Wood Movement and "trapping" between legs - Woodworking Talk - Woodworkers Forum
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post #1 of 10 Old 12-09-2019, 04:31 PM Thread Starter
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Wood Movement and "trapping" between legs

I am designing a coffee table. The table will have a lower shelf. The lower shelf will be made up of three ¾ inch thick solid wood boards, the outer two are of one species and the center board is a different species (see attached sketch; top of the table removed to better show the placement of the lower shelf). My question concerns wood movement of the lower shelf; whether or not I can get away with “trapping” it between the legs as shown in my sketch.

I have seen a number of designs where the shelf (or a seat or even a top) is evidently similarly trapped. Perhaps these designs called for plywood or the part is narrow enough not to be a concern for wood movement.

However, my design calls for the shelf to be 22 inches wide, with 19.5 inches between the legs. I am thinking this is probably a no-no. My question is can I get away with my design and the width I want, or do I have to come up with an alternative arrangement. Also, if I do have to go with an alternative- any suggestions?

Thanks for the help.
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post #2 of 10 Old 12-09-2019, 04:49 PM
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Partly depends on how you also plan to account for wood movement of the top.

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post #3 of 10 Old 12-09-2019, 06:11 PM
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It’s not the legs that are the problem it’s the stretchers that restricts the movement. The easiest thing to do is build it the way you designed it, but leave an small gap between the shelf and the leg that allows for the movement. Probably 3/32. Inch would be enough to keep the shelf from being an issue.

If the 3/32 gap is too ugly, you could house the gap in the leg by routing a dado in the leg that would allow expansion and contraction to remain hidden.
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post #4 of 10 Old 12-11-2019, 06:10 PM
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Depending on where you live, 22" is a lot of wood movement from season to season. Terry offers some good ways to handle movement, leave some gaps or make a slot in the legs for the shelf to grow. There is at least one more way though. I've a coffee table with a lower shelf, although only 16" wide or so. My shelf dies into breadboard ends. Breadboards are tenoned and fit into mortises in the legs, so shelf is still allowed to move as needed. You don't need any aprons below with this method, but could still add them as a design feature if you want. Best of luck.
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post #5 of 10 Old 12-12-2019, 12:53 AM
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First, accept the fact that wood moves and there is nothing that we can do about it, except.
Second, use as a general rule of thumb, wood at equilibrium in the shop will move about 1/8 inch per foot of width.
Third, an old sage of an instructor told me, "When you have a problem call attention to it." Meaning make the edge as nice as possible. A simple round over is enough many times.

I built a similar coffee table. My design was to put a mortise in the stretchers and a tenon on the shelf. The mortise was about 1 inch shorter than the width of the shelf and the tenon was about an inch shorter than the mortise. Only the center inch of the tenon was glued into the mortise. During the past year we have been through some vicious humidity swings but no problem with the coffee table. A similar coffee table went up to the Portland, Oregon area and again, no problems.

Oh yes, the "except". Something 2 feet wide needs 1/4 to 5/16 inch for movement. That is wide enough to drive a truck through. If you fix the center inch or two of your two foot wide shelf in the coffee table, then the movement is restricted to 1/8 on each side of the shelf or half of the total. If you leave 5/32 on each edge wood movement ceases to be a problem.

In furniture 1/32" is a Grand Canyon
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post #6 of 10 Old 12-12-2019, 03:35 AM
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Okay, so I have done this design with ash. It was a quick bedside table that I'd made for my son. I'd left about 1/16" gap between the bottom shelf and the legs, and after many years, it still sits firmly and has never had any problem. And I screwed the shelf on through from the bottom edges of the stretchers.

The thing is, a lot of people here on this site base their answers on the 'theoretical' movement of the wood. However, in reality, wood movement is greatly affected by the climate that the piece lives in, and the wood species. For instance, I had no problem with that piece. But a friend who has a custom made cherry dresser with inset drawers, which only have about 1/32" gap all around the 8" drawer faces, cannot open his drawers for 4 months of the year. My apple dresser has 3/32" gap all around the 11" drawer faces and never gets stuck. And, BTW, neither of us has 'climate control' in our house. Lots of seasonal variation.
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post #7 of 10 Old 12-12-2019, 01:36 PM
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You can do it that way. Space in the dados in the legs a little deeper and attach shelf with either buttons or figure 8 fasteners.

A general rule of them is 1/8" of movement per foot so size dado depth accordingly.
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post #8 of 10 Old 12-12-2019, 04:41 PM
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There is one other solution.

Make the shelf with plywood (Baltic Birch) and edge the shelves with wood moldings.

In furniture 1/32" is a Grand Canyon
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post #9 of 10 Old 12-15-2019, 09:48 PM Thread Starter
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Thanks for all the suggestions. Pretty much confirmed my suspicions; I either have to mortise the legs or accept a small space between leg and shelf (or do a redesign of the table).

I am in northern California. Here we have just two season: one is damp, the other is dry. If I complete the build during the rainy season, I might try dimensioning the shelf so that the width not quite touches the lag. That way the 1/8 inch (or so) gap will only show half of the year.

Seems like it might be a workable compromise. What do you think?

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post #10 of 10 Old 01-04-2020, 09:53 PM
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You have to account for wood movement on bottom as you would on top. Treat the top and the bottom the same and you shouldnt have a problem. Account for wood movement....
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