay out truss locations on the outside walls. Use a tape measure to mark a line 1 1/2 inches in from the porch end wall for the first truss. Measure 23 1/4 inches in for the outside of the second truss. Mark 24-inch intervals from the second line length of the roof. The last interval before the wall may be less than 24 inches.

Set a second truss on the outer end of the porch walls. Plumb it with a level and nail the bottom chord to the wall caps with three framing nails, two on one side, and one on the other on both ends of the truss. Add other trusses at marked locations, setting each one plumb and securing it with framing nails into the wall caps.

Add truss braces, a center ridge board if trusses are cut for that at the top, or diagonal bracing between trusses. Use 2-by-4-inch braces from the top of one truss to another. For example, alternate angles so a truss with a brace at the bottom on one side has a brace at the top on the other side.

Cover the trusses with oriented strand board (OSB) decking, panels nailed horizontal with 8d galvanized nails. Set the full panels in place and trim edges as needed with a circular saw. Lay roofing paper over the OSB, overlapping seams from the top and putting a layer across the peak fastened to both sides. Secure roofing paper with a construction stapler.

Nail metal drip edge flashing to the porch roof edges, under the roofing paper on the eaves and over it on the angled gable ends. Install bent metal flashing up the house wall and over the roof where it connects. Fasten flashing with galvanized shingle nails.

Shingle the porch roof with shingles to match the house roof. Run shingles to the edge against the wall, where siding will cover the seam. Put cap shingles over the peak. Re-install the house siding set flush against the roof top and against the truss chords on the inside of the roof. Cut siding as needed to conform to the angle of the roof.