4 ms·

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

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.
    - DannyBee 1y ago
      "Initial shrinkage is larger when drying from green wood" ? 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://gupea.ub.gu.se/handle/2077/54179 https://www.mdpi.com/2076-3263/8/10/378 https://www.mdpi.com/2076-3263/8/10/378 https://www.sciencedirect.com/science/article/abs/pii/S1296207416304496 https://www.sciencedirect.com/science/article/abs/pii/S12962...
      - exDM69 1y ago
        Yeah, the coefficients are the same but the initial moisture content in green wood is much higher than the wood will ever get to after seasoning, it won't suck that much moisture from the air (unless you're in a swamp or something). So the annual absolute change in millimeters is lower than from green to seasoned. 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.
      - kurthr 1y ago
        Ummm, most wood starts at a much higher moisture content (50%-200% noting that this is as a percentage of fully dried so it can easily be over 1) than it will ever have after drying (typically 5-15%). 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%.
  - bee_rider 1y ago
    Are there designs that exploit this effect? I want a house with walls that intentionally become more permeable in the summer, less in the winter, haha.
roberthahn 1y ago
I think you’re off by an order of magnitude. With those numbers, a 12” board would expand and contract 1.2”, and an 8’ long board would vary by almost an inch. 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.
- DannyBee 1y ago
  Serious woodworker here: 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/1004/1004 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 :)

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.
  - DannyBee 1y ago
    "Initial shrinkage is larger when drying from green wood" ? 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://gupea.ub.gu.se/handle/2077/54179 https://www.mdpi.com/2076-3263/8/10/378 https://www.mdpi.com/2076-3263/8/10/378 https://www.sciencedirect.com/science/article/abs/pii/S1296207416304496 https://www.sciencedirect.com/science/article/abs/pii/S12962...
    - exDM69 1y ago
      Yeah, the coefficients are the same but the initial moisture content in green wood is much higher than the wood will ever get to after seasoning, it won't suck that much moisture from the air (unless you're in a swamp or something). So the annual absolute change in millimeters is lower than from green to seasoned. 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.
    - kurthr 1y ago
      Ummm, most wood starts at a much higher moisture content (50%-200% noting that this is as a percentage of fully dried so it can easily be over 1) than it will ever have after drying (typically 5-15%). 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%.
- bee_rider 1y ago
  Are there designs that exploit this effect? I want a house with walls that intentionally become more permeable in the summer, less in the winter, haha.

roberthahn 1y ago

I think you’re off by an order of magnitude. With those numbers, a 12” board would expand and contract 1.2”, and an 8’ long board would vary by almost an inch. 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.

DannyBee 1y ago
Serious woodworker here: 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/1004/1004 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 :)