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Moisture content of wood and building boxes

22K views 81 replies 21 participants last post by  rmaxwell 
#1 ·
I've just got a bunch of wood from a mill to build about 40 boxes, and I'm wondering about drying - and how that will affect my measurements.

Wood shrinks as it dries, so I've heard that's where the 9 5/8" deep dimension came from - manufacturers building boxes out of not-so-dry wood and allowing for shrinking as it dries - the box stays safely withing bee space requirements. 9 5/8 is about an 8th too big for proper bee space.

So do I need to reach a specific dryness before making final cuts and assembling? Or is it safe to take wetish wood and build to 9 5/8 dimensions?

Thanks,

Adam
 
#2 ·
Well, I don't think that you have to care much about wood movement so much. If all the the wood has the
same/similar moisture content you should be fine.
My only concern would be if the wood is very wet, gluing could cause some trouble. Really wet surface just don't glue
very well.

Cheers
Stefan
 
#6 ·
I believe the wood has been aired dried for some time. I doubt the mill would mill and sell green wood, but I guess it's possible. The wood is dressed, but there are a few boards where you can see it was the end of the log, and those ends are well-weathered and grey from what appears to be a considerable time outside.

I don't have a moisture meter, but I suppose I can do the "weigh-sample-dry-it-in-the-oven-and re-weigh" approach to finding out what it's moisture content is, and then go to the percentages Cleo suggests...

Adam
 
#4 ·
Adam Foster Collins.... I use pine, cypress, and poplar rough saw from saw mills. My experience is that a 10 1/2 inch board will shrink between 1/2 and 3/4 inch over a period of one year If you have green, rough cut, or as you put it, wetish wood, and you cut it to 9 5/8 you would be looking at a finshed board, after shrink, of about 9 inches. I would say unacceptable.

I try for 12 - 18 % moisture. This normally takes 9 months to 1 year, stripped, and air dried.

There is another good thread on this same topic in Equipment and Hardware review.

cchoganjr
 
#7 ·
I built 10's of boxes this year out of hemlock that was only cut for several days. I used rebate joints instead of box/finger joints so that shrinking didn't matter. I had no trouble at all. I made sure that I had planed, cut, built and painted within 3 days though - in hopes the layers of paint would slow down the drying of the wood and keep it more stable. It worked great. No issues at all. I did not measure my boxes after a few months but they may be a bit more shallow. I know frame clearance between boxes is tight on some of them. Maybe add an extra 1/8 or 1/4 to your depth just in case. It won't be enough to matter if they don't shrink.
 
#11 ·
Green lumber from a mill is actually a common practice, it takes a lot of space/time or a kiln to supply dry material..and you normally pay for that additional step. logs lying in the yard rarely drop below fiber saturation point (the moisture level at which the wood begins to shrink) The mills intentionally keep the logs damp and saw them green; it is easier on the equipment and avoids/reduces losses to end checking.

That said, if the material has been surfaced it is likely to have been at least air dried down to 14% which would be fine for outdoor work. It is rare and a bit pointless to run true green lumber through a planer.

Mike is right, just ask the mill, they should have a reasonable idea of the material's moisture content.

Ace, your test doesn't tell you anything useful except that the wood is not at 0% if there is shrinkage. The oven test is done with weight of the wood and then carried to "oven dry" or 0%MC (the point at which the sample stops losing mass) the loss in water weight relative to the weight of the dry wood then allows you to calculate the original MC of the sample. If you don't know your starting point or end points, the amount of shrinkage you see is meaningless. Once you know the MC of the material it is a simple matter to calculate the shrinkage by species and cut that the boards will reach at environmental equilibrium.

If the wood has already been planed to thickness, then Mike's point about cupping certainly comes into play...that's one reason it's best to dress lumber that is at equilibrium with it's intended use...it eliminates those stresses by removing whatever cupping has occurred as it dries to that point. If it is dressed and flat now...I would use it now

Boxes the size of hives are small enough and thin enough to self restrain mild cupping and warping forces. If you know your MC you can likely just cut your material to account for the anticipated shrinkage across the face of the board....not best practice but certainly doable.


The oven test is slow cumbersome job, and simple pin type moisture meters are cheap and reasonably accurate.
 
#14 ·
Ace, your test doesn't tell you anything useful except that the wood is not at 0% if there is shrinkage.
The amount of shrinkage from 14% to oven dry will be considerably less than green to oven dry. The rate at which the moisture leaves the wood decreases as the moisture content decreases. I don't know if 250 degrees for a couple of hours in a dry oven would bring the sample to 0% if the start point was 14%. I would rather doubt it. What do you think?
 
#13 ·
Ace,
Just thought about it more your suggestion could work if you dried the board untill shrinkage stopped (meaning 0% MC) and then used the dimensional change and existing shrinkage tables to back calculate the original MC. I have never heard of doing it that way but it should work. Unfortunately shrinkage is non linear so it would be somewhat inaccurate.
 
#15 ·
my mistake, shrinkage is linear. I was thinking of some strength properties relative to MC.

A couple hours...I doubt it. obviously dependent on species and thickness. You could get there faster with a microwave..... but really I like my moisture meter if you work regularly in wood it is 50$ that instantly eliminates all sorts of questions and guesswork.
 
#16 ·
but really I like my moisture meter if you work regularly in wood it is 50$ that instantly eliminates all sorts of questions and guesswork.
Does it? I am guessing it really only probes the surface. The surface could easily be much different then the rest of the board if the board is not in a controlled environment for a considerable amount of time.
I have used IR scanners that could only read the surface and they were 20K. A nuclear mass scanner can penetrate much deeper into a substrate but they are upwards of 100K. I can't see a $50. instrument telling you much under varying conditions. I think you would have to be very observant on controlling the variables.
 
#17 ·
I built a bunch of boxes years ago from a local mill, and the wood was air dried for about a year. After the wood is worked into boxes, it will shrink, and I found the boxes that I cut 1/8 over size shrunk to being 1/8 to small in every direction. I would not worry about it too much, as these boxes will prove their purpose for the next few years.
which ever way you look at it, these boxes will be a pain, you are going to cut them 1/8 larger than needed to allow for shrinkage, so your bee space is wrong, then they will eventually shrink , to the right size , and many will over shrink and cause you bee space problem

thats the nature of home made boxes

the guy that custom builds boxes here will measure every board before cutting into boxes and frames. quality and consistency is very important and that is why it is sometimes worth the extra money to buy them
 
#18 ·
Ian,
I can't say what you may have experienced or why. 1" softwood lumber properly air dried for a year should be at EMC with outdoor conditions....that is just what you want for hives or any woodwork in an exterior application. Around here that averages 12% MC and that is what my hives read after several years of use. Kiln dried material (6-8%) is going to expand and get bigger unless you live somewhere very dry. And I really don't know how the hive could shrink "in every direction"...wood movement along it's length is negligible. It sounds like the wood you used was still pretty wet.

Ace, pin meters are the industry standard. They work very well when used properly. Admittedly the 50$ units are less accurate than the 500$ but even the 50$ one is really quite useful. Pin penetration can be from 1/16" to 1"+ depending on probe type used. If your local conditions have changed radically over the last few days, the material is very thick, or it just came out of a kiln and you need an accurate reading. You simply cut 2" off the end of the board and stick the pins in the "middle".
 
#20 · (Edited)
I am very much concerned that some may be underestimating the amount of width shrinkage of saw mill lumber, that has not been air dried or kiln dried. I typically buy a lot of 500 board feet, of saw mill wood. As I strip stack it for drying, I normally take a piece and make the front of a nuc, cut to 9 7/8 inch and date it. Sometimes I measure it less than one year, sometimes I don't. I had several of these laying around the shop from past years, so I took a photo of two recent lots of them to show actual shrinkage. I am in Kentucky, so you likely know weather conditions year around here. It may be different where you live.

My experience over the years is, strip stacked, air dried, pine, cypress, poplar, will shrink 1/2 to 3/4 inch in one year. Almost nothing after that.

In the photo below, the nuc front on the left is poplar. It was cut 9 7/8, on 10 October 2011. It was measured on 5 March 2012 and it measured 9 9/16, In 5 months it has shrunk 5/16, Today, 18 Dec 2012 it is 9 5/16. Total shrinkage in 14 months is 9/16 inch.

The nuc on the right is Pine. It was cut 9 7/8 on 20 December 2011. Today, 18 December 2012 it measured 9 7/16. Shrinkage in 12 months is 7/16 inch. Both were cut from 1 inch green, rough saw, saw mill lumber, planed to 3/4 inch. I never know how long the log has been cut, but, I pick up the wood as soon as the mill has it cut.

I don't know if my experience is normal or not, but, it is what I have experienced and I have the pieces to show how much it shrinks. If someone cut their deep to 9 5/8, then one year later, my experience is that that box would be somewhere near 8 7/8 inch, and would be way too short for a deep super.

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cchoganjr
 
#21 ·
>>It sounds like the wood you used was still pretty wet.

yes I bet it was.

but this is my point, and it doesnt look like I said this in my last post

if your wanting to work with wood that has been dried, specify the MC reading on every board, not the average MC of the lot. This way you will get a consistent supply of dry wood. All the box makers buy their wood this way to ensure consistency otherwise we would all be buying mis matching box material. I have never bought wood this way, and dont know what conditions are used for orders like this, but I know for a fact, these guys demand an accurate MC measurement and quality cut wood.
 
#24 ·
Wood move a LOT as it drys, and it does NOT move in a linear fashion. It shrinks more along the grain that across it (hence the cup) and irregular grain will dry into irregular shapes.

Modern KD lumber, especially pine, is rarely properly equilibrated, hence the problem with boxes shrinking (they should actually grow as the MC goes from the 6-8% of properly KD lumber to the approximately 12% that is typical here). Wet lumber should be stacked properly and air dried over a summer and winter, then machined flat and straight. Cut it and assemble at once, you should NOT leave cut pieces laying about changing moisture content and warping. If you must, don't stack them on a floor, put them on one edge and separate them to keep them from absorbing or losing water on one side only, they will stay flat much better.

Wood is live material even when cut, dried, and painted, and will always move a bit with moisture content changes. This movement is fairly small if the wood was properly cut and selected for consistent grain and the wood was allowed to equilibrate properly before machining, but it's still there.

Making bee equipment from wet wood straight from the mill is asking for warped, split, and twisted boxed that are too shallow for your frames, and it's very difficult to predict how much the wood will shrink, every single log is different!

Peter

Peter
 
#26 ·
Just a note that I have learned from buying saw mill lumber, poplar, cypress, pine. Have the wood cut a full one inch to one and one quarter inch. After seasoning, most cupping that you get, can be taken out by planing. If you strip, (I use 4 strips on 10 ft. boards) (see photo attached) about the only ones that will cupp are the top two or three boards. The weight of the others will keep them straight as they cure.

Most saw mills will charge you for 1 inch even if they cut it 7/8. I always specify full one inch, and most often I get 1 1/8. They still charge for 1 inch. Attached photo shows how I stack my saw mill cut wood. This small stack is cyress.

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cchoganjr
 
#31 · (Edited)
Quote from Acebird.... "I think they charge by the log, and from there they tell you what you want to hear". Unquote.


Acebird... No, all the saw mills I deal with, charge by the board foot, cut lumber. They calculate the board foot, but most often consider 1 1/4 as 1 inch. They will also consider 7/8 as one inch in calculating the board feet. 7/8 is really thick enough to finish 3/4 after planing, but 1 inch or 1 1/4 gives you chance to plane out some small imperfections, so, I specify full 1 inch when I order. Requires a little more planing, but, I feel it is worth it.

Length, times width, times thickness,= board feet. I buy lots of 500 board feet at a time, and stack and cure, normally for one year. Mine is all stacked in a big barn.

I run a few square hives (13 frames) attached is a photo of one of the square hives built from saw mill cypress.

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cchoganjr
 
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