Some info/instruction on wood movement please

[Mark Rios]

So, one of my tenants said that they would like a shelf high up on the wall to be able to hang pots and pans off of. I have a butt load of 1 x 6 x 14' oak boards so I cut one up to make the shelf.

I milled the four pieces, 5" wide and 42" long, and cut a slot/dado down the length of each and then milled a spline and glued them up to make a 20" by 42" panel. Should I plan/make allowance for wood movement?

How is this different than a side or back panel of something like a desk, blanket chest, armoire or a table top? I've never built a table but I have read and learned that one wouldn't fasten down the glued up planks but would instead allow for a floating top to accommodate wood movement. The sides or back of a desk, blanket chest or armoire are fixed together while the table top is not. I"m confused about the difference.

For another project, I have five "planks" that are made up of small/short pieces of wood. There are strips, the largest strips are 1 1/2" x 20" and there are small pieces glued up, the largest of which is 3 1/2" x 9". Right now they are glued into "planks, 3 1/2" wide and 40" long. Is the need to worry about wood movement decreased by gluing up smaller pieces?


Could someone safely take a panel, say 24" x 40", made up of small pieces and attach it to something else on all for edges? To use a tangible example, let's say that I was making a torsion box. Could I use a panel made of glued up smaller pieces for the top or bottom?

Thanks very much for any info and advice.

[Steve Schoene]

If I understand this, you have edge joined 4 oak boards to make one broader oak board, using a spline to hold them in alignment.

Is it correct that the grain of the spines run the same way as the grain in the boards? If so, you won't have any wood movement issues between the spline and the boards. It was unnecessary work--just gluing well jointed edges would have been strong enough.

The joined up shelf won't change meaningfully in length. No allowances need to be made.

It will vary in width as moisture levels change. For that you have to make allowances. The fastening of the shelf to the wall or brackets, must allow the shelve to do that changing. The change between the end of a winter(dryest) and the end of summer (wettest) could be as much as 3/8".

That's the problem in trying to "frame" a solid wood panel. On the ends, the grain of the frame runs at right angles to the grain of the panel. The frame won't change in length, but the panel will change in width. That's one reason why things like torsion boxes are so commonly made with plywood, which shows much less dimensional change, and is similar between length and width of the panel.

[Mark Rios]

Thanks Steve. I agree about the strength issue however I used the splines for alignment rather than strength. My planer is only 13" wide and boards were already down to 5/8" in thickness after planing. The splines allowed me to get just about perfect alignment. I only had to do normal sanding to prep the surface.

You mention that there could be up to 3/8" in movement. Is it actually that much across only 4 - 5" oak boards? That sounds like a ton of movement. I wasn't aware that it would want to move that much.

Thanks again.

[Greg Cole]

Mark,
That 3/8'ths Steve mentioned should be the maximum movement (say measured in July and again January) expected for the 4 board glue up.
I've never had great success using biscuits for aligment so I usually just edge joint and glue it up.

Greg

[Cliff Rohrabacher]

I am guessing that Modesto CA has rather stable humidity most the year. Wood Movement will be more or less depending on local conditions. And if the piece is in a HVAC environment then expect even less movement.

[John Schreiber]

. . . And if the piece is in a HVAC environment then expect even less movement.
Is that right? I thought that with winter heating, indoor relative humidity went lower than it ever was in the natural world. Of course, it would be true that with AC, summer humidity would never be as high indoors as it is outside. And it might be different (everything else is) in California.

[Steve Schoene]

It's better to think of it as one 20" wide board. The 3/8" number is based on a change from wood moisture level content from 6% to 12%. Those wood moisture numbers correspond to wood in equilibrium at 29% relative humidity for the low end to 65% on the high end. Those aren't dramatic values on either end of the range. The data come from a very useful web page http://www.woodbin.com/calcs/shrinkulator.htm

Finished wood doesn't reach moisture equilibrium immediately, and consequently may well not reach the ends of the range every year. And air conditioning in summer and humidifiers in winter may narrow the range of moisture contents. But AC systems do fail, usually at bad times, and humidifiers are often the least reliable part of a central heating system.

In general, rather that trying to fine tune how narrow the range of movement might be in good conditions, it is a lot safer, to build to allow worst case scenarios. There are lots of very sound contruction methods that allow for wood movement and don't really limit good design.

[Mark Rios]

Thanks very much again to all who replied.

[Peter Quinn]

Is the need to worry about wood movement decreased by gluing up smaller pieces?

I had not seen this aspect of your question addressed. I'd answer no, the movement is not decreased by gluing up panels from smaller pieces. In terms of math, 1+1+1+1+1+1+1+1+1=9. Making up panels from strips will not add stability in terms of expansion across the grain, though it may decrease the potential for cupping and warping versus a single wide board, particularly if you are careful to orient your stock carefully. By alternating the heart wood and sap wood faces of the boards you can achieve better balance than a wide flat sawn board has naturally, though often at the expense of aesthetics.

The only allowance your shelf needs for movement is that the attachment to the brackets should allow for expansion. Typically an oversized screw hole will allow you to snug things up but still let the panel move, or if the brackets are steel they often have slots rather than holes, and you should apply the screws at the center of the slot.