Things You'll Need
Prefabricated roof trusses
Bracing boards and stakes
16d framing nails
Oriented strand board
8d galvanized nails
Most gable roofs are simple, with a center peak and two sloping sides. Some architectural styles, however, place two gable roofs side by side. A Shingle style, popular in the late 1800s, has front-facing gables on either end of a basic gable peak. Some Craftsman designs from the early 1900s use side-by-side gables with one side projecting in front of the other. Some additions simply add another structure and roof beside a gable house. Whatever the style, building side-by-side gables is framing two roofs over two structures.
Choose a side-by-side gable style and make sure the underlying walls are framed properly to support it. Provide some drainage slope if the two gable sides will intersect at a valley. Shingle styles solve this with a connecting slope between sides; other styles incorporate a flat roof with gables on each side. Determine the gable pitch; both sides can slope the same or have differing pitches, often done in Craftsman houses.
Use prefabricated trusses for both gables; these are installed on most houses today because they are stronger, cheaper and easier to use than individual rafters. Get the width or span and pitch or degree of slope for each gable segment and the length of each roof in inches; divide the length by 24 to determine the number of trusses needed for each roof. Find this information on a house plan or design.
Mark the walls on gable sides, using a ladder, a tape measure, a speed square and a pencil. Start at the back end of one gable. Measure 1 1/2-inches in on the wall cap and draw a line with the square across the cap board. Measure 23 1/2 inches in and make a second line to mark the outside of the second truss from the end. Use that line to mark 24-inch intervals the length of the roof.
Locate truss lines on both sides of the first roof, then duplicate that marking on the second roof segment. Have truss locations align if two gable sides will be connected at a valley. Mark similar truss locations for any connecting roof, like a gable slope between ends of a Shingle style or a flat roof. Install trusses on any connecting segment first, then add trusses on the side-by-side gables.
Erect the first truss on the first roof at the back end. Brace it with boards nailed to the truss and to stakes in the ground and set it plumb with a level. Fasten truss ends to wall caps with 16d framing nails and a hammer. Drive nails at an angle through the truss into the wall cap, two on one side of the truss, one on the other.
Add other trusses one at a time the length of the roof. Install 2-by-4-inch braces, starting with the third truss from the end, with boards laterally on the underside of top truss chords or with braces from the top of the back truss to the bottom of the third one. Follow this pattern on both gable sides. Frame the second gable the same way; nail truss rafter chords together if they join or nail both to wall caps.
Deck the gables with oriented strand board, nailed to trusses with 8d galvanized nails. Lay panels horizontally and start a second row with a half-panel so the seams do not line up. Place full panels and trim overhangs on edges with a circular saw. Overlap panel edges where they meet, at roof peaks or valleys; put one panel to the end of the peak or valley and set an abutting panel against it so both edges are covered.
Install metal flashing on all edges and all peaks or valleys, fastened with shingle nails. Staple roofing paper on both gables with a construction stapler and install shingles. Use cap shingles or 12-inch shingle pieces to overlap the peaks. Bend shingles across valleys, where one roof line meets another, staggering the seams; put one shingle across the valley from the left, the next row overlapping from the right, for instance.
If you are building an addition to an existing house with a gable roof, you will have to remove shingles and decking on the old roof to connect the new roof. You also may have to remove siding from a wall and build a second wall or reinforce the original to support a second gable roof.
Always check local building regulations for required permits or special regulations before starting any roof project. Get services of an architect if joining a new roof to an old one to make sure the existing roof and framing will support the added weight of the new gable.