Trusses usually come with detailed information on how to brace the specific type of truss you are installing. These instructions need to be followed carefully to guarantee that the house will have a strong and stable roof in case of high wind, heavy snow, or earthquake. Don’t try to guess your way through truss bracing.
Having said that, most simple gable – truss roofs are quite easy to brace. Begin by temporarily nailing a long 1x across the rafter chords of each truss, near the ridge point. (I often nail this 1x on the underside of the rafter chord so I won’t have to remove it when sheathing the roof.) This 1x helps hold the trusses stable until you nail in the ridge blocks and other braces. Do the same with another 1x nailed permanently on top and near the center of the joist chords with two 8d nails into each joist. This 1x is the catwalk, like the one nailed to regular joists, and helps hold the joist chords at 24 in. o. c. and keep the roof structure steady.
Once the catwalk is in place, it’s easy to walk along it and nail in the blocks at the ridge point unless the ridge is a tall one. On tall ridges, experienced carpenters walk the rafters to nail in the blocks, which is fairly easy to do with the 1 x nailed alongside the ridge. Hold a block in place between the rafters at the peak. Drive two 16d nails through the rafter into the block on one side and another 16d nail in from the other side.
To help hold a gable-roof structure plumb, you also need to install a sway brace at each end of the roof. A sway brace is a 2×4 nailed in at a 45° angle from the double exterior wall plate to a ridge block nailed in at the peak of the roof. Miter-cut the sway brace at one end, set it flat on the wall running diagonally up to the top of the ridge, and mark it to length. Make the cut and nail the brace in place with three 16d nails at each end (see the photo on the facing page).
To further brace the roof and to tie it to the house frame, use hurricane clips where the joist chords meet the plates. These metal clips are not always required by code, but if you ever happen to get a serious wind, they help keep the roof attached to the house. Nail a clip on each truss with hanger nails.
Despite the increasingly widespread use of trusses, gable roofs are still being built stick by stick. Newcomers to the trade often think there is something magical and mysterious to cutting and building a roof and that the task is beyond their abilities. But if you can draw a right triangle, use a handheld calculator or read a book of rafter tables, use a small rafter square, and handle a circular saw, you can build a gable roof. Of course, other types of roofs are more complex and difficult to build than gable roofs, but understanding how a gable roof goes together is the first step in building these more complex roofs.
You don’t have to be a math genius to be a roof cutter, but the more you know about roof parts and how they go together, the easier it will be. A gable roof slopes in two directions, like two right triangles butted together (see the drawing on p. 142). It has a ridge board at the peak. Common rafters are nailed to the ridge board and slope down to the top plates of opposing outside walls. Here the rafters nail into the wall plates and ceiling joists, effectively forming a truss.
Many homeowners today seem to want more openness to their houses, with larger rooms and higher ceilings. As a remodeler, I have been asked to remove ceiling joists and change a flat ceiling to one that follows the roof pitch. This does give an open feeling, but at a cost. If you remove all the joists, the roof truss is compromised, and weight on the roof can bow exterior walls out and cause the roof to sag. In the absence of a structural ridge beam or collar ties (horizontal members that tie rafters together above the wall plate), a roof generally needs those ceiling joists.
To determine the length of the common rafters, you first need to know the pitch of the roof and the span and run of the rafters. Pitch is the amount of slope a roof has. To say a roof has a 4-in-12 pitch, for example, means that for every 12 in. a rafter runs horizontally, it rises vertically 4 in. (see the drawing on p. 143). A 12-in-12 pitch roof is fairly steep, rising at 45°, while roofs pitched at less than 3-in-12 are generally too shallow for asphalt shingles.
Span is the total distance a rafter travels horizontally (the width of the building from outside to outside). Run is one-half