Kiln-Dried and Engineered Lumber in Panelization
In the production of quality wall and floor panels, selecting the proper raw material is critical.
The single greatest impediment to achieving a quality finished wood frame structure is the high moisture content or the green lumber normally used in site framing. The lack of consistent lumber results in starting with a sill plate that does not provide a level base for floors and walls. This makes achieving quality more difficult when plumbing and squaring your walls, and also requires door and window shimming. These problems are made even worse as the wood dries, changing shape, causing nails to pop, corners to crack, walls to bow and windows to leak.
In addition, the change from arsenic-impregnated to copper-impregnated sill plates now requires using hot dip galvanized fasteners, anchor bolts, hold-downs and straps to combat the corrosiveness of the copper. According to the Simpson Strong-Tie Technical Paper T-PTWOOD06, the new copper process has a corrosion rating of over 3x, which is a significant increase over the ratings on previous methods of pressure-treating wood, demanding more expensive hangers, fasteners and hardware.
In order to address the above problems, automation of the process of manufacturing panels provides the desired superior quality. Leading panel production firms use kiln-dried (KD) lumber for framing lumber and have dramatically expanded the use of engineered lumber in their products.
KD lumber comes in exact dimensions, ensuring level, unbowed walls and greater quality with no change in shape or form after the walls are placed. All lumber is #2 or better without using standard or no-grade lumber. When using longer wall panels, up to 27 feet, the use of KD lumber and automation delivers a manufactured product that does not require job site adjustment. A wall this size would usually be framed on site in three separate sections. The ability to place and plumb only one-piece manufactured walls delivers vastly superior quality at greater speed.
In many areas, the use of engineered lumber also greatly improves quality and performance. In floor panels, the use of I-joist and tongue and groove (T&G) sheathing allows for longer spans and greater open space in the structure, while delivering minimal deflection and without squeaks. The Weyerhaeuser Silent Floor Systems, when properly utilized by a floor panel manufacturer, provides a warranty that is passed all the way through to the home buyer (see www.ilevel.com for details).
To address the new wall sill plate requirement for hot dip galvanized fasteners and hardware, the smart panel manufactures are using a zinc-borate treated LSL (laminated strand lumber) sill plate. The use of this engineered product ensured a straight, level and square base for the walls, but does not require any specially treated fasteners, hardware or imbedded items, dramatically lowering those related costs. Additionally, pneumatic (collated nail gun) fasteners can be used. A few leading panel firms are also experimenting with the use of LSL studs for window and door subcomponents. Since engineered lumber does not expand, contract or change shape, window and door openings remain at the exact desired dimensions without shims. Windows don't leak from expansion and contraction of the wood cripples, trimmers and sills. Doors continue to open and close properly, as well. Finally, the use of LSL lumber ensures better quality balloon-framed walls.
How are leading panel firms able to afford to use more expensive material like KD and engineered lumber, while still being cost effective? Efficient operators of wall and floor panel lines generate dramatically less waste than job site framing does (i.e. 3-5 percent compared to 18-22 percent). This fact, combined with the very large amounts of lumber purchased (5 to 10 million board feet annually) at one site by panel manufacturers, provides lumber costs that are significantly lower than that available on the job site. This advantage allows panel firms to use KD and engineered products to deliver quality at a cost that is lower than the less precise site built competition.
Be sure any panel firm you are considering uses KD lumber at a minimum and as much engineered lumber as possible, to ensure you receive the best possible product on your job site. Quality In, Quality Out!
September 2006 Builder Architect Edition Issue