Wood Moisture Question

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Forum topic by JJDWoodworking posted 08-24-2019 11:04 PM 397 views 0 times favorited 4 replies Add to Favorites Watch
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5 posts in 320 days

08-24-2019 11:04 PM

Topic tags/keywords: maple

Hi All,
New to this site. Amazing community and resource. I have a question regarding a 2×24 x 8’ natural edger maple slab I just bought. I bought it in MY and I live in VA (quite a bit of humidity difference). I know equilibrium moisture content for VA is 13%. I have a workshop that is not air conditioned in the summer so it is open to the humidity Anyway, slab has been in the shop for 2 weeks. It had the pith included in it so I ripped a 4”: section out of the middle. Moisture meter (pin type) reads 16% at the ends of the slab and the middle sides of the pith (freshly cut). The issue is that I have a pine workbench in the shop that is measuring 15% moisture on the ends and it has been in the shop for 3 years. So, my question is, how do I know when the slab is dry enough to build with when the moisture content is the same as wood that has been in my shop for 3 years but is still above ECC (I think that is the right acronym) ? I am planning to build a kitchen table with it. Do I need to move the slab inside the house for a few weeks and see if the humidity changes? Appreciate your help.

4 replies so far

View Steve's profile


2082 posts in 1357 days

#1 posted 08-25-2019 02:43 AM

how long ago was it slabbed up? kiln dried? air dried?

View Lazyman's profile


5428 posts in 2162 days

#2 posted 08-25-2019 03:41 AM

It sounds like either your moisture meter is not accurate or the equilibrium moisture content (EMC) of your shop is actually 15%. I assume that the indoor space is conditioned so it will have a much lower relative humidity than your shop. If I recall, the rule of thumb is to divide the RH by 5 to get the target EMC. So if your indoor humidity is in the 30-50% range, the target (theoretically) EMC is 6-10%. The problem is that a 8’ long 2” slab is going to take a while, stored in your house, to adjust down to the indoor EMC.

I would take them inside and measure the width carefully and after a few weeks see how much the moisture has changed and also how much the width has change. It sounds like after you removed the pith, you may have 2 quartersawn halves? If so, the maple should be fairly stable. If my math is correct, with a radial shrinkage rate of 4.8% for hard maple, they should theoretically shrink about 0.01” per foot of width (quarter sawn) for each percent moisture content change. So if you actually get the MC of the wood down to 8% from 16% (change of 8%), a 4’ wide QS hard maple top would shrink about 1/3” . Put another way, if you build it when the MC is say 12% and it eventually reaches a MC of 8% after it is built, a 4’ wide QS maple top would shrink about 1/6”.

But that is all theoretical and assuming my memory of how this all works and my math is right. Note that I based this on a green MC of 54% which may not be accurate. It could be higher which would mean the shrinkage would be less.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View controlfreak's profile


765 posts in 376 days

#3 posted 08-25-2019 11:02 AM

A dehumidifier in the shop would get you there too. For around $200 you could get one and set the RH% to whatever you want. Tools would probably like it too. I would also pipe in the drain so you don’t need to empty the bucket. It is a fool’s errand to try and keep up with a dehumidifier bucket.

View RRBOU's profile


231 posts in 3066 days

#4 posted 08-25-2019 12:36 PM

I lived in the Richmond area for 15 years. At first I could not beleave the movement. Got real anel about acclimating the wood and still had a problem. Then noticed that seasonal humidity played a bigger part than I had originaly thought in both conditioned spaces as well as unconditioned spaces. I had stuff move as much as 1/4 inch over 4 feet for 1 1/2” thickness. It made me quit spending the extra for kiln dried material. It also changed my joinery, grain orientation and finish choices to allow for more movement. An example is table tops and large panels, can have a grain orientation of quarter sawed to quarter sawed, rift sawed to rift sawed, and flat sawed to flat sawed. I tried to stay away from flat sawed, but material availability can be a problem.

-- If guns cause crime all of mine are defective Randy

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