Posted in Architects

King of the Sill

As the industry embraces wood framing in taller multifamily structures, builders and designers alike need to understand the science behind the use of various types of wood in these applications.

Shrinkage is an important concern for lumber in multi-story structures.  How much shrinkage are we talking about? In typical air-dried lumber, the moisture content may be up to 19 percent. Over time, the lumber is going to dry down to the moisture equilibrium of the finished building—typically 8 to 12 percent—causing the wood to shrink.

As the lumber dries, it’s not uncommon to see a quarter of an inch of shrinkage per story in a typical multifamily building. For a four- or five-story building, that can be as much as an inch of shrinkage, with the wall plates alone accounting for nearly a third of the total. As the framing becomes misaligned, window seals can break and plumbing can be damaged. Ultimately, it can affect shear wall performance and structural capability.

Here’s where LSL comes in. LSL is engineered so that walls start straight and stay straight. Because moisture levels are carefully controlled throughout the manufacturing process, LSL has a 7 to 10 percent moisture content, similar to the moisture equilibrium inside the building. That can significantly reduce plate shrinkage. Plus, LSL is available in longer lengths than traditional lumber, and you don’t have to worry about warping, splitting, or knots, helping minimize waste. LSL can also help save time on the job site as it’s easier to install than steel and concrete.

LP Building Products is currently developing a Zinc Borate (ZB) treated LSL product that will resist shrinkage and will also protect against termite damage and fungal decay. The process for the new ZB-treated LSL will blend the zinc borate, a recognized preservative for engineered wood throughout the product, eliminating the need for field treatment.

For more information on the science behind shrinkage, see our Q&A with Chris Newhouse and the accompanying infographic in our winter issue. Visit WhatIsLSL.com to learn more about LSL applications.
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