Q: Can you tell me what has caused these cracks in the enclosed red oak square? Can you tell me how to avoid this defect? Is our dry lumber storage shed too hot?

A: The red oak square that you sent me contains the defect called deep surface checking, also called interior checking, bottleneck checking and honeycomb. All names are used for the same defect. This defect results because the wood was dried too quickly during the initial drying (above 50 percent MC typically) or because the wood was dried at too high of a temperature (above 30 percent MC). Storage conditions for kiln-dried lumber do not cause or worsen this defect.

Note that "too quickly" varies from species to species. Likewise, thicker lumber must be dried more slowly than thinner. Determining the safe rate of drying can be tricky, but for 4/4 lumber at the higher MCs, the safe, maximum drying rate is 3.8 percent MC loss per day for northern or Appalachian red oak and 2.5 percent MC for white oak. Occasionally, the lumber being dried may not be as strong as "normal" due to growth factors or the presence of strength-reducing bacteria. The safe rate is then much slower, perhaps 1 percent MC loss per day. Such weakened lumber can develop checking even under normal, standard drying conditions.