“The only problem with using wood shims arises when the shims are too small to support the weight/load of the main beams. If the shims are cut from the edges of nominal 2x4s, they are only 1-1/2 inches wide. A single sliver of 1-1/2 inch wide shim will naturally crush under the weight of nearly any beam in a wood floor system. All that the builders need to do is utilize a series (multiple pieces) of wood shims when leveling the main beams in order to reduce the pressure on the shims. Usually, they just use a single piece of 1-1/2 inch wide shim. These shims can be �-inch thick up to 1-1/2 inches thick. Whenever the shim needs to be more than 1-1/2 inches thick, the builder will typically use a piece of dimensional lumber such as a 2x4, 2x6, 2x8 or even 2x10 or 2x12. In these cases, the shims will usually never crush. The crushing problem typically only occurs whenever a single sliver of thin wood is utilized.”
From: The Use of Wood Shims in Residential Construction. Information from JADE Engineering