A. Tim Garrison, a professional engineer, the president of ConstructionCalc.com, and the author of Cracks, Sags, and Dimwits: Lessons to Build On, responds: Wood achieves its greatest compressive strength when it's oriented like a post, so that loads are applied parallel to grain and resisted by its long fibers.

But most shims are cut along the grain so that they don't fall apart; they receive compression loads perpendicular to grain, which tends to crush the wood fibers.

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