by Eric Meier

Allowing lumber to passively sit at a given humidity level in order to obtain a desired EMC (air-drying) may be the simplest and least expensive method of seasoning wood, but it is also the very slowest. Drying times can vary significantly depending upon wood species, initial moisture level, lumber thickness, density, ambient conditions, and processing techniques.

Drying times and kilns

The traditional rule-of-thumb for air-drying lumber is to allow one year of drying time per inch of wood thickness; this adage obviously only takes a few of the aforementioned variables into account, but it’s at least a rough starting point in understanding the time investment required in order to properly air-dry lumber.

In situations where green wood is to be processed into usable boards, (especially in the case of thicker lumber), a kiln is frequently used to control the drying process. While there are various types of kilns used to dry lumber, the basic premise is usually the same: a large insulated chamber or room is used to balance and control humidity, temperature, and airflow to safely and efficiently bring wood down to an acceptable moisture content.

The main advantage of a kiln is that with the increased temperature and airflow—all while carefully maintaining and controlling the ambient humidity—the wood can be dried much more evenly, minimizing any sort of moisture gradient between the outer shell (which dries very quickly) and the inner core (which slowly equalizes moisture with the shell). Thus, a kiln is able to dry wood much more evenly, and it’s this uniformity in drying that allows it to also dry the wood quickly—simultaneously avoiding the drying defects usually associated with rapid, uneven drying.

Drying defects

But kiln drying may also introduce internal stresses into the wood—particularly if an improper kiln schedule is used, or if corrective measures are not employed—resulting in a condition known as case-hardening. This defect is caused when the outer shell begins to dry faster than the core: the shell tries to shrink, but is inhibited by the still-wet core. If the moisture difference between the core and the shell is too great, the shell can dry in a stretched condition. Later, as the core eventually begins to dry and shrink, the condition is reversed, and the stretched shell prohibits the core from completely shrinking. In extreme instances of case-hardening, the core can split and check in an irreversible condition called honeycombing.

This piece of Red Oak (Quercus rubra) exhibits honeycombing, which is among the worst of drying defects, both because it’s irreversible, and it usually can’t be detected by looking at the face of the lumber.

Kiln drying wood at elevated temperatures also has many other secondary effects as well, such as killing powderpost beetles (a destructive wood pest) in all stages of their development. However, it can also cause some woods—such as Black Walnut (Juglans nigra)—to lose the vibrancy of their heartwood colors, resulting in a more uniform and/or washed-out appearance.

For most woodworkers, running their own kiln to quickly dry lumber may be impractical or excessive. In most instances, simply storing project lumber at a targeted humidity level is the best option to ensure it will be at the correct EMC when building time comes. However, in some cases, such as when processing logs or other green wood into lumber, a more meticulous procedure will need to be followed.

Home air-drying tips

  • Process logs in a timely fashion. If a tree has just been cut down, or there has been recent storm damage, it’s best to process the logs into lumber as quickly as possible; doing so will help to open up the wood and aid in drying, which can prevent rot or stain from marring the wood. Bark on whole logs can act as a natural moisture-barrier, and if left unsawn, can contribute to fungal decay and deterioration in some species. A hallmark of poorly processed, do-it-yourself lumber is the presence of spalted or partially rotted wood.
  • Cut the wood slightly oversized. Remember that wood shrinks as it dries. This, along with the material that will inevitably be lost when the boards need to be jointed/planed smooth, mean that green wood should always be cut larger than the desired finished size. (And you usually don’t need to bother jointing/planing the wood prior to drying, since it will no doubt distort at least slightly during the drying process, and the edges should be dressed after the wood has dried to EMC—an exception to this is that two surfaces of a log should be jointed level to facilitate getting even and predictable cuts on the bandsaw.)
  • Seal the ends. In addition to processing logs in a timely manner to prevent stain and decay due to excessive moisture, the opposite is also to be avoided: allowing the wood to dry out too quickly will result in splits and endgrain checking. It is important to remember that moisture escapes from wood about 10 to 12 times faster on the ends than through other surfaces. Sealing the endgrain forces the moisture to exit in a slower, more uniform manner. If this is neglected, the ends will tend to shrink faster than the rest of the wood, creating tremendous stresses on the piece that’s ultimately only relieved with endgrain checks—a very common drying defect. (Although there are specially formulated endgrain sealers on the market, just about anything will do in a pinch: paraffin wax, polyurethane, shellac, or even latex paint can be used to seal the endgrain surface. The key is to build up a thick, obstructing film that will inhibit moisture from escaping at the ends of the board. In order to minimize the risk of checking, it is best practice to coat lumber ends within minutes—not hours or days—after coming off the saw.
  • Stack and sticker. Having lumber of uniform lengths and thicknesses greatly aids and simplifies the stacking process; once a log is sawn up into planks of satisfactory dimensions, it’s crucial to stack them in such a way that they will be exposed to air on all sides—stickers are typically used for such a task. Stickers are small pieces of wood (usually about 3/4” x 11/2”) that are used to add space between sawn planks, which increases ventilation and aids in a more uniform drying process. Sticker spacing varies depending on the species and thickness of the lumber being dried; a conservative spacing scheme would be every 12”, though usually 16” or 24” spacing can be safely used on thicker pieces.
  • Add weight. Once the stack of wood is stacked and stickered properly, it’s helpful to add weight to the stack. The lumber at the bottom of the stack is probably weighed down sufficiently by the wood on top of it, but boards near the top greatly benefit from added weight. Weighing the stack of wood down helps to prevent warping or distortion, which is especially important during the initial drying phase when going from green to an ambient EMC. Neatly and properly stacking, stickering, and weighing wood will go a long way towards ensuring that the drying process will result in flat, stable, and usable lumber.
This small stack of Buckthorn (Rhamnus cathartica) half-logs has just been cut, stacked, stickered, and sealed with a water-based wax emulsion coating.

This small stack of Buckthorn (Rhamnus cathartica) half-logs has just been cut, stacked, stickered, and sealed with a water-based wax emulsion coating.

  • Add heat once EMC is reached. It’s important not to rush the drying process too quickly, but once a wood pile has safely reached EMC, it may be necessary (especially during humid summer months) to bring the MC down even further for a specific project. This can be as simple as moving the lumber stack from a garage or shed into a heated basement indoors. In cases where shorter pieces are used, a drying cabinet can be used to gradually reduce the MC down to 12% mc, 6% mc, or any other level that an application may call for.

A drying cabinet can be nothing more than a simple wood cabinet with an incandescent lightbulb on a dimmer to finely control the light output—which in turn dictates both internal temperature and consequently relative humidity. Many thermometers (both traditional and digital) sold by big-box retailers also feature a hygrometer with a somewhat accurate readout of the relative humidity; the ability to know the rh of both the drying cabinet and the wood shop proves to be a helpful and prudent investment.

Warp and distortion

When a wood species has a high T/R ratio, it will tend to shrink in one dimension more than another while drying, causing distortion or warp. A good way to visualize the tendencies of wood during drying and shrinking is to picture the arc of the growth rings trying to flatten themselves out. (This of course is not actually the cause of the shrinkage, but it serves as a good memory tool to help visualize dimensional changes.)

This endgrain view of Plum exhibits cupping. The board was initially cut flat, with the top and bottom originally being parallel. Further machining will be necessary to ensure the board is flat and square.

This endgrain view of Plum exhibits cupping. The board was initially cut flat, with the top and bottom originally being parallel. Further machining will be necessary to ensure the board is flat and square.

The results of uneven shrinkage vary depending upon the particular shape and grain orientation of the board; flatsawn boards become cupped, riftsawn square stock becomes diamond-shaped, and circular dowels become ovoid.

Additionally, there are a number of warping issues that can occur which are not solely related to uneven shrinkage. In certain cases, a pre-existing flaw is present in the wood itself, which is only brought out and made apparent by the drying process. This can result in defects such as: bow, crook, twist, or a combination of two or more defects simultaneously.

Regardless of the specific names that can be applied to distorted lumber, most drying-related warping issues can at least be minimized using a few simple guidelines:

  • Use proper stacking techniques. As mentioned previously, by far the most important deterrent to warp is the adequate stacking, stickering, and weighing of a lumber stack.
  • Avoid juvenile wood. Juvenile wood is wood that is formed during a tree’s early years of growth, and can be thought of as an extension of the pith. There is no officially determined width of juvenile wood, (usually excluding the first few central growth rings is sufficient), but generally, the further the wood is cut from the pith, the better. Much like the pith itself, juvenile wood is very unstable, and has an elevated rate of longitudinal shrinkage; this increased shrinkage rate pulls against the mature wood and causes it to contract and deform either along the face of the board (bow), or along the side of the board (crook).
  • Avoid processing branches or leaning trees. Wood that has been growing at a slant doesn’t have uniform growth ring spacing and varies from the topside to the underside. This abnormal wood is called reaction wood, and it can cause a number of unpredictable warping problems during drying. In softwoods, reaction wood forms on the underside of a branch or trunk, and is called compression wood. Conversely, in hardwoods, just the opposite is true: its reaction wood forms on the topside and is called tension wood.
  • Avoid knots. Simply put, knots are sections in the trunk where limbs once grew. In addition to shrinking unevenly or possibly coming loose during drying, (leaving a knothole), knots can also create areas of concentrated abnormalities in the wood grain, and consequently impact its shrinkage properties. The presence of large knots can result in dramatic and exaggerated warp during drying.
  • Handle spiral or interlocked grain with care. Some wood species have what is called spiral or interlocked grain. Just as the name implies, the wood fibers grow in a twisted or interlocking manner. Not surprisingly, this can result in drying problems, most commonly twist—where one of the corners of a board is raised up out of the plane of the other three corners. Careful drying, along with proper stacking, stickering, and weighing can help alleviate difficulties caused by irregular or spiral grain.

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  1. Brian January 12, 2019 at 10:52 pm - Reply

    I had a small tornado hit my place and knocked down alot of trees cedar, oak,hickory ,walnut ect i want to mill the cedar into 1 inch boards and Shiplap side my house I have a big shop area with a wood stove In it my question is how long will it take the cedar to dry and what is the optimal temp to keep the shop while drying??

  2. Doug Coyle November 25, 2018 at 11:44 am - Reply

    I recently cut 4 white paper birch trees to use as braces from my cabin’s gable end to the loft approximately 10-12 feet. The poles will not support anything, just brace the outer wall from movement when the patio door is closed and add aesthetic value to the open area. The trees will remain intact with the bark. How do I dry these birch poles to ensure they do not rot and the bark stays put? Thank you.

    • Nate R January 14, 2019 at 1:27 pm - Reply

      Curious if you ever got a good answer on this anywhere, Doug?

  3. Pam D Davis November 16, 2018 at 12:55 pm - Reply

    I have a question… My husband has been drying out wood for a few years and I am trying to have a wall created (similar to shiplack but not tight or finished) just propped that way on the wall for storage purposes.

    Will it damage the wood at all to put it on it side like this? It will still get air. It will just be stored on its thin side and not wide side. Does this make sense?

  4. DanTheMan November 10, 2018 at 9:33 pm - Reply

    Great article – I have a question. I cut some rounds and some 45 degree slabs from a black cherry to make cutting boards. Do you think i can apply salad bowl finish right away to reduce any cracking and splitting? I was wondering if applying the finish would displace the moisture and then i could sand them down when a bit drier.

  5. Joe klep October 25, 2018 at 10:59 am - Reply

    I am remodeling my house and bought red oak retreads for my steps and they are 1 inch thick. I double coated the topside (not the underside) with polyurethane and left them in the garage (about3 weeks ). Unfortunately the all occurred during prolonged periods of rain and they cupped upward. The installer cannot use them.
    I am looking to salvage the retreads if at all possible.

    Can I use a kiln to dry out the retreads and will they straighten out? Is there any advice that anyone can offer to help me out of this expensive error. I was told after the fact that I should have coated both sides.

    • Eric October 25, 2018 at 12:02 pm - Reply

      Ouch, that sounds like a very unfortunate turn of events! It’s been my experience that cupped boards can be pretty hard to straighten back out. I do not believe that simply drying them (or otherwise adjusting the moisture content) will be the standalone solution. You might try a combination of weight and lowering moisture content. Maybe someone else can chime in with better ideas.

    • SScott October 31, 2018 at 3:13 pm - Reply

      Cut the boards into 2 in strips alternate the cup and glue back together. Run through a planner/jointer when your done. Lots of work but the only way to eliminate cup.

    • Tyler November 1, 2018 at 5:06 pm - Reply

      Honestly I would advise getting them as wet as possible then weighing them down heavy and letting them dry, steaming would be most effective followed by weight. I’ve straightened tons of wood up to 2 inches thick by soaking then then re drying with weight.

  6. Wil Cooley September 21, 2018 at 2:30 pm - Reply

    What does “EMC” mean? “Eric Meier Cured”? :)

    I’m sure I can find a definition elsewhere but it would be good to define it when it is first used.

    • Eric September 24, 2018 at 11:24 am - Reply

      Equilibrium moisture content.

      • Adam October 23, 2018 at 11:50 pm - Reply

        At what mc can I do the heat treatment? Looked all over and can’t find this info. I only mill Burl and usually a/d but just started kiln drying to kill powder post beetle. My first batch is down to 8% and the larger turning blanks are at around 12%. I don’t need it any dryer and in a rush for the next load.
        I work exclusively with madrone/big leaf maple Burl. I should note most of this Burl is air dried in the whole from 3yrs-40yrs, it’s very stable but I worry that the high heat may be harmful if done too wet. I’d like to do long heat cycles to insure the heat will penitrate into the larger blanks(12-16 hours)
        Thanks in advance

    • Julianna Middleton October 21, 2018 at 8:44 pm - Reply

      I just picked up 4 , 6 x 6 posts 10 foot long to replace front porch pillars. Green treated. 2 are very wet. What is the best way to dry these before use?

      • Cody November 13, 2018 at 6:57 pm - Reply

        Do you mean its Pressure treated and it’s wet? this article is in reference to green lumber that’s been cut from a recently live tree. “green” is usually used in reference to Pressure treated in home construction. If so, leave that bad boy in the sun for an hour or so, the slime will dry off quick, just be sure to install them not long after.

  7. Charles September 15, 2018 at 12:41 am - Reply

    What would you recommend for drying roughed out green Bowls that I’ve turned on my lathe?

    • Eric September 17, 2018 at 10:51 am - Reply

      Drying bowls is a different process than flat slabs of wood. I’ve always used the double paper bag method. Keep rough turned bowl inside a paper bag (or double bag for sensitive species) and then weigh it intermittently until the piece stops loosing (water) weight. After that, it should be ready for final turning.

  8. David Pedigo September 12, 2018 at 11:18 am - Reply

    I have some large Black walnut trees in my yard & occasionally they lose limbs of big enough size to have what I would consider usable material. Meaning if the bark were removed I would still have a piece with a minimum Diameter of 4″ I am just a hobbyist but would like to make some Pistol grips or even gun stocks with some of this lumber. I am certain I haven’t dried it out properly over the years as I just kinda piled it up out of the way. But this year there have been some bigger limbs coming down and I may need to trim the tree as one of the big branches est 16″ DiaMETER and probably a good 12′ long are hanging over my driveway and am afraid that at some point it will fall and damage my vehicles. So I would like to properly dry this wood out. or if necessary take it somewhere to have it kiln?.?. If it makes a difference in how it should be done I live in North Central Texas DFW area. so I’m looking for someone with 1st hand knowledge of this wood and how to achieve the best results for the types of projects I intend to use it for. I have played around with some of the smaller stuff from the piles of previous years but found it to be as described in an earlier article Kind of a washed look in regards to color and pop, it was very disappointing and I have waited a long time to get some of the bigger material harvested and don’t want to leave it to chance that I get the same results as the piled material. Thank you !

    • Eric September 12, 2018 at 3:01 pm - Reply

      I believe that kiln drying walnut actually causes some colors to be lost, your best bet, with regards to color, is to air dry the material. Walnut color is highly variable depending on growing conditions. Are you sure that the other stuff that you’ve used was heartwood? Sapwood is a pale gray color and can take up at least a few inches of the outer edge of the stem.

  9. Tristan August 29, 2018 at 5:15 am - Reply

    im planning to purchase rough cut bunya pine to build a furniture piece which will be stained eventually, would i need to let it dry or acclimatise? any information is greatly appreciated

  10. Steven wilson August 9, 2018 at 3:06 am - Reply

    We are currently slabbing Norfolk pine and we’re wondering what the recommended drying time for the slabs would be? They’re 2.5 inches thick, 2 metres long 900 wide

    • barney September 6, 2018 at 3:40 am - Reply

      A year for each inch in thickness is the standard drying time for all timber. But it will depend on were it is to be used as to the target mosture content around 11% for internal use and 15%- 20% for use outside.

    • Peter L November 22, 2018 at 9:13 am - Reply

      Hey there Steven. Just wondering what saw was used to slice cut the tree?

  11. L Borrowman June 16, 2018 at 2:01 pm - Reply

    I just bought rough cut lumber to build a wall in my basement is there anything I need to do to prepare it

    • Yan July 12, 2018 at 6:08 am - Reply

      good thing would be to acclimate them for a week or so in the room where they are going to be used

  12. Karen May 29, 2018 at 11:31 am - Reply

    How to home dry fresh planks with crusty bark? Seal ends with something?

  13. Karen May 29, 2018 at 11:29 am - Reply

    I just got 2 8 ft long planks of wood with crusty bark on the edges. I’d like to preserve the bark. Should I seal the ends with something while it dries?
    I plan slow on drying it in my house and then making shelves with the wood. Any other advice about drying or working with these planks is appreciated. is appreciated.

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