Q: We specify 6 to 8 percent moisture content on the 5/4 poplar we buy. There have been a number of failures in dining tables as far as warping. The 5/4 that came in today had an average MC of 8.3 percent, with the highest at 9.1 percent and the lowest at 7.8 percent. We realize this is too high, but could you tell us the max and min we should be demanding?
A: You are correct in thinking that the warp is the result of excessively high MC. The MC in a home during the heating season will seldom be over 30 percent RH or 6.0 percent MC. We would call 30 percent RH a 6 percent EMC condition. As a guideline, we can have up to a 2 percent MC change in wood before we see substantial problems. (Dining room tables with leaves would be less, if you want to avoid gaps.) I would think that you should specify 7.0 percent MC maximum for lumber you will use in the wintertime. This would give you a little cushion, as probably 7.5 percent MC will still be okay.
On the other hand, avoid over-dried (under 6.0 percent MC) lumber as you will have brittleness when machining and some difficulty gluing.
Moisture measurement is critical, as it must be done correctly. Make sure that your supplier and you agree on the technique, as different techniques and different meters will give different answers.
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