How to Frame an Open Gable Porch

Set posts at the four corners of the porch and at 8-foot intervals in between if the porch is large. Install metal post holders for 4-by-4-inch posts. Fasten these to the porch floor with lag bolts if it is wood, with concrete anchors if it is concrete. Use a level to get posts plumb and a tape measure to make sure the tops are the same height.

Put 4-by-4-inch beams atop the posts on the long side of the porch. Fasten these with 3-inch galvanized screws driven with a screw gun diagonally through the beam into the post. Add 4-by-4 end beams on each end of the porch, fastened to the top beams with diagonal screws. Make 4-by-4 braces, cut with a miter saw at 45-degree angles on each end, to support the beams on each side of each post. Fasten the braces to the bottom of the beam and the side of the post with galvanized screws.

Order prefabricated gable roof trusses to match the width or span of the porch roof, with any desired overhang. Get trusses to cover the length of the roof, spaced 24 inches apart. Use simple king post trusses for a basic roof, with bottom chords that lay on the beams.

Mark the beams for truss locations, using a tape measure, speed square and marker. Measure 1 1/2 inches in from one end of the roof and mark the first truss spot. Measure 23 1/4 inches and mark a second line, for the outside of the second truss. Use that line as a base to mark 24-inch increments across the roof.

Erect the first truss on the house side of the roof. Set it plumb with a level and fasten it with 16d framing nails and a hammer, driven diagonally through the bottom chord into the side beams. Put two nails on one side of the truss, one on the other. Set other trusses in place at the marked locations. Brace the trusses laterally with 2-by-4-inch purlins, nailed on the bottoms of the angled top chords the length of the roof.