5 ms·

Wood expands and contracts with moisture content. More moisture makes the fibers "fatten up". The interesting thing is that this is anisotropic: the expansion/

Wood expands and contracts with moisture content. More moisture makes the fibers "fatten up". The interesting thing is that this is anisotropic: the expansion/contraction occurs across the grain, NOT along the grain. The rate of expansion also depends on the local characteristics of the grain itself (hence the effects of warping due to uneven expansion) ... Also there's a big difference between the direction "across the growth rings" (i.e. radially when it was still a tree) and tangentially to the growth rings. And these surfaces are curved, of course. But one thing we can always say is: the wood doesn't significantly change size along the grain. Design and construction methods can make wooden artifacts more or less susceptible to cracking and distortion from this. For example dovetail joints can be pretty good as all the wood expands/contacts together the same way. Especially if the pieces are joined together from the same piece of wood. Stuff like that. Or at the other extreme, metal fixings like nails don't move with moisture at all, which can cause problems with relative movement and stress can accumulate. Edit: and the repeated cycling of moisture content induced stress can eventually lead to cracking, in a similar way to metal fatigue. Old wood just cracks sometimes, this is probably why.

arturocamembert 1y ago
Small addendum: some traditional wooden joinery is deliberately prepared to account for the varying rates and effects of drying across the timber. This is particularly relevant in timberframing, where you want to work with the wood when it is as green as possible. Green pine, though heavier to lug around, is significantly more receptive to a chisel than drier lumber. In a classic mortise and tenon joint [0], it's common to leave the outer edge of the shoulder slightly raised from the inner edge to account for the natural warping as the exterior of the beam dries more aggressively. Although it's more outside my area of experience, I believe fine carpentry also has a few techniques that see a higher frequency of use in areas that enjoy seasonal swings in humidity. The split-tenon is the only one that comes to mind, but, now that I think of it, I realize my mental model isn't great. More surface area to account for seasonal swelling / shrinkage? Maybe someone else can chime with a better explanation of this one. [0] https://www.barnyard.com/sites/default/files/styles/full_page/public/uploads/2021-02/mortise-tenon-0.jpg?itok=9IFdY9sN https://www.barnyard.com/sites/default/files/styles/full_pag...
- ofalkaed 1y ago
  Timber framing uses dry wood as well, slightly different techniques but in the softwoods and some of the hardwoods its is not all that harder to work dry than green and in some ways easier. It depends on the tradition and location as to the exact process and technique, some preferred dry timbers, some green, some something between. In US farm country it was common to fell the trees in late fall/early winter after the harvest was all taken care of and then leave the trees where they dropped until the ground froze. After the ground froze you haul them to the build site, much easier to drag logs on hard frozen ground than on soft wet ground. Then you would forget about them until after the spring planting is taken care of and build in the summer. Those big timbers would be far from dry but they will have lost a fair amount of weight and will be more stable which makes everything easier.
  - HeyLaughingBoy 1y ago
    I read a biography of the earthmoving equipment maker R.G. Le Tourneau, and it was really eye-opening how much this was a thing before mechanized equipment was readily available. A lot of moving was put off until winter because it was so much easier to drag logs, boulders, buildings, etc. over ice than over thawed ground.
    - potato3732842 1y ago
      Or waiting for things to freeze real good so you can dig the kind of hole or trench that would make HN clutch its pearls or simplify de-watering problems.
      - lazide 1y ago
        I’ve never tried to dig in frozen ground - isn’t that going to require blasting equipment or techniques closer to mining? (Heavy pneumatic jackhammers)
        HeyLaughingBoy 1y ago
rollulus 1y ago
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.
- exDM69 1y ago
  This is correct but the numbers are off by an order of magnitude. The annual movement of wood is maybe 2% width wise and almost negligible lengthwise. 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 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.
  - DannyBee 1y ago
    See my other comment - they are closer than you think in some sense, but there are too many missing variables to say anyone is right or wrong. 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.
    - exDM69 1y ago
      The figures I gave are annual movement. Initial shrinkage is larger when drying from green wood. The numbers (from the calculator) match my empirical observations very closely. 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.
exDM69 1y ago
A good mental model for wood is that trees are a bunch of stacked cones (growth rings) on top of each other. In the spring it fills with water and the diameter grows but the tree does not get longer because it needs to support a large mass on top and the lengthwise fibers are not able to grow and shrink (they need to be stiff to carry the weight). Because of this, the circumference of the outermost growth rings need to grow more than the inner ones. Now cut a board out of it and look at the end grain. Think what happens when the rings closer to the outside need to shrink more than the inner ones for the same humidity change. For a flat sawn board, you will always see it cup so that the concave side is on the outside. This doesn't explain why boards twist or bow but cupping is the most prevalent wood movement in typical flat sawn boards.
- jermaustin1 1y ago
  Both are actually explained the same way because in bowing, it dries slower in the middle of the board creating the bow, and twisting is just different type of uneven drying typically due to some open grain drying faster.
rags2riches 1y ago
A panel door is basically designed to minimize warping as the wood expands and contracts. There is leeway for the panels to move inside the edge pieces (sorry, not sure about the terminology here) and the edge pieces have the grain along the sides of the door. Stuck doors or doors that will not close are no fun.
cardamomo 1y ago
One nit: there are times when you do want to use a metal nail or screw in a joint, particularly if there is some sort of cross-grain joinery going on and you want to allow for wood movement. Chris Schwarz (publisher of this article) makes the point himself: https://blog.lostartpress.com/2015/07/11/the-bare-bones-basics-of-nail-technology/ https://blog.lostartpress.com/2015/07/11/the-bare-bones-basi...

arturocamembert 1y ago

Small addendum: some traditional wooden joinery is deliberately prepared to account for the varying rates and effects of drying across the timber. This is particularly relevant in timberframing, where you want to work with the wood when it is as green as possible. Green pine, though heavier to lug around, is significantly more receptive to a chisel than drier lumber. In a classic mortise and tenon joint [0], it's common to leave the outer edge of the shoulder slightly raised from the inner edge to account for the natural warping as the exterior of the beam dries more aggressively. Although it's more outside my area of experience, I believe fine carpentry also has a few techniques that see a higher frequency of use in areas that enjoy seasonal swings in humidity. The split-tenon is the only one that comes to mind, but, now that I think of it, I realize my mental model isn't great. More surface area to account for seasonal swelling / shrinkage? Maybe someone else can chime with a better explanation of this one. [0] https://www.barnyard.com/sites/default/files/styles/full_page/public/uploads/2021-02/mortise-tenon-0.jpg?itok=9IFdY9sN https://www.barnyard.com/sites/default/files/styles/full_pag...

ofalkaed 1y ago
Timber framing uses dry wood as well, slightly different techniques but in the softwoods and some of the hardwoods its is not all that harder to work dry than green and in some ways easier. It depends on the tradition and location as to the exact process and technique, some preferred dry timbers, some green, some something between. In US farm country it was common to fell the trees in late fall/early winter after the harvest was all taken care of and then leave the trees where they dropped until the ground froze. After the ground froze you haul them to the build site, much easier to drag logs on hard frozen ground than on soft wet ground. Then you would forget about them until after the spring planting is taken care of and build in the summer. Those big timbers would be far from dry but they will have lost a fair amount of weight and will be more stable which makes everything easier.
- HeyLaughingBoy 1y ago
  I read a biography of the earthmoving equipment maker R.G. Le Tourneau, and it was really eye-opening how much this was a thing before mechanized equipment was readily available. A lot of moving was put off until winter because it was so much easier to drag logs, boulders, buildings, etc. over ice than over thawed ground.
  - potato3732842 1y ago
    Or waiting for things to freeze real good so you can dig the kind of hole or trench that would make HN clutch its pearls or simplify de-watering problems.
    - lazide 1y ago
      I’ve never tried to dig in frozen ground - isn’t that going to require blasting equipment or techniques closer to mining? (Heavy pneumatic jackhammers)
      - HeyLaughingBoy 1y ago

rollulus 1y ago

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.

exDM69 1y ago
This is correct but the numbers are off by an order of magnitude. The annual movement of wood is maybe 2% width wise and almost negligible lengthwise. 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 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.
- DannyBee 1y ago
  See my other comment - they are closer than you think in some sense, but there are too many missing variables to say anyone is right or wrong. 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.
  - exDM69 1y ago
    The figures I gave are annual movement. Initial shrinkage is larger when drying from green wood. The numbers (from the calculator) match my empirical observations very closely. 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.

exDM69 1y ago

A good mental model for wood is that trees are a bunch of stacked cones (growth rings) on top of each other. In the spring it fills with water and the diameter grows but the tree does not get longer because it needs to support a large mass on top and the lengthwise fibers are not able to grow and shrink (they need to be stiff to carry the weight). Because of this, the circumference of the outermost growth rings need to grow more than the inner ones. Now cut a board out of it and look at the end grain. Think what happens when the rings closer to the outside need to shrink more than the inner ones for the same humidity change. For a flat sawn board, you will always see it cup so that the concave side is on the outside. This doesn't explain why boards twist or bow but cupping is the most prevalent wood movement in typical flat sawn boards.

jermaustin1 1y ago
Both are actually explained the same way because in bowing, it dries slower in the middle of the board creating the bow, and twisting is just different type of uneven drying typically due to some open grain drying faster.

rags2riches 1y ago

A panel door is basically designed to minimize warping as the wood expands and contracts. There is leeway for the panels to move inside the edge pieces (sorry, not sure about the terminology here) and the edge pieces have the grain along the sides of the door. Stuck doors or doors that will not close are no fun.

cardamomo 1y ago

One nit: there are times when you do want to use a metal nail or screw in a joint, particularly if there is some sort of cross-grain joinery going on and you want to allow for wood movement. Chris Schwarz (publisher of this article) makes the point himself: https://blog.lostartpress.com/2015/07/11/the-bare-bones-basics-of-nail-technology/ https://blog.lostartpress.com/2015/07/11/the-bare-bones-basi...