Q. I run a small shop, building furniture and custom doors in the Midwest. This past January, I rejected about 10 percent of a red oak order. There were surface checks. After contacting my supplier, who said that it was the dry shop conditions that caused the checking, I started to look at my shop humidity. My questions: Should I try maintain a shop humidity around 50 percent RH? Is 35 to 40 percent RH too low? Did this low RH contribute to the red oak check problem?
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A. Here are some short answers. Let me know if you need more info.
1. All surface checking (unless liquid water is put on the lumber) is formed above 50 percent MC for red oak. The wood is too dry and too strong at low MCs to create new checks. So, the checks you see were in the lumber when you got it and the supplier should be willing to replace the wood.
2. Your customer is likely, especially during the heating season, to have 30 to 35 percent RH (average) in their home or office. So, your shop should have the same to avoid any issues involving moisture content change (and shrinkage) after delivery. Using a high humidity of 50% RH will likely swell the checks closed so they will not be seen until the wood is exposed to low RH.
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