Mixing lumber has mixed results

By Dave Anderson Professional Engineer Trus Joist MacMillan

I F YOU have ever shuffled two differently sized decks of I cards together, you soon realized they were difficult to work with. Similarly, there are situations where using solidsawn lumber with engineered wood can create problems, including undesirable aesthetics, system performance problems and structural inadequacies, that both you and your customer must recognize.

The manufacturing process of engineered wood disperses and minimizes naturally occurring wood defects and maximizes the potential of the wood fiber by employing efficient and economical shapes, such as the engineered wood I-joist. And emerging technology, such as that being used by Trus Joist MacMillan to make PSL 30O LSL (laminated strand lumber), enables use of fast-growing, abundant species such as aspen to further reduce the ptessure on old growth timber resources.

The problems mentioned earlierpartially stem from a size differential between engineered wood products and solidsawn lumber. Engineered wood is manufactured in widths and depths that generally differ from solid-sawn lumber dimensions.

For instance, if you use a9-112" deep l-joist and a solidsawn 2x10 measuring 9-114" n depth in the same floor system, you'll end up with an uneven floor surface or a squeaky floor caused by a gap between the 2x10 and the plywood.

A more likely situation is when a solid-sawn rim joist or blocking is used with an engineered wood l-joistfloor orroof system. In this instance, structural deficiencies can occur because the solid-sawn blocking or rim joist, being shallower than the l-joist, won't transfer loads from walls, floors or a roof above. This load would instead be trarnferred through the I-joist and, in fact, could easily crush it at the end bearing support.

Problems also occur when you mix engineered wood with solid-sawn lumber because the two products differ in terms of uniformity. With engineered wood, you always know what you're getting. For instance, if you install a 16foot-long garage door header, you will find the two pieces of L-314" wide, Ll-718' deep LVL are virtually identical in size, strength and stiffness. On the other hand, the multiple pieces of solid-sawn lumberthatthe LVL isreplacing won't always measure the same and defects may cause differences in the strength and stiffness of the various pieces.

The other major problem in mixing engineered wood (particularly wood I-joists) and solid-sawn lumber is that the two products react differently to changes in atmospheric conditions such as temperature and humidity. Solid-sawn lumber is more susceptible to dimensional change from variations in environmental moisture conditions than engineered wood I-joists.

The moisture content of solid-sawn lumber varies considerably depending on the type of wood, location of the tree it was cut from, type of drying and duration of the drying process.

It is impossible to accurately predict the actual dimensional change due to varying moisture content in a given piece of wood. However, studies have produced data which represent the average shrinkage for several species of wood. For instance, Douglas fir may shrink from3-ll2 to 7 -Il2Vo from green to oven-dry depending on which part of the log the lumber was cut from. As an example, a green Douglas fir 2x 10 with a moisture content of 3OVo placed into an environment with an equilibrium moisture content of 9% may shrink nearly ll2" in depth. In addition to shrinking, the solid-sawn lumber may warp, check or split.

Storyata Glance

Pitfalls of using solid-sawn lumber in coniunction with engineered wood .. . size differentials between two framing methods prevent successtul intermingling . . . variation in moisture contentcauses problems.

Engineered wood, on the other hand, experiences little variation in moisture content thanks to several controls in the manufacturing process. Thorough drying of the wood fiber and efficient cross sections of many engineered structural wood products make them much less susceptible to moisture-related dimensional change.

The drying processes during the manufacturing of engineered wood reduce the moisture content to a point where it is similar to the average atmospheric moisture content anywhere in the country-approximately 6 to 11%. This similarity between beginning and in-place moisture content further reduces the possibility of shrinking or swelling in the engineered product.

The dimensional stability, uniform strength characteristics and minimal defects exhibited by engineered wood create a high level of predictability in the performance of these products. However, these benefits vanish when technologically superior, new-generation products are mixed with solidsawn lumber.