More Wood Movement

[rrich]

Does wood move due to changes in MC? Absolutely!

Do we, as woodworkers, need to allow for this wood movement? Absolutely!

As Cody implies, do we need to go crazy when dealing with wood movement? Absolutely NOT!

Let me try to explain it this way:

The airline mileage between Los Angeles International (LAX) and Dulles International (IAD) is about 2280 miles. Or from fully saturated wood to completely dry wood the percentage of moisture can be 30% to 0%.

So what about Rich going to Las Vegas? From Huntington Beach to Las Vegas is about 300 miles via freeway. So what is the point?

Well, I'm not starting at LAX and I'm not arriving at LAS. It's the same thing with wood, moisture content and wood movement. The wood that we work with is not at saturation and the wood will not get to a completely dry state.

With wood, we need to be concerned with 'Where are we now' and 'Where do we expect be'?

With dried wood, we should be starting at about 6% to 8%. And where do we expect to go to could be as low as 3% or 10%. The difference being Arizona or Florida. In both cases we're looking at about a change of about 3% in moisture content up or down.

In a chart from school, a 12" FLAT SAWN will shrink or expand (In INCHES) as follows:
Red oak .26
White oak .32
Black Walnut .24
Red maple .25
Sugar maple .30
Black Cherry .21
Mahogany .15

For Quarter Sawn you can safely assume about half the expansion or shrinkage.

You are all thinking what is the starting point and what is the ending point? OK the STARTING moisture content is 7%. The ENDING moisture content is either 0% (Not realistic) or 14%. (Not very likely, but close.)

So how do you use this information?
If you are starting with Kiln Dried wood, assume the moisture content 7%. If you move the piece to a dry climate (e.g. Winter indoors) assume a drop of 3% in moisture content or shrinkage. If you are moving the piece to a wet climate (e.g. Summer w.o Air Conditioning) assume an increase of 3% or 4% in moisture content.

The chart shows wood movement for a 7% moisture content change and we're planning for about 50% less of moisture content change. So use about half the number shown in the above chart. For a 24" wide white oak table top, we need to plan for 5/8" of wood movement from DRY to WET. (2 feet x .32 per foot = .64" or 5/8")

Whoa! The 5/8" is way too much... Well you are starting at 7% MC and the table top can expand 5/16" or shrink 5/16" FROM WHERE YOU ARE STARTING or 7%.

Well that's better but still too much. You can solve that problem by doing what everyone says not to do, glue it so the top doesn't move BUT ONLY IN THE CENTER. Fix the center of the table top to the frame below so that the center (one inch or so) can not move. Then allow for 5/32" of wood movement on each side of the top.

We just went from over a half inch of wood movement to just barely more than 1/8". All we did was to bring wood movement into the real world and use a little common sense math.

Yes, we have to allow for wood movement but it is not the extreme that everyone thinks. All it takes is understanding the wood, how to apply the numbers and allowing for movement in a controlled environment.

[TDHofstetter]

I might interject one thing here to Rich's fine writeup... actually, it's a tiny rebuttal of one point. Many of us can't assume that our "kiln dried" lumber is anywhere near 7%, even if the rack label reads "6%-8%". You see, that was the moisture content when it left the KILN. Since then, it may have been trucked many miles on the back of a flatbed semi, parked in nobody knows how many truckyards, unloaded & fork-stacked anywhere, left in any sort of weather while it waited to be brought indoors to be racked, and then permitted to stand in open air in an open-bay warehouse fully exposed to outdoor air while it waited to be picked & purchased. In short... for many of us, we can't honestly expect "kiln dried" lumber to have any lower moisture content than quite ordinary air-dried, time-dried lumber. I dunno what moisture content that is 'round these parts, 'cause I don't have a meter. I do know that it's purt'near the same moisture content it'll have through most of the year in a typical Vermont house, midwinter notwithstanding.

Quicker words: I plan for very little expansion when I build, but I do plan for contraction. Others, in different climates & with different wood available, may do the opposite - planning for expansion but not contraction... or planning halfway for either. 'Pends.

[rrich]

Tim,
We just have to get you moved to Huntington Beach! ;D ;D ;D

[Leo Voisine]

Whoa Rich - hold on a minute!!!

Clear this up for me.

Your chart says that a 12 piece of oak can move a 1/4 inch??

I am having a hard time with that!

Are you SURE about that?

OK - which direction, Length? Width? or Thickness? or All??

At what moisture content change. Starting at 7% = 12 inches.

You are stating that my dresser I just built can move 1/4 inch per 12"

If that is true - then this dresser is going to tear itself to pieces.

Yes it is going to move - some. But 1/4 inch per foot is HUGE movement.

[Ruffnek]

Leo, that's the movement across the grain (tangential) with a 7% moisture change...that's huge, too.

Over 12 inches of width:

7% to 0% = 1/4" (shrinkage)
7% to 14% = 1/4" (expansion)

There is no way the piece is going to see that much change in MC unless you sit it in the rain or put it in a sauna. Forget about it reaching 0% MC...that ain't gonna happen.

Even when the humidity changes dramatically, it deoesn't stay there and the wood takes a long time to reach the EMC of whatever humidity it's in.

Remember that RoT that states 1 year of air drying per inch of wood thickness? Wood MC doesn't naturally change very rapidly and what happens is that before it reach EMC, the humidity has changed the other way so the wood reverses what it was doing. The end result is that overall movement is minimimal (assuming the wood is properly dried to start). Adding a finish to the wood slows down the moisture vapor exchange even more.

[rrich]

May 16, 2010 14:30:57 GMT -6 Ruffnek said:

Over 12 inches of width:

7% to 0% = 1/4" (shrinkage)
7% to 14% = 1/4" (expansion)

There is no way the piece is going to see that much change in MC unless you sit it in the rain or put it in a sauna. Forget about it reaching 0% MC...that ain't gonna happen.

Hey guys,
The numbers are out of the USDA document, "Wood as an Engineering Material". The instructors at school produced the table from the document. And, yes, the numbers seem realistic to me.

Remember you're going from 7% to either 14% or 0%.

AND, twelve inches of tangential cut wood. Almost 3/8 of an inch of white oak across 12 inches doesn't seem to be that much. It's only slightly less than 3-1/8%.

Remember in early granite quarries, they would drill a hole and stuff the hole with very dry wood. Then pour water into the hole to cause the wood to expand. The wood would expand enough to crack the granite into blocks.

[Stretch]

Here's another way to think of it.

We all know that if you record the moisture content of an unfinished piece of wood at equilibrium to it's environment the moisture % will fluctuate up and down with the seasonal changes in humidity. If you graph these percentages over time, the graph will look something like the graph of a sine wave. The amplitude of the waves represents the change in moisture content from the mean. Now if you finish that piece of wood the moisture content will still fluctuate but the amplitude of the waves will be much less because the finish will slow the rate of change in moisture content. If the furniture is built on site and never leaves, movement due to changes in moisture content can be cut to almost nil by using a good sealer. In fact, you would be hard pressed to find geographical location where the humidity fluctuates enough through the course of the year for the moisture content of the piece of wood will fluctuate the 7% that gives you the 1/4" of expansion/contraction in the graph. The only way you'll get that much change would be to ship a piece of furniture from a dry region to a humid region or vice versa. In this case, even using a good sealer will not prevent the problem, it will just take longer for the wood to adjust to the new environment, but it will eventually adjust.

Edit: Here's an article written by Bob Flexner discussing this. It has a graph of the concept I'm talking about, but the graph is kinda small.

www.masterpiecefurniture.com/design_notes/flexner/antiques_roadshow.html

[Ruffnek]

Stretch, the only thing I have to add to what you wrote is that the wood never reaches the humidity level of the environment. You can take a piece of wood that's at 7% MC in the NE, move it to Arizona where the average humidity is 3% and the wood will still never reach 3% MC. Conversely, a piece that's at 7% in Arizona that is moved to Florida will never reach the average 50% humidity (guessing) of the environment there.

It WILL eventually reach EMC, finished or not, but that is a reflection of the average humidity, not a level equal to it.

[Stretch]

May 16, 2010 19:15:45 GMT -6 Ruffnek said:

Stretch, the only thing I have to add to what you wrote is that the wood never reaches the humidity level of the environment. You can take a piece of wood that's at 7% MC in the NE, move it to Arizona where the average humidity is 3% and the wood will still never reach 3% MC. Conversely, a piece that's at 7% in Arizona that is moved to Florida will never reach the average 50% humidity (guessing) of the environment there.

It WILL eventually reach EMC, finished or not, but that is a reflection of the average humidity, not a level equal to it.

Yes. This is an important point. Thanks for filling it in.

[Leo Voisine]

Well I watch my firewood dry. No I don't really just sit there and watch it dry. But I buy it green and stack it in my firewood shed. Member I built that a couple of years ago. It shrinks a lot. I don't measure moisture content - but is is green when I get it. In 4 feet plus stacked up - it shrinks maybe 3 inches in height. NOT - to be considered a scientific analysis - but just to make a point that it does shrink.

I am sure it moves - and I trust the engineering data. It just seems a bit much to say 1/4 inch per foot, and I didn't read the part about the difference in change in moisture content. I may run a test one day. First I would need a meter - which I don't have.

[Stretch]

May 16, 2010 21:13:51 GMT -6 Leo Voisine said:

Well I watch my firewood dry. No I don't really just sit there and watch it dry. But I buy it green and stack it in my firewood shed. Member I built that a couple of years ago. It shrinks a lot. I don't measure moisture content - but is is green when I get it. In 4 feet plus stacked up - it shrinks maybe 3 inches in height. NOT - to be considered a scientific analysis - but just to make a point that it does shrink.

I am sure it moves - and I trust the engineering data. It just seems a bit much to say 1/4 inch per foot, and I didn't read the part about the difference in change in moisture content. I may run a test one day. First I would need a meter - which I don't have.

Got a scale? With a decently accurate scale that reads down to a gram would allow you to calculate moisture content from the change in weight at least as accurately as a meter.

[TDHofstetter]

Reloading scale. Reads in grains. Ridiculously accurate over a wide range. Handy for other purposes.

[dicklaxt]

Okay I got the answer,,,,,,,,,,,,,,listen to this,,,THAT WOOD IS GOING TO MOVE,,,who cares how much.

The wood in the test moved 1/4 inch and it more than likely did but those are the extremes.

It has oft times been said by the old furniture makers to design for 1/8 inch movement after your lumber has acclimated to its resting place(this is across a dresser top or a bedstead not per/foot) and leave it in these friendly surroundings and all will be well for many tears to come.If you move it from one extreme to the another then all bets are off,there are not any rules at this point because methods of construction come into play,,,,,its impossible to even guess what may happen.

dick

[fredbelknap]

A few years ago I read up on constructing a log home. The amount of space that has to be allowed over doors and windows was a lot. If my memory is correct it was like 6" or so. That is for horizontal stacking of green logs. Even using standard construction lumber there is a lot of shrinkage. Nail a metal strap across a 2x12 with a nail on each side and in a year or so you can stick a pencil under the strap. Furniture lumber is a lot more stable.

[TDHofstetter]

Fred, you've called up something that bothers me about a lot of modern construction - very often you see solid blocking above doors & windows: 2x8s, 2x10s, or 2x12s, running horizontally and standing on edge. As the house is closed in & heated & dries out, those moderate-moisture-content headers shrink in place.

A good header, one that doesn't shrink nearly so dramatically and will still bear the load it's asked to bear, is built up from 3/4" plywood & 2x4 stock instead. Side benefit is that it adds a little insulation value there, too.

[fredbelknap]

Tim I see what you mean, but I normally use 2x12 headers over windows up tight against the plate for the top of the rough opening. Don't remember having any problems.