How much will soft woods (ie, pine) typically expand and contract over, say, 20"?

[Luke Dupont]

I'm considering a bench-top design that would be, in total, 20", roughly half (or a little less) of that being a thinner board for a tool well.

For the tool well board, I was going to cut a rabbet (half lap) into the underside of the thicker top piece, and leave a gap large enough for the expansion and contraction of the entire top, as the skirt would be long-grain around the width of the top, and won't contract or expand with the top.

The problem is how much space to leave! 1/8"? 1/4"? I don't actually know how much 20" of softwood will typically expand. The changes in humidity here are kind of large.

I don't want to try to calculate this, because I can't measure the change in humidity over the course of the year. Rather, I'm trying to just get everyone's "best guess" as to how much of a gap is probably enough.

My totally uneducated guess is that something like 3/16 should be plenty, but I rarely ever build large things with wide tops or panels, so I'd like to know if that sounds close enough, or really off the mark.

[Richard Hutchings]

Depends on the wood but this should answer your question.

477401

[Luke Dupont]

Depends on the wood but this should answer your question.

477401


Yeah, I came across such charts but I have no idea how many percentage points the humidity here fluctuates indoors throughout the year, so I can't hope to calculate correctly.
I was hoping for a general "rule of thumb that should work in most situations"

Edit:
Looking this up further, it seems there's a distinction between "Moisture Content" in percentage terms, of the wood, and "Relative Humidity" -- a distinction that I wasn't making as I first tried to calculate.
https://www.thisiscarpentry.com/2010/09/03/moisture-content-wood-movement/
Okay, but I still have no idea how to even measure moisture content, nor do I have a year to do so with a given piece of wood before I start building...
Nor does Google seem to understand any of my conceived ways of wording the question "how much does furniture typically change in moisture content over the course of a year?"

I'm just amazed that every article I come across just assumes you have all of this data and can therefore calculate everything and put a plan into action, lol

[Luke Dupont]

Oh, awesome! Nevermind, that last article I found actually did the work and offered the following chart. Putting it here for everyone's convenience:


5. Species affects the amount of movement
Wood movement depends in part on the species. A 12-in. wide western red cedar board will fluctuate 1/8 in. while the same size maple board will fluctuate 1/4 in. The formula for calculating wood movement is complex and extremely accurate, but tedious.
One simple rule of thumb serves as an approximate guide to predicting wood movement: “Most species of flat grain material will change size 1% for every 4% change in MC.” Applying this formula to a situation where the seasonal EMC ranges from 6% to 10%, a 12-in. wide board will change dimension 1/8 in.

I’ve put together a rough chart (see below, click to enlarge) that offers approximate movement values for various widths and commonly used species of wood. These values are based on flat sawn lumber, and offer a general idea of anticipated annual in-service movement. The movement values for quarter sawn lumber are approximately 1/2 the flat sawn values.

http://www.thisiscarpentry.com/wp-content/uploads/2010/07/Species-Movement-Values-e1280334653977.jpg (http://www.thisiscarpentry.com/wp-content/uploads/2010/07/Species-Movement-Values.jpg) If you want to know exactly how much the material you’re using is going to shrink or expand, use this online shrinkage calculator (http://www.woodweb.com/cgi-bin/calculators/calc.pl?calculator=shrinkage). Simply enter the high and low MC values and the width and species of the board.

[Rafael Herrera]

As you research this keep in mind that some of the figures are for the lumber as it dries, not after it has reached equilibrium.

[Kevin Jenness]

Here in the US the NOAA has records of relative humidity variations in exterior air for all the states. Japan probably has a similar resource. You have to use your judgment and experience in relating that to indoor conditions, which can vary a lot depending on heating/cooling requirements and methods. My guess is that Tokyo has a relatively high and stable rh compared to my location but you are the one who is on the ground. Are there other woodworkers there you can consult?

You aren't likely to have a failure using a high movement value in this situation. In the future you may want to narrow your calculations though, for instance when sizing drawer parts to ensure smooth operation and minimal gaps. There's no substitute for having and using a hygrometer to track the humidity in your shop and where your work will end up. Weather records and a moisture meter can help. Bruce Hoadley's Understanding Wood is an invaluable reference for understanding water and wood.

Finally, are you sure you want a tool well? Mine was just a chip and dust accumulator and tool hider until I put a lid on it. If I ever build another bench it will be omitted.

[Richard Hutchings]

Finally, are you sure you want a tool well? Mine was just a chip and dust accumulator and tool hider until I put a lid on it. If I ever build another bench it will be omitted.

I'm with you on this. They're big mess collectors but I've always had one so I'm not sure if I could live without one. I'm tempted to remove it and add a saw and chisel holder in it's place.

[Luke Dupont]

Finally, are you sure you want a tool well? Mine was just a chip and dust accumulator and tool hider until I put a lid on it. If I ever build another bench it will be omitted.

With regards to the tool well...

Well, I've found I like a shallow one that is easy to sweep clean. They do fill up with junk, but their most important role is keeping my tools from rolling off, or getting knocked off, the bench.
In the past I've used small benches without a tool well and was always afraid of tools falling off -- and for good reason. I think I had at least a few chisels and squares crash onto the floor (!)

[scott lipscomb]

I have a board in my shop, which is of a type of wood that I very commonly use...its about 15" wide and about 1/2" thick. It's in the pile of scraps that I keep. Whenever I see it, I take it out and look at it, and measure the width, and write it down on the board with the date, and perhaps a note about the weather.

Its a very informative board, especially when it comes to sizing panels. I should probably do a few more species.

[Daniel Culotta]

First off, I think 3/16" should be fine :).

In general, when considering wood movement I find it useful to also consider context:
- Has the wood dried to its equilibrium moisture content based on where it's stored AND where the finished product will be placed? If it's wet construction lumber, it's going to shrink a bit more as it dries no matter where it is. And if your living room (or wherever the final location is) has a lower average humidity than your shop space, it's going to shrink more in that final location even if it was at equilibrium in your shop.
- What are the humidity conditions in the final location? The humidity level and swings in an insulated, HVAC-equipped home are typically more stable and less dramatic than the average outdoor readings. So, the board may not grow/shrink as much as the seasonal outdoor humidity swings would imply.
- What season are you building in? If you build in the season with the highest humidity, a board that's dried to its equilibrium is (pretty much) as wide as it's going to get. If you build in the season with the least humidity, it's about as narrow as it's going to get. Size your gaps accordingly for when the seasons change and those conditions flip.
- There are probably other considerations that I'm forgetting, but you get the idea.

[Tom Trees]

Radial vs tangential grain might be worth noting also, should you want a QS top, if you be able to acquire perfectly flatsawn planks and stand them on edge would
result in the least amount of movement.

[John Jardin]

Lee Valley has a wood movement calculator which should be a must for most woodworkers. $11.50

https://www.leevalley.com/en-us/shop/tools/books-and-dvds/46281-lee-valley-wood-movement-reference-guide?item=50K2401

[Mark Gibney]

That is a useful looking guide from Lee Valley. Think I'll get one, thanks.

[Richard Coers]

Depends on the cut of the wood and humidity. Rift or quartersawn are much more stable than flat cut. In my early years I would buy wide pine boards at the lumberyard and cut the rift and quarter sawn off the edges and leave the heart section for secondary wood or burn it. I usually use 1% for seasonal movement. But the time of year you do the work factors into the cutting. Here is Central IL I fit things tight in the humid summer, and loose in the dead of winter.

[Mel Fulks]

I don’t see anything about whether it’s for air dried or kiln dried.

[Charles Guest]

Face laminated lumber (think 2x4s or 2x6s glued face-to-face to make a benchtop) move through their thickness, and very little through the width as a glued top. This is why face laminations make great benchtops. Use your endcaps, screw the well board tight to the rabbet, and don't worry about it. The end caps and well board will move a little with the top as it expands and contracts through its thickness. Don't bother with glue on your endcaps (it's end grain gluing anyway) and of course no glue on the well board - just screws and not gigantic number 12s. 8s should be plenty, 6s even.

[John Jardin]

You are right, but most of us who build furniture consider wood movement the most important factor...not whether the wood is air dried or kiln dried.

[Tom Bender]

I have a tabletop glued up and waiting for the table to be finished. (taking my time on it) The top has moved 1/32" over 24" in 6 weeks. I'll have to fasten it in about 6 more weeks and have a total of 1/4" of play available. Just need to decide where in the play it's at then.

[mike calabrese]

Yeah, I came across such charts but I have no idea how many percentage points the humidity here fluctuates indoors throughout the year, so I can't hope to calculate correctly.
I was hoping for a general "rule of thumb that should work in most situations"

Edit:
Looking this up further, it seems there's a distinction between "Moisture Content" in percentage terms, of the wood, and "Relative Humidity" -- a distinction that I wasn't making as I first tried to calculate.
https://www.thisiscarpentry.com/2010/09/03/moisture-content-wood-movement/
Okay, but I still have no idea how to even measure moisture content, nor do I have a year to do so with a given piece of wood before I start building...
Nor does Google seem to understand any of my conceived ways of wording the question "how much does furniture typically change in moisture content over the course of a year?"

I'm just amazed that every article I come across just assumes you have all of this data and can therefore calculate everything and put a plan into action, lol

This chart may help it lists values in equilibrium moisture for locations in the US and for cities around the world. file:///C:/Users/tflit/Downloads/WEBSITE_DOWNLOAD/FOR%20VIDEO%20EMC%20Equilibrium%20Moisture%20Conte nt%20of%20Wood%20in%20Outdoor%20Locations%20in%20t he%20United%20States%20and%20Worldwide.pdf
The value of this chart and other interesting details can be see on this you tube video by Jonathan Katz-Moses https://www.youtube.com/watch?v=EOr06I9JoAU
mike calabrese

[Rafael Herrera]

Yeah, I came across such charts but I have no idea how many percentage points the humidity here fluctuates indoors throughout the year, so I can't hope to calculate correctly.
I was hoping for a general "rule of thumb that should work in most situations"

Edit:
Looking this up further, it seems there's a distinction between "Moisture Content" in percentage terms, of the wood, and "Relative Humidity" -- a distinction that I wasn't making as I first tried to calculate.
https://www.thisiscarpentry.com/2010/09/03/moisture-content-wood-movement/
Okay, but I still have no idea how to even measure moisture content, nor do I have a year to do so with a given piece of wood before I start building...
Nor does Google seem to understand any of my conceived ways of wording the question "how much does furniture typically change in moisture content over the course of a year?"


"Moisture Content" is a percentage value of the moisture content inside a piece of wood. You can buy meters for this.

"Relative Humidity" is a measure of moisture content in the surrounding environment, in this case inside your house. Most household thermometers have a RH scale.

You don't need an exact measurement of any of these values since they're varying constantly throughout the year. In a temperature controlled environment, furnace and A/C, the fluctuations of RH are not as wide as those outside. Your estimates need to use typical values for your house and the wood you're using. It'll give you an estimate of movement. Double it and design accordingly.

Charts of movement of wet wood until it reaches equilibrium are not necessarily the same as the movement of wood already in equilibrium and just fluctuating due to ambient moisture changes.