there are words and phrases that tell you, regardless of the subject, you’re talking to someone who makes furniture. we don’t talk about “portable” — it’s “knockdown”. we don’t “trim” when we can “pare” or “resurface” when we can “finish”. nothing’s “sharp” until it’s “screaming-sharp”, “scary-sharp” or even “sharp-sharp”. but the phrase that strikes fear into the heart of everyone who ever sent a piece of fine furniture out the door to a client and dreaded the first six or eight months of its like is “seasonal wood-movement”. we know it happens. we prepare for it and compensate for it and, as anyone used to working in metal, ceramics or plastics will immediately attest, obsess over it. but this is the curse of working with the otherwise-blessing-filled organic material we have made our source of joy and emotional fulfillment. wood moves. the fact that we’re aware of it, though, doesn’t necessarily mean we understand exactly how, when and why it happens. and i think that’s unfortunate.
so let’s see what we can do to fix that. it’s not an unsolved mystery, something too complex for all but the brainiest of rocket-scientists and experimental engineers to grasp. it’s incredibly simple. you just have to understand a few very basic concepts and much of that tension linked to the threat of movement will dissipate. of course, nothing mitigates the need for good joinery and allowing space for the wood to actually move. but when your head is firmly rooted (yes, i know) in the moisture cycles of the material, it just becomes second-nature to allow for these things.
why is wood wet?
trees are living things. i think we sometimes forget that, even if we’re aware of it in theory. they grow, live and adapt to their environments. windstorms and sun angles determine tension and twist and each year brings new rings, new layers, new pores and a thick bulk of new material for us to turn into desks and chairs. but when was the last time you took a moment to contemplate what the tree as a living object was like — the tree before it met the saw or even the axe?
when you were in school, you likely learned about flowers and vegetables. sinking roots into the ground then gradually springing through the surface to produce layer after layer of organic material through cellular duplication. what was the trigger? sunlight, water and sugar. a tree is no different. there are very few complexities in the life of a plant — no confusing organ systems or communication pathways to deal with, though there is certainly an inherent systemic structure, especially for self-repair (pitch pockets, knots, crotch-figure and such demonstrate these reactions rather explicitly — in fact, anywhere the grain deviates from straight-lines is typically referred to as “reaction-wood” for this reason). the pattern is simple. water and sugar (yes, sugar is a simple way of putting this but it’ll do for our purposes) are absorbed through the roots and sunlight comes in through the canopy and the interaction produces cellular growth. i’m not an organic chemist or botanist but this level of understanding is plenty for what we need to know as woodworkers. just remember the purpose of the tree isn’t to provide our raw materials. that internal structure is to facilitate the distribution of water and sugar through the entire wood structure and produce leaves, nuts, berries and fruit.
as new layers of cellular growth appear, though, what you see from our perspective is yearly rings. these radiate from the center of the tree. let’s begin with some basic terminology. the center is called the pith, surrounded by the heartwood. the outside area, the part with the soft growth, is the sapwood and this is protected by an outer shell, the bark. when we cut a board, we generally remove the pith and bark plus as much of the sapwood (as it’s soft and usually another color) as possible. what you’re left with is heartwood, the hard, dense material generally well-suited to construction of stable objects. but, when it’s first cut from the tree, it’s saturated — these pieces are literally dripping with water. if you had a moisture-meter to stick on the freshly-cut pieces from a newly-felled log, it would read in the neighborhood of 100%.
before i go on from that point, perhaps it’s good to explain a little about how wood-moisture is calculated and why it will sometimes show as greater-than-a-hundred-percent, which drives math students crazy (me included) but is perfectly-sensible when you think more deeply about it. the percentage of moisture is calculated based on the quantity of dry wood. so the amount of wood if it was dry (0% humidity, which we never reach) was 1kg and it has 500g of water, that’s 50%. in some species, though, 1kg of dry wood might have 1.5kg of water in it. why, you ask, isn’t it calculated the other way around, from the 100% point? because it’s far harder to measure relative quantities that way because the saturation point varies by temperature and other environmental considerations while the dry weight is a fixed, known entity. the important part is to remember it’s a function of weight. you don’t need a fancy moisture-meter (though you should get one and i recommend the wagner ones if you want one that’s relatively price-effective). all you need is a table of dry volumetric weights, a scale and an understanding of basic arithmetic. measure the board. if your board is supposed to be 10kg at 0% and you’re aiming for 10%, that means your target weight is 11kg. if your board is at 14kg, its humidity is currently 40% and you have some serious waiting or baking to do. if it’s at 11.5kg, you’re almost there.
how do you dry wood?
wood, when it is no longer attached to a living tree supplying it with moisture from a root-system, naturally loses moisture in ambient air as long as that air can flow around the wood. if you make sure all six sides of a board can “breathe” — have access to flowing air — it will gradually drop in moisture content until it has reached equilibrium with the ambient relative-humidity of its location — if your shop has a relative-humidity of 50%, though, you’re likely working in a swimming pool and your wood will never dry in there to the point it’s no longer green (wet).
if you have a reasonably-dry place to leave it, though, and it’s got a chance to have the wind or at least air movement touch all its surfaces, you can dry a board from raw to usable (10-ish percent) in a period of years — a 2.5cm-thick board takes about a year to fully-dry, a 5cm board about three years, a 7.5cm board more like six years and so on. the thicker the board, the longer it takes — and the relationship is not linear. it’s a compound because the interior moisture takes longer to get out as it has to be shifted a greater distance from the center. what does this mean? a board should be, at the earliest possible moment, cut to rough length, width and thickness and it will dry far more quickly. know the sizes of your parts? do your rough-milling as soon as the tree is chopped into boards and you’ll be able to get to work much more quickly than if you leave them as slabs to dry.
there are a few ways to dry wood more quickly, of course. the natural way might be the easiest but it’s rarely the most convenient. for small pieces of wood, you can simply stick them in the oven and bake out the wood the way you would a cake or loaf of bread. if it doesn’t fit in your oven, though, you can simply build a bigger oven — it doesn’t have to get all that hot but the combination of heat and reduced humidity will force the moisture out of the wood at a much increased pace.
we call those large-scale ovens “kilns”. there are realistically two ways this is achieved. a “solar kiln” is simply a greenhouse with good ventilation. allow the sunlight to enter, get trapped and raise the temperature. now you have a hot environment to extract and evaporate the water and the wood will lose its moisture to the air that is then moved away from its surface to exchange the water with the outside environment. this can significantly reduce drying time — and it’s very inexpensive to build if you have the space. a solar kiln is a weekend construction project for an amateur woodworker and is realistically only dependent on having a place to build it. if you have access to freshly-milled logs and some extra space on your property, this is probably the most cost-effective way to get lumber — green lumber is usually either free or inexpensive because of the huge delay and urban logging is becoming more and more popular — in fact, if you’re willing to go and chop down the tree to take it away, many homeowners will actually pay you to acquire the lumber. even if you rough-mill it with a chainsaw and a homemade jig and the results are less-than-spectacular, once those pieces are dry you can refine them with your other tools and you’ll never be able to tell the initial milling was amateurish. all this approach needs is a small shed structure, preferably with transparent panels (cheap, thin plastic is a good way to achieve this with minimal cost) and a fan or two.
a more-involved kiln might simply take up a section of your existing shop — frame an area or use an existing storage-closet — or can be a standalone structure. instead of using the sun to raise the temperature, though, you add heating as well as ventilation. this can be as simple as a space-heater or two or as complex as a central-heating system like radiative heat or thermostatically-controlled elements. in a small kiln, this is sometimes achieved using lightbulbs. space-heaters are generally a safe and easy approach. paired with a dehumidifier and potentially a fan for circulation to extract the moisture, this can turn wet slabs into usable lumber in a small fraction of the time. if you have access to green lumber and have the space to devote to drying it at speed, this is another effective way to turn free-ish materials into the raw input for your craft. if you have lots of space and access to more trees and milling equipment, this might be a way to support your woodworking and maybe even make a good living, milling and drying slabs and boards for others who don’t have either the time or space to do it — slab furniture, while not my general taste, is extremely popular and people are willing to pay stupidly-high prices for a well-dried slab of oak, maple or (especially) walnut.
the next step up is something you’re likely not going to be building yourself even if you have the time and space — a pressure or vacuum-kiln. this is the same idea as what we just looked at — heat, humidity-reduction, ventilation — with the addition of pressure. you can certainly do it for small pieces with a pressure-pot or even in a drainage pipe (yes, this works but can be quite dangerous if you’re not careful and don’t understand the physics) but for most people it means something large enough to put whole 3m x 1m x 10cm slabs in in a stack and take them out a few hours dry. it’s extremely lucrative if you have one and this is a hobby you want to pursue. but it’s no longer a sideline. it’s a huge investment and they’re not to be operated by the faint-of-heart as they’re complex machinery in a way a solar or homemade kiln just isn’t. i’ve included it to be somewhat-comprehensive, though, not as a suggestion for a building project next time you’re bored.
there are other methods to increase the speed of drying, while we’re at it, though most don’t have much impact. tip the board on its end and raise it from the ground. leave it for a few weeks and the water will drain through the engrain far more quickly and the floor under the board will be wet. this usually only gets you to about 25% humidity but it can save you a few months on the front-end of drying time if you don’t have a kiln.
why does wood move?
this is where the real confusion happens, though. almost all material can absorb moisture and heat. metal isn’t a stationary object when its environment changes as anyone who’s worked with steel or copper will tell you. heat it and it swells. cool it and it shrinks. so what makes wood so special and why are we obsessed by wood-movement to the point every conversation we have with other woodworkers seems to return to that point eventually? uniform-volumetric-expansion.
that’s just a fancy way to say most materials grow and shrink with temperature and humidity change in a proportional way. they swell when it gets hot and contract when they cool in every direction the same. wood doesn’t. and wood doesn’t grow and shrink in a linear pattern, either. it bends. we refer to this as twist, cupping, dishing, warping and by many other words that strike fear into the heart of any joiner. but they all really come from the same two places.
the first is reactivity. when the tree grows under tension (a branch has weight and it levers against the trunk so the wood in the trunk has to develop strength in one direction through compression to compensate) and that pressure is released, the wood can move more freely and the compression expands. this generally only happens once, though, unless it doesn’t get to finish its restoration procedure. by the time you have the part milled and the furniture built, this is probably a thing of the past. unless you rushed or weren’t careful. don’t do that.
the other is ring-flattening, though. remember the rings we talked about when we discussed how a tree grows? they’re radial. the ring goes around the tree. but we don’t cut boards radially. we cut across the tree. which means the rings aren’t flat in the boards when we apply the saw. there are three loose ways to classify cutting a board — flat, rift and quarter. if a cut is flat (also called plain), it is cut across the log. the rings enter low on one side, curve across the board and exit at another height on the other side (it can be balanced or unbalanced and the effect is much the same). when the wood dries, the board will bend to make them as flat as possible. if you look at the endgrain and see a smile, remember the smile will become a straight-line when the wood dries (at least, it will try to and never gets all the way there). rift and quarter cuts are more stable because they are either vaguely-tangent-to (rift) or approximately-perpendicular-to (quarter) the radial pattern. the rings will still try to flatten but they’re far closer to already flat and the board will have to move far less to accomplish that goal. why isn’t all lumber rift? vastly more waste than flat-sawing. quartering wood is the middle-ground but it neither solves the problem nor uses the wood most efficiently. so you’ll need to play around with your milling procedure to see what the best balance between efficiency and stability is — build table-legs from quartered stock, for example, and they’ll have nice, straight grain on all four sides. there are many considerations beyond even the movement to take into account when choosing what cut to use but that’s a discussion for another time.
the important part, though, is to remember that wood moves because it expands and contracts radially — not radially according to the board but according to where it used to be when it was part of a living tree. if you want a shorthand guide and you’re using flat lumber, this is a good way to think of it. it will cup in whatever direction makes the rings on the endgrain as straight as possible. it will expand across its width but not in any significant way in its length or thickness. the relationship between expansion of width, thickness and length varies by species but you can generally imagine it will expand less than a tenth as much in thickness and a hundredth as much in length so these are simply not considerations when building a piece of furniture — the expansion in length along a 2m board is so insignificant it can be ignored. if it was a 200m board, it would have to be taken into account but you’re probably not building a table the length of a city-block, just part of a room.
how much does wood move?
this is where everyone really wants to jump to, of course. understanding the how and why wood moves is important. but when we’re making things it’s most relevant to think about how much it moves. of course, this varies by species. but here are some approximate guidelines to get you started — there are tables of exact shifts you can calculate for the species you’re working with but this is a good way to conceptualize it for most hardwoods available in the west.
for a 30cm-wide board, between winter (extreme dryness) and summer (extreme humidity), you will see an expansion of about 6mm if it’s cut flat. if it’s quarter or rift, about 2-3mm. when you are sizing parts that have to move against each other with different grain-orientations, this is a good way to think about it. if you have a drawer-front, for example, and you’re building at the driest time of year, if it’s 15cm you probably need to leave about 1.5mm on each side for expansion at the height of summer. you can certainly ignore some of this if the piece is never going to leave its air-conditioned environment but a/c fails and furniture often moves place to place. i suggest wood-movement is ignored at great risk in all circumstances. you don’t have to go crazy — 1.5mm is a pretty tiny space! you don’t need a gap in your drawers you can drive a truck through. and you don’t need any gap at all on the sides where the wood won’t significantly move with the seasons. it’s just good to remember you don’t want swelling drawers to shove the joinery apart or make your drawers stick shut or fail to close — that’s a way to lose clients and money in a hurry.
the joys of organic materials
this is all the complex stuff about wood movement but there is another side to this organic basis for our craft. the beauty. the fluctuating rings and natural structure that transfers water and growth through the tree gives us incredible patterns that show up as figure. yes, it makes the material more complex to work with and we have no choice but to accept movement as a natural side-effect of building with wood but the cost is, from my perspective, worth it when you see beautiful grain on the finished piece.
there are a few ways to avoid having to deal with wood movement, though, and i would be sadly-lacking if i didn’t mention them, especially as they’re so easy to accomplish if it’s what you want. the first is veneering. if you use a stable core (plywood or mdf, for example), you can veneer a 1-2mm thick piece of wood to its face and there will be no real movement to worry about. you can assume it will remain stable year-round. don’t want to deal with wood movement? no problem. just build with veneers and the issue disappears.
the other option is to simply build with plywood. don’t build with mdf. you’ll kick yourself. it’s awful and you’ll spend your entire life coughing from the dust — or you’ll wear a respirator every moment of every day in your shop and start to feel like darth vader after a few weeks. plywood, though, is an amazing product. thin layers of veneer stacked with the grain of each layer perpendicular to those surrounding it means the potential for expansion in each is eliminated by the stability in that direction of the next. it simply doesn’t move. each layer wants to move in a particular direction but the next layer wants to move only in another and the internal tension and glue-bonds between them enforce stability — it’s like wearing compression-tights if you’re a tree. multiple layers of them.
yes, many woodworkers have issues with plywood for reasons i’ve never understood. ethical? religious? ritualistic? i love working with plywood. and if you’re against machine-made plywood, rip the stock yourself into veneers and glue up your own. shop-made plywood with a beautiful veneer top is probably the most functional product for building you’ll ever work with and it can be both beautiful and dimensionally-stable. if you’ve never really done serious furniture from plywood, i think you owe it to yourself to make at least one piece and try it out. the material really will reward you — even if you’re a handtool-only woodworker. it’s still wood. it works the same way. yes, you might have to think a little differently about grain when you plane it but you can plane and chisel and scrape and hand-sand and drawknife and spokeshave plywood the same way as any solid stock. if you’re paying attention.
this, i hope, is useful for those who haven’t really thought too much about the mechanics of wood-movement until this point. other than the fear of it happening and the obsession with leaving space, that is. let me know if you’d like more detail on any of what i’ve discussed today. thanks for reading!