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 Today, over 75 percent of all new homes are constructed with trusses. Lightweight and needing no on-site assembly, trusses give builders a bigger bang for their buck. Truss-framed roofs can be erected faster and with less skilled labor than stick-built roofs. Often, trusses go up and sheathing down on the same day, so closure against the weather comes sooner. Trusses' long, clear spans offer greater flexibility with floor plans. And since interior bearing walls aren't needed, their expensive underpinnings aren't needed either. Highly efficient in their usage of lumber, trusses help conserve forest resources. Most often made of 2x4s and spaced 24 in. o.c., a truss-framed roof uses less wood than one stick-built from 2x6 or 2x8 rafters and joists 16 in. o.c.
  Residential roof trusses range from 15 to 50 feet long, and from 5 to 15 feet high. Roof pitch and span plus cantilever, if any, determine truss height and length. Tall trusses are sometimes made as two separate trusses so that they can be shipped over the road. Called piggyback trusses, the two parts are joined on site with plywood or metal gusset plates during erection.

How trusses work
Triangles are naturally rigid geometric shapes that resist being distorted when pushed on. In the upright position, a truss is rigid for the same reason. Regardless of its overall shape, all its chords and webs form triangles, or triangulate. Stick-built roofs operate on the same principle, with rafters, ceiling joists, and collar ties forming the triangles.

Under the weight of sheathing and roofing, a roof truss as a whole is stressed in bending. Its chords and webs, however, are stressed principally in either tension or compression. Top chords, which are in compression, push out at the heel and down at the peak. The bottom chord, firmly fastened to the top chords, is stretched in tension to resist the outward thrust. The result is a stable, self-balancing structure.

One important difference between stick-built and truss-framed roofs is that ceiling joists rarely span the width of the building. Instead, they bear on interior partitions, as well as on exterior walls. Trusses are almost always designed to bear only on exterior walls, with the webs connecting the top and bottom chords providing intermediate support. That's why webs, depending on their location, are stressed in either tension or compression.
Fabrication
Trusses are made mostly from southern pine, Douglas fir, and the woods of the spruce-pine-fir group: eastern and sitka spruce; lodgepole, red, and jack pine; and western and balsam fir. With each of the two common truss assembly methods, chords and webs are first crosscut to the precise length and angles needed with computer-controlled saws. The kind, size, and grade of lumber for each chord and web on the cutting list is based on how great a force each has to resist while under load. Highly stressed as a rule, chords are made from machine stress-rated lumber that has been nondestructively tested to ensure performance. Usually subjected to lower stress, webs are more likely to be No. 2, No. 3, or even Stud grade.

A truss' integrity depends on the integrity of its metal connector plates. Stamped from 16-, 18-, and 20-gauge structural steel coated with zinc, plates have numerous integral teeth 5/16 in. to 3/8 in. long. With about 8 teeth per square inch, plates are sized during design according to the level of stress they have to transfer between members. At panel points in tension members, plates may be stressed in a combination of compression and shear, or tension and shear in the plane of the plate. Here loads are transferred between members from wood-to-metal-to-wood. In top chords and other members stressed in compression, loads are transmitted across joints primarily by wood-to-wood bearing.
Bracing, bracing, bracing
After heels are nailed, the top chord of the truss must be secured by temporary lateral bracing. Starting at the heel, 2x4 bracing is usually installed at about 8-foot intervals along the top chord. Bracing should span four or five trusses and be fastened to each truss with two 16d nails. Its ends should overlap on at least two trusses. Bottom chords need to be braced too, at intervals of about 15 feet across the span.

While helping to maintain on-center spacing, lateral bracing won't prevent connected trusses from tipping over as a unit. To prevent this catastrophe, trusses must be braced diagonally, either across the top chords or through the webs, about every 30 feet starting at the gable end. With the first option, bracing is laid at 45( across several trusses between lateral bracing on both sides of the peak. When run through the webs, bracing starts beneath the top chord against the web closest to the center of the gable end truss. Descending at 45(, it crosses several trusses, terminating above a bottom chord. Through-the-web diagonal bracing is sometimes left in place, becoming part of the system of permanent bracing. Bottom chords are braced on the diagonal in each corner of the building, with other diagonals placed below those on the top chords. In all cases, 2x4 bracing is fastened with two 16d nails to every truss it passes.