Dimensional Lumber: The stuff that gives your house its basic shape
Advances in building technology haven’t eliminated the need for good old sawn lumber—2x4s, 2x6s, 1x strapping, and the like. It’s still the dominant material in a house’s skeleton. Dimensional lumber (as distinct from lumber made from fibers or veneers) has good compressive strength—it stands up well to force pressing down on it when it’s vertical—so it makes excellent studs. But it’s the least expensive framing material, so it’s also used for the horizontal parts of a wall frame, such as the sole plates at the bottom, the top plates, and the blocking between studs. Interior walls are predominantly made from 2x4s, which are deep enough to fit plumbing and wiring between the studs, while 2x6s make better exterior walls because they leave more space for insulation.
Most dimensional lumber is milled from softwoods like spruce, fir, and pine, then kiln-dried for stability. There are stronger versions, such as straight-grain fir. When combined with metal ties it can be turned on the flat, with its broad face parallel to the wall, wherever there’s limited space for a stud.
Strong beams that allow for open spaces with no posts
Engineered lumber is made from wood veneers and particles, glues, and resins to create large structural elements that virtually never fail if used correctly. Manufactured in a controlled environment, the load factors for these materials are precisely calculated for every size. Engineered lumber also saves trees by using more of the whole tree—typically 30 percent more than sawn lumber—so fewer need to be cut down.
The two most common engineered wood products used in modern framing are LVL beams and I-joists. Laminated veneer lumber (LVL) is just what it sounds like: wood veneers (typically poplar, pine, or fir) laminated together under heat and pressure with a moisture-resistant resin.
Because the grains all run in the same direction, LVLs are extremely stiff and stable. They come in thicknesses up to 3 1/2 inches, depths from 3 1/2 to 24 inches, and lengths as much as 60 feet.
Because of their tensile strength—meaning they can hold up a lot of weight along their length without sagging—LVLs make great door and window headers, stair stringers, ridge beams, cantilevered roof supports, and other carrying beams. And as they have the ability to span long, open spaces, LVLs can eliminate the need for posts in basements and garages.
I-joists are engineered beams made in an I shape. They are made up of a vertical web of dense oriented strand board (OSB) in the middle, with a horizontal flange of dimensional lumber or LVL above and below. They’re used for joists and rafters, because they’re lighter than sawn lumber and able to span greater distances. And unlike lumber, they can take large holes for plumbing and ductwork without compromising strength. All these factors add up to higher ceilings, because a smaller I-joist will carry the load of a deeper one made from dimensional lumber, and systems don’t need to run under the ceilings in added framed channels, or soffits.
The flanges in I-joists are also wide—up to 3 1/2 inches—providing more room to glue and nail subflooring. “Any time you get more fastening surface, your floor will be stronger,” says Tom, who makes sure to use stiff I-joists under floors with rigid finish materials like tile or stone.
The benefit of engineered lumber is its stability and strength. But you can’t generalize about the strength of engineered wood versus dimensional lumber—in part because the latter is so varied in quality. “The real advantage,is not so much that engineered wood is stronger, although in many cases it is, but that it is more consistent and predictable. It’s worth noting that engineered lumber is generally more costly than dimensional lumber.
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Trus Joist: Building Support
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While dimensional lumber handles downward pressure, and engineered lumber braces against push/pull forces, sheets of plywood or oriented strand board (OSB) fight side-to-side movement by spanning and connecting framing members.
Less-expensive OSB is wood shavings glued together with resins. Plywood is thin sheets of veneer or “plys” glued together in layers (typically five), with the grains perpendicular to each other. This maximizes strength and minimizes shrinking and swelling, and makes plywood the stronger of the two. But both are used for exterior sheathing, subflooring, and sometimes even interior wallboard.