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young guy needs urgent help with wood slab questions

10/16/12       
swisswood Member

Please help. Unforeseen circumstances find me in a do-or-die situation with limited industry knowledge. Specifically, relating to eliminating problems in custom-built solid wood plank tabletops and countertops via proper up-front planning: (1) Is it true that if you buy wood dried to 8% and seal it equally on both sides, assuming there are no variables such as radiant heat, plank width of up to 6" is comparable in warp and crack resistance to plank width of 1"-3"? (2) I have seen installations of tops with 10"-12" wide planks with no steel bracing below. Is this reasonable? (3) What are the pros, cons and pitfalls of thicker countertops (up to 3")?
My background is in particleboards and laminates. I've been chucked in the deep end here. For objectivity, assume we are talking about a Black Walnut table top 48" x 120" with Oil finish, Our family business depends on doing things right the First time so as to have the maximum safeguard agains future problems.

10/17/12       #2: young guy needs urgent help with wo ...
Jon

I can't follow what you meant by "assuming there are no variables such as radiant heat, plank width of up to 6" is comparable in warp and crack resistance to plank width of 1"-3"?," so you might want to clarify that part of your question. There's no reason a large solid top has to be made from many narrow pieces, but doing it that way can sometimes help prevent problems. It's entirely possible to have very large tops made from single plank, if it's done carefully. It's best to have the same sort of finish on both sides of a solid top, so that both sides take up or lose moisture at roughly the same rate, so that the top is less likely to cup.

Solid wood shrinks and expands with changes in moisture content. The main reason it's recommended that lumber be dried to 6-8% is that when it's used inside houses with central heating and/or air conditioning, it's likely that the wood will eventually get down near that moisture content by itself. If it starts out much wetter than that then it will shrink and possibly distort as it moves towards equilibrium with its permanent environment. So when you build something out of solid wood, you want the wood to already be as close to it's long-term moisture content as you can get it.

When you're building something with solid wood joinery (M&T joints, face frames, frame and panel doors, etc.), it's better to have the MC at manufacture a bit lower than the long-term EMC, so that the parts expand once the thing is in its permanent home and joints tend to get tighter. If the parts are wetter than they will be over the long term, the shrinkage as they dry will tend to pull joints apart. This is a less important consideration when you're just dealing with slab tabletops.

All solid wood changes size as MC changes, but the ways and degrees in which it changes size or warps depends on the species and on the orientation of the annual rings. Fortunately for you, black walnut is one of the more stable domestic species. Quartersawn planks are more stable than flatsawn.

Regardless of how dry the wood is at construction, the design of whatever you're building has to allow wide solid surfaces to shrink and expand a bit with seasonal humidity changes. You can't just screw a solid table top, which will shrink and expand seasonally, to a table base that will stay the same size; if you do, either the top or the base will be damaged by seasonal changes.

As to thickness, the main problem is that it's difficult to find wood thicker than 2" that has been appropriately dried. You'll find that thick, dry planks are quite expensive. You'll be tempted to buy something cheaper. The cheap stuff will be cheap because nobody spent the time and money to dry it. That will cause problems.

10/18/12       #3: young guy needs urgent help with wo ...
swisswood Member

Thanks, that's very helpful. I feel like in my case perhaps misinformation I had acquired throughout my life has been blocking out the truth, which is perhaps simpler than I thought.
I really don't know as much as I should about wood movement principles.
I do understand the basics of surface moisture imbalance (I think, anyway), e.g., if a house floods, the concentration of water vapor upon the bottom side of a tabletop will cause the bottom to swell which in turn causes cupping on the top.
To take another stab at the question I didn't phrase very well originally: Assuming normal conditions (no atypical spot concentration of moisture or heat) and assuming that the lumber is dried to 6-8%, can I then expect it to remain fairly stable from that point on? And if I understand you correctly, whether that lumber has been seamed in 3-inch or 8-inch or 16-inch planks is not so important, as long as that lumber was properly dried?
The objective of where all this is leading is that I want to do more with wide planks and maybe even live-edge but I want to be sure i do it correctly and to prevent cracks if at all possible. Because wwe ship nationwide, EMC/climate varies a lot.

Here are two more questions:
- What are the pros and cons of edge-grain-up as opposed to plank-style for stability of edge-glued-panel tabletops?
- How does Iroko compare to Walnut for stability?

10/18/12       #4: young guy needs urgent help with wo ...
Jon

"if a house floods, the concentration of water vapor upon the bottom side of a tabletop will cause the bottom to swell which in turn causes cupping on the top."

Let me tweak that a bit. The reason a lot of tables will cup upwards in a situation like that is that, against all the standard advice, most tables get a much lighter finish on the bottom than on the top (if any at all). It's not that there's more water vapor closer to the floor, or anything like that; it's that the lightly-finished or unfinished bottom surface absorbs water more readily than the heavily finished top side.

"Assuming normal conditions (no atypical spot concentration of moisture or heat) and assuming that the lumber is dried to 6-8%, can I then expect it to remain fairly stable from that point on?"

No, not really. Wood moves whenever its moisture content changes, and that happens whenever there's a significant change in relative humidity. This process of expansion and contraction with environmental changes does not stop because the wood was once dried to 6% MC. What you accomplish by properly drying the wood to as low an MC as it's ever likely to see again is that joinery is more likely to stay tight, and you eliminate the possibility that the wood will shrink dramatically as it acclimates to its new indoor environment. I met a guy once who lived in Denver, and who ordered a custom-made table from a shop located in some very humid area. Evidently the wood was not fully dry, because the table was circular when he got it, but within a few months was visibly oval.

"And if I understand you correctly, whether that lumber has been seamed in 3-inch or 8-inch or 16-inch planks is not so important, as long as that lumber was properly dried?"

As long as it was properly dried, and as long as it's then not exposed to extreme conditions like being stored in a damp basement, or a flood or exposure to extreme heat... then, yes, the plank width isn't so important.

The objective of where all this is leading is that I want to do more with wide planks and maybe even live-edge but I want to be sure i do it correctly and to prevent cracks if at all possible. Because wwe ship nationwide, EMC/climate varies a lot.

Shipping finished, wide, thick planks nationwide is a bit of a tall order. Average humidity in, say, Denver, compared to Baton Rouge, is going to be very different, and it will have an effect on large pieces of wood. You should read Hoadley's "Understanding Wood." Also, get a good moisture meter and learn to use it. You can't prevent wood movement, but you can anticipate it and design furniture so that it can move without damaging itself.

"What are the pros and cons of edge-grain-up as opposed to plank-style for stability of edge-glued-panel tabletops?"

To a tree, there's not really such a thing as "edge grain." It's all just wood. You're describing butcher block, which is a way to build up thick, heavy tabletops from thinner, less-expensive lumber. It does help with stability, in an extreme version of the way that gluing up a top from narrow planks does, by randomizing the orientation of the annual rings so that if one piece warps in one direction, it's countered by a neighbor trying to warp in the opposite direction, so that the overall surface stays fairly flat.

"- How does Iroko compare to Walnut for stability?"

I've never worked with Iroko, but just going by the numbers I'm finding, my guess is that Iroko is a little less stable than walnut, but not terrible.

10/18/12       #5: young guy needs urgent help with wo ...
swisswood Member

Thanks. I am very grateful for the help. As you can see, what little I thought I knew even is questionable.

Is there anything to be gained from recessing a sheet of plywood or MDF or even steel strips into the bottom of the tabletop to maintain flatness?

If we go the extra mile to get one really high-build sealer coat on the underside and then do 2-3 finish coats on top, is that balanced enough, or should coat count be the same on both sides?

Can the expansion/contraction of the solid wood core lead to cracks in the finish, or is this unlikely? (probably a modified urethane or hard Oil).

If we use this same edge-glued solid wood material for tops of furniture pieces such as dining room buffet cabinets or entertainment centers, does the fact that the underside is enclosed by a cabinet lead to additional problems, as opposed to the dining table which is open below?

Do you have any suggestions for clamping when one only needs to do 3-4 pieces per day and doesn't want to buy a JLT windmill-type clamp system just yet? We have a good supply of sturdy bar clamps.

Thanks much?

10/18/12       #6: young guy needs urgent help with wo ...
swisswood Member

error fix. Question mark at end of last message was a typo, sorry.

10/19/12       #7: young guy needs urgent help with wo ...
Jon

"Is there anything to be gained from recessing a sheet of plywood or MDF or even steel strips into the bottom of the tabletop to maintain flatness?"

Generally no. A stable sheet good like MDF would make warping worse, not better. Do you understand how a bimetal strip in a thermostat works? A bimetal strip is a lamination of 2 different metals that expand and contract at different rates, causing the strip to curl as temperature changes. The curling of the bimetal strip is used to actuate a switch. Attaching a stable sheet good to a moving solid wood plank would have the same effect. In a thick top it's conceivable that you could use something like a piece of angle iron as a stiffener, but that steel would have to be rather large and heavy to be able to restrain a thick plank, and shouldn't be necessary anyhow. You're building furniture, not a machinist's surface plate.

"If we go the extra mile to get one really high-build sealer coat on the underside and then do 2-3 finish coats on top, is that balanced enough, or should coat count be the same on both sides?"

This is outside my expertise, but I think you're likely to see diminishing returns the more finish you apply. The difference between a solid coat of sealer and no finish at all is probably significant. The difference between 1 heavy coat and three, probably not so much. Worth noting that true oil finishes don't provide much resistance to moisture anyhow (though a build of thinned varnish, which is what most products marketed as oil finish really are, can provide decent resistance).

"Can the expansion/contraction of the solid wood core lead to cracks in the finish, or is this unlikely? (probably a modified urethane or hard Oil)."

Shouldn't be an issue.

"If we use this same edge-glued solid wood material for tops of furniture pieces such as dining room buffet cabinets or entertainment centers, does the fact that the underside is enclosed by a cabinet lead to additional problems, as opposed to the dining table which is open below?"

Also shouldn't be an issue. Think of it this way: finishes and enclosed spaces do not prevent MC changes, they just slow them down. Unless you keep a panel in a dark, climate-controlled closet, no panel will ever be perfectly balanced all the time. Solid wood *will* move. Panels will get wider and narrower, and you must design your furniture to accommodate that. Generally you deal with the tendency to cup by:

1. Use wood that's properly dried in the first place.
2. Choose planks that are less likely to warp (species, orientation of rings).
3. Keep your wood storage and shop at a humidity level similar to that where the finished furniture will be used. Store the lumber for a while, in these conditions, before you use it. This is not the place for JIT ordering of materials.
4. Where feasible, restrain wide planks from curling by fastening them to sufficiently robust stiffeners. (i.e. don't expect a delicate little apron to restrain an 8/4 single-plank top.)

"Do you have any suggestions for clamping when one only needs to do 3-4 pieces per day and doesn't want to buy a JLT windmill-type clamp system just yet? We have a good supply of sturdy bar clamps."

You can just build a clamping table, with a bunch of parallel bar clamps lined up next to each other. For thick glue-ups, you'll need to also have loose clamps to put across the top, too.

10/19/12       #8: young guy needs urgent help with wo ...
swisswood Member

Thanks, Jon. I feel i have a much better understanding of this now, thanks to your help. This should get me started.

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