Moisture, Movement and Meters

[JohnM Martin]

I'm just getting started in woodworking and have built my first dining room table out of 4/4 walnut slabs. It looked great until the past couple of months and some of the edge glued seams started separating, and it seems like they are just getting worse. Picture below.

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That seem was tight when I first glued it up. From what I've read it seems that the wood not being dry enough is the most likely culprit.

So, my question is what moisture meter should I own? I felt like I should own a moisture meter, so I went out and bought a Dr. Meter MD-812 moisture meter, but I'm now wondering if I shouldn't have spent more money on a more expensive model. Being a new woodworker, I have a hard time knowing what to spend premium money on and what is good enough. What do you all recommend for moisture meters? It seems like a really important part of the equation... or maybe I'm being too paranoid about it?

[David Eisenhauer]

Where did the wood come from? Was it supposed to be dry? Can you describe/show us how you attached the top to the base? And did you add a breadboard end to the top? A moisture meter may not be the answer here.

[JohnM Martin]

Where did the wood come from? Was it supposed to be dry? Can you describe/show us how you attached the top to the base? And did you add a breadboard end to the top? A moisture meter may not be the answer here.

I bought the wood from a local sawmill who assured me it was "good and dry" and ready to use. I've since learned it was air dryed, but never measured for moisture. It was my first large project and I followed the instruction of a buddy for construction. It included ripping the boards on a table saw with plywood used to get straight edges. Yes, no edge jointing - did I mention it was my first project?

The boards were then glued to a 3/4" plywood (red oak plywood) base board and nailed with brads from the bottom up. Each board was then edge glued side by side to complete the top. The top of walnut planks and plywood was then attached to a steel base with 1/4" lag bolts going up through the base. Here's what the completed table looked like when first completed.

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I assumed moisture is what would cause the boards to shrink, but I'm more than happy to learn how the table should have been constructed. It was my first build so go easy on me.

[Jamie Buxton]

Air-drying rarely dries the wood down to the level it will get to in a home. (And often sellers overestimate the dryness..) The lumber likely dried further, and shrank after you fastened it to the plywood, and that opened the gaps.

What you should have done would have been to edge-joint the boards, and glue them together. Do not add plywood. It only complicates things, and doesn't help at all. Fasten the lumber top to the steel base firmly in the middle of the table. Away from the middle, fasten the top to the steel with some sort of sliding joinery. This will allow the lumber to shrink without splitting. One example of sliding joinery would be screws which go up through slotted holes in the steel. Tighten the screws up, and then back them off a bit to allow the sliding you need. If you can't make slotted holes, you can make bigger round holes, and put washers under the screw heads.

[Jerry Miner]

I think your buddy gave you some bad advice. Even if the lumber were not perfectly dry, your table would have performed better, IMHO, if you had followed more traditional table construction methods.

Gluing the solid lumber top to a plywood "sub-top" is unwise: The lumber shrinks and moves. the plywood does NOT shrink at the same rate, so the shrinkage of the solid stock leads to splitting.

The "trim piece" across the end of the table is also a problem, for the same basic reason. The end piece is attached "cross-grain" to the top, and will not shrink (or expand) in length, but the boards of the main top will (and have) shrink across the width.

I don't understand how you can glue and nail plywood to the bottom of the boards and then LATER edge-glue the boards together. (How do you get glue into the joint?) I'm guessing you did not get a good glue joint in the edge-gluing process

So next time:

1. Prepare the stock ("4-square" the stock) so the edges meet tightly together without force.

2. Edge glue and clamp.

3. Remove excess glue squeeze-out and flatten the top surface.

3a. If you want a breadboard end, make and install one that allows for movement. (Search on-line for "breadboard end" and you will find illustrated examples)

4. Attach to the base in a way that allows seasonal movement (buttons, table clips, figure-8 fasteners, slotted holes, ....)

[andy bessette]

Agree that buying a moisture meter would not have helped faulty design and edge joining technique.

[JohnM Martin]

Thank you all for the suggestions. The encouraging thing is that all of the suggestions line up with what I have learned in my research since the build. I completely agree that the construction was fraught with errors and I have learned a lot of mistakes the hard way - as one often does when taking up a new craft.

Having said that, my original question is one I still have - how do I know wood is dry enough to use? I've been researching and the closest kiln I have found in my area is 3 hours away so relying solely on a dryer isn't really a good option. Even if one was closer, I would like to be able to reliably tell the moisture content of wood up to 2" in thickness. Otherwise, I'm just setting myself up for more failure by working with wood that's too wet. Any suggestions there?

[Kevin Jenness]

Lignomat and Delmhorst are two reliable suppliers of pin (resistance type) meters. These allow you to check the moisture content at different points through the thickness of your lumber, but leave visible holes. Wagner and others make non-invasive field type meters that indicate the average MC in a piece of wood.

[Brian Holcombe]

Wood moves, no matter how dry. You need to account for that in every aspect of design, especially wide table tops. Either slotted holes for the mount or other arrangement.

The idea of wood movement and kiln drying is often mistaken in its context by new woodworkers. Wood must acclimatize to its environment, workshops want this to occur before they do the work because they do not want to have wood movement issues during the build, however they're accounting for wood movement in their design because they know that conditions change throughout the seasons.

Here is a typical scenario, I'm working on art frames at current;

Buy 6" x 8/4 material, the material is newly shipped in from British Columbia, and so it is kiln dried but to an unknown moisture content.

Bring the material into the shop, allow it to acclimatize for a few weeks.

Face joint and edge joint. Then cut the material into strips that I need for my project.

Allow the material to acclimatize again, this time not nearly as much time is needed since it is now down to 7/16" x 2".

Face joint, thickness plane (I do this by hand) then edge joint.

Saw to width. Edge joint again.

Each step causes some wood movement, and the steps where the wood is sawn from a larger size down to a smaller size the wood must reacclimatize because wood is an insulator and so it takes a very long time for the wood at the center to be at the same moisture content as the wood on the outside. This is the reason why wood cracks on its own doing, if the entire board shrunk and expanded perfectly you would never have problems with cupping, bowing or cracking.

[Steve Demuth]

Having a good moisture meter is very helpful. See Kevin's advice above on brands.

But "dry enough to use" isn't a single number that you need or want to hit. You can build beautiful, sound and functional furniture with air dried lumber (probably 10 - 12%, depending on where you live) depending on the design and target environment. Air dried is preferred, e.g., for outdoor furniture, almost required for steam bent pieces, and perfectly acceptable for many small items, or pieces without wide expanses of flat wood. I strongly prefer air cured American Black Walnut over kiln dried in almost any application because kiln drying this gorgeous wood muddies its sometimes spectacular color patterns. Red Oak's appearance, on the other hand isn't much affected by artificial drying.

Kiln dried wood that has subsequently stabilized to an environment similar to its intended home is easier to work with in other cases - but you still need to design for wood movement. A table top 30" wide made from "settled" wood at 7% moisture in a dry, winter shop may well expand by 1/2" in width in many areas of the country and depending on the species, if placed in an air conditioned home. But it'll - shrink back to its build dimension in winter.

[Brian Tymchak]

Kiln dried wood that has subsequently stabilized to an environment similar to its intended home is easier to work with in other cases - but you still need to design for wood movement. A table top 30" wide made from "settled" wood at 7% moisture in a dry, winter shop may well expand by 1/2" in width in many areas of the country and depending on the species, if placed in an air conditioned home. But it'll - shrink back to its build dimension in winter.

+1. I figure your edge joints might have stayed together regardless of the moisture content of the boards, if not attached to the plywood backing. Those boards wanted to shrink and would have shrunk as a single unit on their own, but the plywood did not want to shrink. So, a crack (or cracks) occurred at the weakest point(s) in the table as the lumber acclimatized to its local environment. Never sandwhich hardwood with plywood. As others have said, hardwood expands/contracts with moisture, plywood doesn't. Another issue with the construction appears to be the gluing process or joint prep. Joints with modern PVA glues usually are stronger than the wood itself. The fact that your glue joint separated instead of getting an actual crack in the wood itself tells me that you either used too little glue, or the joint was not properly milled and there were significant gaps in the joint. Possibly you used heavy clamp pressure to pull those gaps together, but with too little glue or poor faces on the joint, the joint became the weakest part of the board when the board shrank.

With a good rip blade on the table saw and good technique, you can glue right off the rip cut. I usually take a pass on the jointer anyway before gluing up a joint like that.

Also, you don't mention if you allowed the lumber to acclimatize to your environment before milling, but that is a good practice as well, that will help minimize wood movement during the construction process. If you do heaving milling of the lumber, particularly planing to thickness, it is good to "rough mill" the lumber to close to final dimension, stack and sticker the lumber for a few days to allow it to balance with the local environment, and then "final mill" the lumber to final dimension. I like to use it immediately after final milling.

[David Eisenhauer]

What is being said here is that knowing the moisture content by using a meter is not necessary, if you can even get an accurate moisture reading. The moisture content of the wood is most likely going to change some as it acclimates to it's new home so you need to let it acclimate and then plan for it to continue to move some as you work it and subsequently use it. I don't own a meter and don't have problems with stuff coming apart after I build it. I have had problems with wood changing drastically after acclimation (or so I thought) and becoming unusable before I was able to complete the construction. Don't worry about this walnut-to-plywood join up excessively because if mating the walnut to the plywood is the only error you crank out of your shop, you are way ahead of the rest of us. I would remove the plywood base and then re rip the walnut to get rid of the previously glued edges and then fab up a new top with the walnut.

[Kevin Jenness]

" knowing the moisture content by using a meter is not necessary"

I agree with this statement in general. If you have plenty of time to let your wood settle and know the conditions it is acclimated to, you can predict its movement in service.

I use a meter primarily to check wood that I acquire for a job and want to use immediately and in checking for moisture gradients when drying wood. It is handy for checking the accclimation of wood brought into the shop from an unheated storage shed. Much of what I use a meter for can be accomplished with a weight scale- when a piece's weight levels off in a stable environment I know it is at equilibrium moisture content (very useful for thick turning blanks and the like that are not easily read with a meter). The meter gives an instantaneous reading that doesn't require monitoring over time.

Keep in mind that meters can get out of calibration, and may give anomalous results for various reasons. For instance, if one is checking a thick piece with a meter with short pins it may indicate that the wood is drier than it really is.

[andy bessette]

Forget about the meter. Learn about design and joining.

[JohnM Martin]

I appreciate the feedback. It's especially helpful for someone like me that is trying to learn the craft via the internet and trial and error.

The last few comments are interesting - essentially downplaying the significance of moisture content and a meter to a point. So, if you don't have a meter, how do you know when enough time has passed to allow sufficient drying and/or acclimation? For example, I just got back a couple of walnut logs I had taken to the sawmill and milled into a mixture of slabs, 4/4, and 6/4 lumber. Currently, that is sitting stacked and stickered inside a barn. But without a meter, how do I know when that wood is dry enough to move into the shop for acclimation and eventually for use? Everything I've read says you don't want to build furniture with wood that is above 7%-8% (actual value varies based on source). How do you know when wood has reached that level without a meter?

[Jerry Wright]

Air dry for 1 year per inch of thickness. You probably won't get to 8% without kiln drying. Air drying is more like 12%. Perfectly acceptable as long as you pay attention to basic rules of expansion and contraction. Lots of fine antiques were built with air dried lumber.

[andy bessette]

Your thread was about a failed attempt at table joinery, which a meter would not have avoided. Now it seems to be more about finding rationalization (milling green logs) for buying a meter. I have been a woodworking professional for over 40 years and have never owned a meter.

[Kevin Jenness]

" So, if you don't have a meter, how do you know when enough time has passed to allow sufficient drying and/or acclimation?"

You can use a scale to monitor weight loss. Kiln operators typically weigh samples to monitor drying rates. When piece of green wood stops losing weight it is in equilibrium with the ambient relative humidity. A meter is a useful tool but not essential.

[Doug Hepler]

John,

Sorry to hear about you gappy table top. I respect your interest in really understanding this issue. To answer your question about moisture meters, see this article: http://www.popularwoodworking.com/american-woodworker-blog/moisture-meters. It discusses the subject but does not tell you which meter to buy. I have a Timber Check moisture meter (pin type) which has been satisfactory. Precise moisture measurement is not necessary IMO because small changes (a few points) are not associated with large changes in dimension.

Your biggest issue for furniture, unless it will be shipped to another climate, is the change from workshop to indoors, especially summertime outdoors to wintertime indoors. I prefer kiln-dried wood but have used air-dried wood for many projects with no problem.The significance of changes in equilibrium moisture content (EMC) is controversial for a number of reasons, so reasonable people may disagree. My philosophy is always to calculate the likely change in dimension and then allow for it when I design a piece. See this article: http://www.lonniebird.com/wood-moisture-content-by-lonnie-bird/

I think that your problem is mainly the result of attaching the boards to plywood, as already discussed above, and the way in which you glued the edges together. I agree that you would probably not have seen this problem if you had simply edge glued the panel. You would, however, need to account for dimensional changes with EMC when you attached the top to the base. I have seem screws that had been ripped out of their holes by wood expansion and contraction over the years. Specialized clips are made for this purpose to let the wood slide horizontally while keeping the panel tight against the base.

Finally, I have briefly addressed these issue in my book, which is available (free) on the web. Just go to http://plaza.ufl.edu/chepler. Look at chapters 4, 8 and 35

Doug

[Brian Holcombe]

I appreciate the feedback. It's especially helpful for someone like me that is trying to learn the craft via the internet and trial and error.

The last few comments are interesting - essentially downplaying the significance of moisture content and a meter to a point. So, if you don't have a meter, how do you know when enough time has passed to allow sufficient drying and/or acclimation? For example, I just got back a couple of walnut logs I had taken to the sawmill and milled into a mixture of slabs, 4/4, and 6/4 lumber. Currently, that is sitting stacked and stickered inside a barn. But without a meter, how do I know when that wood is dry enough to move into the shop for acclimation and eventually for use? Everything I've read says you don't want to build furniture with wood that is above 7%-8% (actual value varies based on source). How do you know when wood has reached that level without a meter?

We're talking about something entirely different now, as Kevin pointed out above. In furniture making we're making design choices which allow for wood movement. It is typical that a few weeks are needed to acclimatize kiln dried stock to a shop, obviously depending on how thick it is. Generally speaking, the more the better and so many of us will buy stock far in advance of using it so that it is acclimatized without question.

If you are bent on buying a meter, buy one. I've got plenty of tools that are of limited use and when I use them, I'm happy I have them. That said you need to work on your designs.

A new woodworker is lucky to learn this lesson in a not so condemning fashion, but one significant enough to make certain you never make the mistake again.

[John TenEyck]

Sorry to read about your problems. Number one - stop listening to whoever told you to glue/nail your lumber to a plywood backing! Get a book or two on furniture design/construction. Also, just study existing pieces of furniture you already own, at the store, hotel you are staying at, etc. You can learn a lot about what works, and occasionally about what doesn't, by just being observant.

Do you need a moisture meter. No, of course not - but you should have one. I said of course not because people having been building furniture for thousands of years so it's certainly not a necessity. Furniture has been failing for just as long, too, so clearly lots of folks got some things wrong. Even now that we know everything there is to know about wood (not really) pieces still fail for one reason or another. So get yourself a moisture meter and learn how to use it and when to use. If you are going to buy green and/or air dried lumber I'd say it's a must if you want to know what's going on.

I mill a lot of my own lumber so I've seen how long it takes for wood to dry - using a moisture meter, and cutting then weighing and oven drying samples, too. Where I live in the NE 4/4 lumber cut in the Spring is down to 12 - 14% MC in 4 months; usually it wants to dry so fast I have to slow it down to avoid checks. If I cut it in the Fall it will take all Winter plus another 2 to 3 months in the Spring to dry the same amount. OK, fine, it's down to 12 - 14% so I can bring it inside. Uh, how long before I can work it you ask. Good question, and w/o a moisture meter you won't know unless you want to cut samples and weigh and oven dry them. I now have a kiln to finish drying my wood so I know it's 6 - 8%. Before I built the kiln I would check the lumber every week or two and plot the MC vs. the EMC in my shop, which I know from the RH meter hanging on the wall. When it gets within 2% I know I can use it. I still do that with lumber I buy.

I also use my moisture meter when I buy lumber. Just because someone says lumber is kiln dried doesn't tell you much unless you know the person telling you can be trusted. I've bought KD wood that turned out to be 12% moisture. It turned into a potato chip when I resawed it. That episode is what finally prompted me to spend the big bucks on a moisture meter - all $100 or so I think it was. People will spend hundreds or thousands of $'s to have an incrementally better piece of machinery but don't see the need to understand the MC of the wood they are running through those machines. To each their own.

You sound like someone who is eager to learn about wood working. Good on you. Learn as much as you can - including about the wood you are using. Buy a moisture meter. And buy Hoadley's book "Understanding Wood". Best $30 or so you'll ever spend on woodworking.

John

[JohnM Martin]

Thank you, John T. that was great.