Most of the wood-frame houses built in the last hundred years have been assembled of dimensional lumber, the standardized two-by-fours, two-by-sixes, two-by-eights, and the rest that you encounter at your lumberyard. You shouldn't be surprised to learn, given the finite number of trees to be harvested, that new products have been developed that take better advantage of the trees we have. Thus, a number of factory-made wood products have begun to appear at the construction site.
Perhaps the most common variety of prefabricated structural members are trusses. Trusses are carefully engineered arrangements of triangles that can carry large loads over broad spans. They're most often used in home construction to form the triangular gable roof, but other roof shapes and even interior floors are being framed today using trusses.
Trusses use the inherent rigidity of the triangle. Most are assemblages of two-by-fours. At the same time that they conserve materials, they also give the designer the option of creating larger uninterrupted spaces. Trusses are fabricated elsewhere, delivered to the job site oh a flatbed truck, lifted into place by a crew or even a crane, and nailed in place much like traditional solid-wood joists or rafters. The costs are roughly comparable, especially when savings in labor are considered.
Made of thin layers (veneers, roughly Vio inch thick) of wood that have been glued together, LVL is extremely strong and stable. Unlike the veneers in traditional plywood, all the layers in LVL are glued together with the grain in parallel. This produces a very consistent and uniform product suitable for use as beams, joists, and headers.
LVL actually costs a bit more than solid lumber but there can be labor economies in its installation. The price may also come down as these products become more commonplace, but one argument for the use of LVL is the material's uniformity (there's less spoilage than with solid lumber, where a loose knot, check, or twist can render a piece unusable). Another is its strength-structural members made of LVL can reach across much tifoader spans. LVL can also be purchased in lengths of 60 or even 80 feet, allowing the builder to span an entire structure without overlapping joists.
LVL comes in two basic configurations. As their name suggests, I-joists are a cross between steel I- beams and traditional wooden joists. When looked at in cross section, they have the shape of the letter I The vertical portion is called the web, the horizontals the flanges. J-joists are made of LVL and can be used both as joists and rafters. Micro=Lams are structural lengths of LVL without flanges that are used for rim joists, headers, ridge beams, and other applications.
The advantage of all these members is they're strong but light. One man can lift, and two men can position, lengths of 60 feet or more with ease. Trusses and LVLs are all very stable when kept dry in delivery and installation. Shrinkage is minimal, unlike with traditional kiln- dried lumber where shrinkage routinely separates baseboards from floors and cracks plasterboard walls when moisture content is too high. Trusses have the added advantage that their open structure makes the installation of wiring, plumbing, and HVAC systems easier.
Don't be surprised if your designer specifies trusses or LVL in designing your addition, particularly if you are creating a broad open expanse within the house.