How to Build a Free Standing Gable Porch Roof

Install 4-by-4-inch posts at the porch corners and every 8 feet in between if it is a very large porch. Install posts with metal brackets fastened to the porch floor with long galvanized screws if it is wood or with concrete screws or anchors if it is concrete. Use a level to get the posts plumb. Lay a board across the tops to get them level.

Put 4-by-4-inch beams atop the posts on the long sides of the roof. Secure them with 2-by-4-inch boards across the short sides of the roof to make a frame to support roof trusses. Brace the beams, if desired, with 4-by-4-inch supports, cut with a miter saw to 45-degree angles on the ends, from each post to a beam. Fasten all these components with 16d framing nails and a hammer.

Use prefabricated roof trusses, which are easier, stronger and cheaper than rafters. Get simple king post trusses if the porch span is 16 feet or less. Use a tape measure, speed square and pencil to lay out the truss locations on the side beams. Measure 1 1/2 inches in from the outer end and draw a line using the square for the first truss. Measure 23 1/4 inches from the end and make a second line, for the outside of the second truss. Use that line to space trusses 24 inches apart to the house end.

Set the first truss in place at the outer edge of the porch. Lift it upside down, erect it on the marked space and fasten it with 16d framing nails driven with a hammer through the truss chord into the beam. Use a level to get it plumb and brace it with boards nailed to the truss and to stakes in the ground. Put two nails on one side of the truss, one on the other.

Lift other trusses in place one at a time and nail them to the beams, taking care to get each one plumb. Brace trusses starting with the third one with 2-by-4-inch boards nailed from the top of one truss to the bottom of the next. Nail these to the king post for that style or to the side webs if a different truss is used.

Deck the trusses with oriented strand board. Lay panels perpendicular to the trusses, with the rough side of the board out. Use as many full panels as possible, but stagger the seams; put a half-panel atop a full panel, then a full panel, for instance. Trim the edges as needed with a circular saw. Let the tops overlap at the peak, so the edge of one panel goes over the edge of the one on the other roof side.

Lay roofing paper over the OSB, starting at the bottom on each side. Overlap sheets by at least 6 inches and fasten one sheet across the peak. Fasten the paper with a construction stapler. Cut it as needed with a utility knife or big shears. Nail metal drip edge flashing on all roof sides, under the paper on the eaves, over it on the gable ends.

Nail on shingles, beginning with a starter strip at the bottom of each roof side. Lay the first full row with the tabs over the starter strip. Cut a tab width off the first shingle on the second row so seams do not line up. Fasten shingles with four shingle nails about 3/4 inch below the top of each shingle. Work to the peak on both sides and cover the peak with cap shingles fastened to the roof on both sides.

Put 1-by-4-inch fascia or facing boards on the ends of the trusses. Nail these in place with 8d galvanized nails. Leave the gable ends open or sheath them with OSB, cut to fit the triangle and nailed to the truss sides. Put 1-by-4-inch trim boards along the rake or sloped angles of the gable ends, nailed to the truss chords or to the OSB sheathing, and around the post bracket tops on the floor.