Q. Ideally, pier/beam connections are always properly aligned, with the pier at the right elevation to fully support the beam above. But in the real world, a little shimming always seems to be required. Are shims ripped from framing lumber really adequate for supporting beams, or should I use plywood?
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.
Or, to put it another way, Doug fir's allowable compression parallel to grain is around 1,500 psi; its allowable compression perpendicular to grain is around 625 psi.
Thus, the answer to your question depends a lot on how well the shim is cut. A 3.5-inch-square Doug fir shim at the top of a 4x4 post in a crawlspace can take approximately 7,656 pounds of load (625 x 3.5 x 3.5 = 7,656 pounds) as long as the shim seats perfectly. That's a lot of load — probably more than will ever be applied by a floor beam.
The rub comes, of course, when shims are not cut carefully and do not seat properly. Most shims are not perfect, uniformly thick wafers. They're wedges, hacked from a stray board by an overworked framer toting a worm drive. If such a shim, cut from a 2-by, is driven into the gap at the top of the same 4x4 post and bears on half its length and its complete width, the load capacity is only 1,640 pounds (625 x [3.5/2] x 1.5 = 1,640 pounds).
When this shim gets overloaded — during the annual Thanksgiving gathering, say — it will crush and the beam will settle a bit. Once the feast is over and the in-laws go home, the floor will either have a permanent dip (if the post was at a beam end) or be bouncy or squeaky (if the post was located midbeam).
Plywood may seem like a reasonable alternative, but its allowable stress perpendicular to grain is around 300 psi, making it only about half as strong as Doug fir for this purpose. Unless the loads are light and you are sure the shim is seated completely, I wouldn't recommend making shims from plywood.
Steel-plate shims are the best choice, provided they seat properly and are available in the right thickness. The allowable compressive stress for steel is on the order of 25,000 psi — far more than for any wood.
