Framing with Engineered Wood

By Katherine Salant

If your childhood home is the base line when assessing the construction materials on new houses, you're likely to be upset. Wood is still the principal framing material used in new houses today, but it doesn't look the same as it used to.

Most home builders in the U.S. now use engineered wood for a substantial part of their framing. When first encountered, this materials looks undeniably odd, and many buyers doubtless suspect it's cheap and inferior - chunks of wood randomly mixed with brown gook and pressed into sheets like plywood or strange looking floor joists that are shaped like steel I-beams. But engineered wood is more expensive, of better quality and more environmentally benign than sawn lumber of the kind used on your parents' house that comes straight from the tree.

Those "randomly placed wood chunks" are actually thousands of stands of lumber that were precisely shaved off a log, coated with wax and resin, aligned to provide maximum structural strength and shaped variously to be a beam, column, joist, subfloor or exterior sheathing.

Home builders like engineered wood because "what you see is what you get." Unlike dimensional lumber that may warp, crack, twist or check as it goes through several heating and cooling cycles, causing problems such as drywall pops or uneven flooring for both builders and homeowners, the engineered wood product is very stable.

For environmentalists, engineered wood also has many pluses. It is made from poplar and aspen, species that many loggers call "weed trees" and consider useless. Besides using wood that would otherwise rot in the forest, the aspen tree even regenerates itself. "You can cut the tree in the fall and the roots send up new growth in the spring," noted David Johnston, an environmental building consultant in Boulder, Colo., and former Washington area builder.

Another plus: Engineered wood products can be made from small trees. A 2-by-12 sized engineered wood floor joist can be made from trees that are only six inches in diameter, but a 2-by-12-inch solid wood floor joist requires a tree with at least a 12-inch diameter, said Mike Baker, an engineer with Truss Joist MacMillan in Boise, Idaho.

Although an entire house can be framed in engineered wood, very few builders in the country are doing this. At current prices, it's often more cost effective to use dimensional lumber for some parts of the house such as wall studs, and non-wood materials such as laminated fiberboard for the 4-by-8-foot sheets used to enclose the stud framing.

The one engineered component that the majority of production builders do use, however, is the engineered wood floor joist, which most builders call a TJI or and I-joist. While most of the framing may only be of passing interest to most buyers, this is one structural component that they should pay particular attention to because the size and spacing of floor joists, whether they are I-joists or dimensional lumber, in combination with the subfloor sheeting will determine "floor performance" and whether or not a floor will vibrate excessively. Anyone who has ever lived or been in a house and felt the chair they were sitting in or the floor they were standing on shake as another person walked by knows that this can be extremely annoying if experienced on a daily basis.

Many buyers assume that vibrating floor problems are handled by building code requirements, but the codes address joist sizes, spacing and spans only as these affect safety, not comfort. A floor can be very safe but still vibrate.

Conventional wisdom among builders holds that the spacing between the joists is a major factor affecting the vibrations or bounciness of a floor, but the critical factor is the depth of the joist, not the spacing, noted Professor Frank Woeste of Virginia Tech University in Blacksburg, Va., who has studied floor performance issues.

Field testing the floors in the builder's model can be informative, but the only way to know how your floor will perform is to test a floor in a finished house that replicates as closely as possible what you intend to buy, advised Woeste.

A house that differs from the model in seemingly minor ways can have significant differences in floor performance. For example, increasing the length of the floor joists to get a slightly bigger room can produce perceptible negative effects, Woeste said, "In one instance a friend added a bay window to a dining room that increased the span by two feet and the vibration is awful. You sit at the dinner table and when someone walks through your bottom shakes." The diner's discomfort could have been avoided if deeper floor joists had been installed, Woeste said.

When field testing a house for floor vibrations, Woeste suggests putting the member of the household who is most sensitive in the center of large rooms and then walk around them briskly as if walking on a giant tic-tac-toe board. If the stationary person feels vibrations that are annoying, second thoughts on the house may be in order.

Woeste also recommends the "heel drop test" for dining areas. With the sensitive person sitting in a dining chair, the other person should raise his heels and then drop them to the floor. If the seated person can feel the chair shaking, this may also be objectionable, Woeste said.

In many areas of the U.S., notably Southern California, Texas and Florida, houses do not have basements and the first floor is built on a slab. For such a house, vibrating floors for the main living areas will not be a problem, but buyers still need to check the second floor bedroom areas.

Many building codes allow a builder to assume there will be less weight on floors used for sleeping rooms, and to build a floor's supportive framing accordingly. But a floor built for a lighter load will be more flexible and floor vibrations can be even more of an issue, Woeste said.

To test the second floor for vibrations, Woeste recommends one or two people walking around the second floor while a third watches and listens from below for shaking ceiling fans and light fixtures and audible footsteps. If you plan to have an exercise room upstairs in which you will likely make different types of repetitive movements, try to mimic these as well, he added.

If you have a waterbed, Woeste suggests finding out exactly what it weighs and what live load standard the builder is using. This type of bed is frequently much heavier than a standard bed and may be too heavy for the floor to support adequately.

Copyright 1999-2006 Katherine Salant. Distributed by Inman News.