I’ve been taught that in the length it can expand/contract at most 1%, but in the width at most 10%.
This is also why properly designed tabletops are attached to the frame with a “floating” construction that can handle those changes.
This is also why properly designed tabletops are attached to the frame with a “floating” construction that can handle those changes.
This is for wood that is dried and stabilized, the shrinking is a bit more from green wood to seasoned lumber (but not an order of magnitude more).
You can use online calculators such as this one for estimates based on the species of wood and your location: https://kmtools.com/pages/wood-movement-calculator
The numbers here match my experience, a 600mm wide spruce table top shrunk and expanded by about 12mm during a year of being outdoors but under a roof at temperatures from -25C to +30C. The structure had sliding dovetails to allow growth but keep it flat.
The annual movement of wood depends (basically) on the local RH swing, thickness, absorption/diffusion rates, and swelling coefficients.
So giving any percents here without more data is just incomplete.
This is assuming bare wood too, with no coatings/etc.
A lot of the bare percents you see are making assumptions of various sorts. Usually they ignore the diffusion rates/etc and shoot for EMC at some parameters (the calculator you linked does) because doing it for real require more complex math. The calculator you linked is better than most for sure, but it is still a simplification of reality where it may be off by orders of magnitude depending on thickness.
It will be much closer to reality for thinner pieces than thicker ones.
By the way your relative humidity figures assume constant temperature. Wood cares about absolute humidity (mass of vapor per volume of air), and temperature is the dominant factor in absolute humidity. Rainy day at +1C (100% RH) is less absolute humidity than a sunny day at +30C.
This matters to me a lot because half of my woodworking projects are outdoors or not temperature controlled indoors.
?
It's not - it's exactly the same as anything else. The wood doesn't know it's green.
The calculator you gave is shortcutting it, and has an entire article in how they shortcut it the same way as anyone else, based on the swelling coefficients/etc, but assuming thickness is small enough to not matter.
If your projects are outdoors, you will be affected by more than just humidity - UV will also have a significant effect on the properties of your projects :)
The moisture transport is also not as simple as you are making it out to be, and has a not insignificant effect.
See:
https://gupea.ub.gu.se/handle/2077/54179
https://www.mdpi.com/2076-3263/8/10/378
https://www.sciencedirect.com/science/article/abs/pii/S12962...
I have my woodworking projects in temperatures ranging from -25C to +100C (sauna) and extreme humidity changes from near zero to 100% RH. It is a form of art to make wooden things survive that, and I don't always succeed.
Frankly, the idea that a piece of wood after initial drying was moving even an in/ft (eg "only" 8%) would be pretty shocking. Even good joinery won't deal with much more than a quarter of that ~2%.
Also, most green hardwood starts at about 30%.
No reputable lumber supplier is kiln drying hardwood that is 200% moisture content. That's crazy town.
Even if they dried it super slowly, it would end up as mostly checked/warped garbage that they couldn't sell.
Beyond that, wood is moving a lot, sorry you don't believe it, but its still gonna do it.
Rather than say it's "pretty shocking", and dismiss it, care to present any studies that back up your assertion?
I sent plenty, both in here and other comments. I'm not aware of any sourced, actual scientific research that says anything other than what i did, since I was careful to use cited figures from actual research studies, and not random pages on the internet about "wood movement"
I think you are also assuming a lot about how it moves and what 8% radial swelling/shrinkage really means that isn't necessarily true.
Also your point about joinery doesn't seem to make a lot of sense. While it's true that most joinery can't handle lots of flex, if everything expanded or contracted uniformly, it wouldn't be a problem.
You seem to be assuming the opening will not expand the same as the thing going into the opening. It will. That is why you try not to mix conflicting grain directions in joints, and why you see so many joints that go out of their way to do that (IE 90 degree mated dovetail boards are not made by two conflicting grain directions, the pins and tails are made of the same grain direction that happens to mate at an angle)
https://cad.onshape.com/documents/3e489410fcf65e1f0f82663d/w...
I made two tabs for you, one with a 25% transform and one without.
Notice the opening gets larger when scaled. So would the mate. They would still fit fine. The same is true if you made a dovetailed box. It would just become a bigger/smaller box.
I didn't bother to scale it differently for tangential vs radial but it wouldn't matter as long as the same scaling factors apply equally to the mate, and the mate is made the same way. As is true of most woodworking joints, on purpose.
So the only issue is if (assuming 2% was the limit) the non-uniformness lead to >2% difference somewhere that mattered.
All of this is also why wood glue has such expansion/contraction characteristics. if wood was only changing 0.1%, it wouldn't matter.
So far i've seen a lot of doubt but nobody else actually seems to be bringing any real scientific rigor to that doubt, or saying some silly things.
Please feel more than free, i'd love to see papers with real measurements that suggest something else.
https://extension.oregonstate.edu/catalog/pub/em-8600-wood-m...
Fiber saturation is a thing and it rarely exceeds 30% for usable lumber.
I'm mostly going on what I've read though I went outside to measure a 30" fir just now and it's 100% (REED Pinless). The 100 yr old apple trees were over 80%, but that's not peer reviewed (nor is it pay walled). Maybe people can cut down trees and toss them straight into an Alaskan for exactly this reason? Firewood sitting covered outside is <10%.
As for pretty shocking. Yes it would be, if the 36" door (flat sawn book matched) to my house didn't open, because it was an inch too big (it accommodates <1/4"/ft). It's about a hundred years old, and I'm pretty sure that's never happened.
I don't even know what you are arguing and why - it seems to change with every post.
So I give up - you still haven't actually shown me a study that says it's wrong, and now your argument is "my door would be too big".
This is a silly discussion.
Since you still haven't given me a single scientific study suggesting the movement doesn't actually occur, I guess i'll offer you this and then walk away:
Is your door surrounded by brick or something rigid? Or is it surrounded by wood and blocking, like most doors? What species is it? What are the radial/tangential shrinkage rates? Is it painted or otherwise sealed in a way that would affect rate of absorption, like most doors?
As an aside, did you know that basically no door company will warranty unpainted doors because of exactly the issue you say doesn't happen? Just about every single one will say something on the order of "this door must be painted or stained within x days or the warranty is void", where x is usually <7, and will unequivocally state that unpainted and unstained doors will warp. Because they do! Like potato chips, a lot of the time.
There are some made to be bare unpainted wood, but it's not common and it requires different construction techniques. Most of them are not solid wood either, they are 1/4" or 1/2" veneer pretending to be solid wood. Otherwise, doors left exposed to the elements often totally fall apart in years. All the time. I can show you one that fell apart due to movement in <5 years.
Beyond that -
Doors surrounded by brick or rigid things frequently become too large to open/close at various times.
My home was built in 1929, and the doors are painted, but the jamb is surrounded by limestone or brick on all sides. Not a facade. The jam is up against well-set brick or limestone. This is actually a super-bad construction technique, since in most cases, the brick/limestone is a facade to avoid this issue. I can send you videos if you want to see what happens.
In the winter, it is about 1/2-3/4 inch smaller than it is now overall. I've measured it. It does in fact, become unopenable in the summer. It actually is right now. I plane it until it can be opened again. It will show a very large gap in the winter.
This is on a painted door, so not even one that is totally exposed to the elements.
This is uncommon, again, because most doors are not surrounded by highly rigid materials. If they are, it's a facade instead of structural. Those doors that are structurally unable to move, will in fact, break apart. This is one of many reasons totally solid wood doors are uncommon (besides weight and cost)
Since you seem big on anecdote, and your door is your baseline, there's a door for you.
Most people with historic homes would laugh at what you are saying. Since you say your door is >100 years old, i'm sort of shocked at your view.
For example, my wife's interior office door, is wildly out of square and plumb. By about 2 inches. The concrete foundation and tile is exactly in the same place, and perfectly level and square. No tiles have broken or cracked, and they are original to the home. Only the things made of wood are no longer where they should be. The two exterior doors in her office on opposite sides were built identically ~100 years ago. They don't even close to line up any more, and are easily 1" off. Again, foundation is exactly where it should be. only the wood has moved.
But still, i'm out since we aren't actually having a useful discussion that involves more than vibes about doors.
Please reread the section about fsp and measured MC, because it explains clearly why wood does not expand beyond it's fsp ~30%. Then look at the simplified MC% vs RH% table and read data for shrinkage vs MC for various wood types. No pay walled university papers required, it's not that complicated.
If your wife's door is 2" out of square you probably have a house framing issue not door expansion due to indoor humidity.
I've sawn enough timber and built enough decks out of them that I know wood moves.
Much more reasonable would be 1% across the grain and 0.1% along it. You can confirm this in some of the wood movement calculators found online.
To those learning about wood movement, these ratios are decent but approximate; if you end up caring about these things you’ll want to check the species of the lumber you plan to work with.
They aren't off by that much. You are further off if you assume some standard parameter ranges :)
But in the end, it depends on factors i didn't see listed.
Overall, the percents are usually calculated by swelling coefficient. Swelling coefficient is percent change in radial/tangential for each 1 percent of moisture change. There are well-known sources for these that calculated them in sane ways. The US forest service is one of them, and they publish their methodologies/etc for how they determine them. See, e.g., https://wfs.swst.org/index.php/wfs/article/download/1004/100...
Take standard flat sawn red oak. The swelling coefficient is 0.001-0.002 for radial (0.1% per 1%), and 0.004-0.005 for tangential (0.4% per 1%).
So in initial drying, which is usually 30%->15%, it will move 1.5-3% radial and 6-7% tangential.
Without humidity control, houses swing from 30%<->60%. Sometimes per day, sometimes per month, sometimes per season. So even more than initial drying. But because the swing varies, depending on thickness/etc, how much moisture change you get in the wood, and how fast, will vary a lot.
If you assume it causes a 10% change in moisture content over the year, throughout the wood, we get 1-2% radial movement, and 4-5% tangential movement for red oak. But that is both swelling and shrinking, not solely one or the other.
So the GP would be off by a factor of 2 in one, but not off in the other.
It's obviously trickier in practice to calculate the actual rates because the moisture is going to diffuse through the wood at some rate, and as long as the RH is changing faster than the diffusion rate, the wood will not really have a consistent moisture content all the way through. To be accurate, you'd have to slice it into enough pieces to capture the different moisture levels in the wood, apply the coefficients to each slice, and, etc. Worse, because boards are rarely square, and instead often much wider than they are thick (IE 12"x1") , you'd have to slice and calculate it one way to deal with this for radial, and slice and calculate it the other way to deal with tangential.
I'm too lazy to calculate how coarse/fine of a slice you'd need to get within say 5% of the "real" number.
I'm also assuming you are trying to do it by hand, since this is obviously an integral of some sort that you could also just directly solve. I'm sure it's in a paper somewhere.
This is all for bare wood too, with no topcoats. The topcoat would seriously affect absorption rates, etc, even assuming you applied it to all sides.
Nobody does any of this calculation in practice, we just accept large error bars and build floating tables :)
https://www.wagnermeters.com/moisture-meters/wood-info/how-r...
This table shows up to a 4% moisture content seasonal difference in a climate controlled house (20-50% RH).
The 10% number was not meant to be real, i just was giving an example :)
Real is much harder.
4% is not a horrible guess from as best i can calculate (but see below because this page has some crazy claims). Studies suggest that wood RH tracks RH pretty closely, slowing down with depth. Transport also appears to depends on temperature, independent of humidity itself. But if you assume it's going to track RH closely and throw out the rest, you can just assume the wood will always fall within the EMC range for the RH range.
If you look at
https://www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapte...
You can see that between 30-60% RH, you really don't get more than like a 7% span (i'm eyeballing it) of EMC that the wood could vary around at any temperatures likely to exist in your house.
So 4% is probably not a horrible guess.
However,the site you link to says some very wrong things, interestingly:
"Temperature Has No Significant Effect on Wood MC"
This is 100% wrong, in more ways than one.
First actually even wrong if you ignore humidity entirely, because studies suggest wood moisture transport changes at high/low temperatures, even ignoring humidity. The exact mechanisms are not pinpointed (AFAICT from skimming), but that's what real data says.
Second, the temperature affects the EMC (and relative humidity).
It's very weird for them to go on and on about how humidity affects would but then say temperature doesn't matter at at all.
You can't actually separate these things, and say humidity level matters but temperature doesn't, because they are linked.
If you want real data/simulations to try to figure out more, here's some references - i didn't read all of them, busy morning, but i did at least look at most of them.
https://www.sciencedirect.com/science/article/abs/pii/S12962...
https://gupea.ub.gu.se/handle/2077/54179
But all things being equal, yes, they generally can only help keep moisture content more steady over time.