Effects of humidity

[Stan DeVaney]

I’ve been reading some of the threads on the effects of humidity because of a project my daughter has for me. She wants me to build a dining table for her and has already purchased the wood. The design is rather troublesome in as much as the legs will extend through and be flush with the top. The legs are to be 4x4 Cherry and the top will be 2 inch Alder. To make matters worse I live in dry eastern Washington where we will construct the table and she lives on the coast. High humidity here may be 40% . I’m guessing it will reach 100% where she lives. Will the Cherry and the Alder shrink and swell at different rates? I would love to be able to marry the top snugly up to the legs but am guessing I will need to leave a gap between the vertical Cherry legs and the horizontal Alder top to allow for expansion. Is there a way to determine how much space to leave or is it just a “guess and a hope“.
Thank you

Stan

[Myk Rian]

Maybe suggest to her that a traditional design might be a better idea.

[John Coloccia]

I think you're right to be concerned, Stan. I'm with Myk. Go with a traditional design. I think if you take large legs and stick them through a table top, you risk all sorts of bad things happening...either large gaps in the winter, or an eventual cracked top. Maybe I'm not envisioning what you're trying to do correctly.

[Van Huskey]

You could look up the change coefficient but it really doesn't matter here. Since even if the change coefficient were the same you are dealing with long grain and end grain...

It will be more of a factor in high humidity WA since I am betting they leave the house open a lot during the high humidity months where as here in South Louisiana the humidity gets very high but we are usually conditioning the air and sucking the water out of the indoor air here.

I deal with my wifes "unusual" design ideas and usually try to accomodate them even when I shouldn't, she cheerfully allows me to spend LOTS of money on my hobby so the least I can do is try to accomodate here. In this case my first thought is to go unconventional. I would cut a thin endgrain veneer and inlay it where the legs are supposed to come through, I think it would work and nobody you didn't tell would know any different unless they got under the table to inspect the construction. Again I have never done this but from years of doing what my wife says instead of what my experience and gut tells me makes it easy for me to accept the compromise.

[Rick Fisher]

I have seen it done on Vancouver Island which is humidity central .. The feller had the legs stick up 1/4" higher than the table top, and did dado's all around the leg ..

The top fit loosely into the dado's .. In his application, the top was not really supported well by the legs in that place, but the top could move freely.

Rounding over the tops of the legs with a router, before cutting the dado's would be ideal..

[Jamie Buxton]

The usual way to build tables is that the table top is not structural. That is, the table base (for instance a legs-plus-aprons base) is completely self-supporting, and the top just sits on the base. To do what you're proposing, I'd use the 2"-thick table top as part of the table structure. I'd make the top, cut the through mortises, and plug the legs into them. There would be no aprons. There would also be no issues with hygroscopic expansion. The whole top would expand and contract, but nobody will ever notice.

[Larry Fox]

I'm with Van on this one - I would do it as inlay.

[David Giles]

Shouldn't be any problem that a good application of epoxy wouldn't fix in a year or two!

[Stan DeVaney]

I guess I didn’t explain this very well. She saw this particular table at a Crate and Barrel furniture store. The table is called the “Big Sur”. The edge of the top is even with the outside of the legs. The end of the top is also even with the edge of the leg. It’s like you notch each corner of the top to fit against the leg which extends to the top surface. This is still a cobbled up explanation, sorry. It seems like I’m going to have to leave a gap between the leg and the top to allow for expansion but I haven‘t got a clue as to how much. I know I’ll have to figure out a way to fasten the top to the frame work so that it will float, it’s the expansion due to humidity and dissimilar materials that has me stymied. And there are no plans as far as I know, just a picture. I guess I’ll find out, she’s on her way here with the wood. Thank you for the help.

[Van Huskey]

Just for reference here is the table:

[Van Huskey]

Details here:

[Van Huskey]

If she like the rustic look of that table it should be no big deal, leave a nice gap similar to whats on the oak original and be done. It has an inside apron so it shouldn't be a problem.

[Stan DeVaney]

Thanks for the picture. That certainly shows a gap around the leg. Just have to figure how much.
Stan

[Rick Fisher]

I looked it up before I found Van's post..

I would just build it .. I doubt it would fail, the legs aren't " through " the top .. I bet it would be fine ..

[Van Huskey]

I looked it up before I found Van's post..

I would just build it .. I doubt it would fail, the legs aren't " through " the top .. I bet it would be fine ..

You know the construction on that table is so simple I might do the same. I might just build it with tight fitting joints and modify it later if I needed to, it wouldn't be an issue to knock do and re-engineer. Though hauling a table that size across the state back to my shop might temper this. Although I have been wrong before this isn't something I would loose much sleep over even if there was no sizable gap.

[John Coloccia]

Let's say your table is 48" wide. A change in RH from 40% to 70% will cause the table to expand anywhere from 3/8" to 3/4" (depending on wether you're flatsaw or quartersawn)

http://www.woodbin.com/calcs/shrinkulator.htm

If you're going to have a frame, I believe those gaps are necessary or summertime expansion will tear that table apart. If you're just going to stick legs in the corners, it doesn't matter because the legs will just move with the table. Keep in mind, though, that since tangential and radial expansion are not equal, unless your wood is perfectly flatsawn or quartersawn, any movement will cause the top to warp slightly. This is not a big deal but with the legs attached to the top, it can cause the table to wobble depending on the season.

I'm not trying to be negative, but I'd like to make you aware of lessons learned over the years and give some insight as to why the traditional design (frame and floating top) has evolved the way it has.

[glenn bradley]

A picture is worth a thousand words. With the rough fit of those corner legs and the purposeful-looking gap for movement, I see no problem. I would build it in the traditional style with the legs and aprons being self supporting and the top just setting on this structure. Use clips (commercial or shop made) to attach the top and allow it to float. I have also used pocket holes through the apron with oversized screw holes that allow for movement as poorly shown in the last pic.

[Jamie Buxton]

... If you're just going to stick legs in the corners, it doesn't matter because the legs will just move with the table. Keep in mind, though, that since tangential and radial expansion are not equal, unless your wood is perfectly flatsawn or quartersawn, any movement will cause the top to warp slightly.. .

His proposed top is 2" thick. If it feels like warping, a little apron at each end isn't going to prevent it.

[John Coloccia]

His proposed top is 2" thick. If it feels like warping, a little apron at each end isn't going to prevent it.

The point about the warping was the next point that it will drag the legs with it and can cause the table to wobble if the legs are firmly attached to the top. If you use the standard clips, they can flex some and help keep the legs flat where they belong.

[Bill Petersen]

After seeing the pics and thinking about it...what about securely attaching the legs to the long edge grain with mechanical means or stout dowels and to the end grain with a floating tenon? I think that as wood does not significantly expand in length as opposed to cross-grain this might work.
Bill