Drying Lumber

Some miscellaneous facts about drying lumber.

General Info

The rate that lumber dries is based upon it species, thickness, moisture content, and environmental factors (temp, RH%, and air flow rate).

For every inch that you increase the thickness of a board beyond one inch, the kiln drying rate decreases exponentially by 60% per inch.

The so called “one year per inch of thickness” rule is inaccurate except for certain slow drying species such as oak.

Species such as pine, poplar, and cedar dry very quickly. In the summer time in the south a 1” thick pine board will frequently air dry to 14%MC within 60 – 90 days. A poplar board will air dry in about 90 days. Species such as maple and walnut dry at an intermediate rate, ie a 1” walnut board air dried in the south in the summer time will dry in about 120 days, and species such as oak, mesquite, and hickory dry very slowly (ie 1” per year).

It is best to store lumber “in the tree” until you are ready to mill it. When lumber is stored “in the log”, it will degrade by attracting bugs and stain. Certain decay resistant species such as eastern red cedar, white oak and black walnut may be successfully stored in log form for 1 – 3 years, but the sapwood will decay and the heartwood may develop some stain. Soft species such as pine and poplar will start decaying quickly.

Lumber dries very poorly in “log” form. It is best to dry lumber as dimensional boards.

Rounds, or “cookies” as they are called in the industry, almost always crack severely during the drying process unless they are treated with a preservative such as Pentacryl.

Air Drying and Stickering Info

Green lumber can be successfully dried by a hobbyist as long as some basics are observed. To dry properly, a stack of lumber needs gentle airflow across it. Thus, the best location to air dry lumber is where it is under cover but with good access for air flow. Open sided carports or carport structures can be excellent places to dry lumber because they offer cover from the weather but good air flow through the stack. Totally green lumber does not dry well in an enclosed space where it does not have airflow, nor does it dry well out of doors if the stack is placed longitudinally near the side of a building or anything else that may impede air flow through the stack.

Wood is hygroscopic, which means that it will absorb and release moisture based upon the surrounding relative humidity and it is always trying to reach equilibrium with it’s environment. In most parts of the US lumber dried and/or stored in a non-climate controlled environment such as outdoors or a non-climate controlled warehouse will air dry down to 12% – 16% moisture content (MC%). This is called “equilibrium moisture content” and is based upon temperature and relative humidity. Lumber destined for interior use should be dried between 6% – 10% MC, with 6% – 8% being ideal.

Stickers are small strips of wood that separate the boards while they are drying. Industry standard kiln drying stickers are 3/4″ thick and 1-1/4″ wide and are a dry hardwood material. Softwood such as pine, cedar, etc has also been successfully used for stickers. A good “store bought” sticker are “furring strips” or “1 x 2’s”. They are typically around 3/4″ thick and 1-1/2″ wide and come in 8′ strips. Cut them in half to make 48″ long stickers and you are good to go. A 1″ thick sticker is a good choice for air drying. The width should be about 1/2″ wider than the thickness so that they are not accidentally placed on edge (which is easy to do with a 3/4″ x 1″ sticker).

It is important to protect the top of the stack from rain and snow during the drying process. If left uncovered on top (or not placed under cover), rain/snow will cause excessive degrade on the boards – especially the top boards in the stack. Many air driers use old roofing tin or make a simple flat panel that they will place on top of the stack. Overhang the panel on all four sides of the stack at least 12″, and 24″ is better.

Do not tarp the sides of a stickered stack of green lumber while it is drying (unless it is on a very short term basis such as during a rain storm). Stickered lumber requires air flow in order to dry, and tarped lumber may develop severe surface mold. Using proper stickering techniques will make a significant difference in the quality of air dried lumber. Adding weight to the top of a stickered stack of green lumber will promote flatter boards. Ratchet straps can be successfully used to maintain pressure on a stack of stickered lumber. If you go this route, you will want to make “Spreader boards” that are a few inches wider than the stack for your ratchet straps to go over. They allow the pressure from the strap to be equally distributed across the entire width of the stack. Because lumber shrinks as it dries, you will want to tighten your ratchet straps about “one click” per week to maintain pressure as the stack dries.

Using green stickers (the spacers used to separate the layers of boards in a drying stack) can often lead to “sticker stain” which is a discoloration on the board where the sticker was located. It is important to use dry stickers to prevent sticker stain. With some hard to dry (without stain) species like maple, sticker stain and gray stain (an enzymatic oxidation reaction in the wood that occurs when the wood is exposed to air, high temp, and high humidity) is a big problem. So, it is key that there be good air exchange around the drying stack so that the evaporated water vapor from drying is removed from between the layers of the wood to lower the humidity. Here is a pic of gray stain blotch in maple used on a drawer side.

You do not want to try to air dry wood in an enclosed space where there is no air exchange. For example, a basement is a poor place for drying green lumber. The water vapor from the drying wood has to go somewhere, and you will be amazed by how much water will result from drying wood. All that water ends up in your basement. Not good.

The closer that you space the stickers, the flatter the resultant lumber will be. A lot of good lumber has been ruined because of poor stickering. Boards will take on the shape of the foundation of your sticker stack. If the foundation is not consistently level, your boards will not turn out flat. The stickers in a layer need to line up with each other as you build subsequent layers. There should also be good support in the foundation under each line of stickers. 

End Sealers

Ideally lumber should be dried from the face of the boards, as opposed to the ends of the boards. Due to the cellular structure, lumber loses moisture much more quickly through the ends of the boards than the face of the boards; however rapid moisture loss from the ends of boards can result in unequal shrinkage rates between the end of the board and the portion of the board 1′ or so from the end. This rapid shrinkage frequently results in “end checks” on the lumber and reduces the yield from your boards.

End sealers were developed to reduce the loss of moisture from the ends of the boards while not damaging post processing equipment such as jointers and planers. One of the most predominantly used end sealers is Anchor Seal Classic available from US Coatings or an authorized reseller. Bailey’s also sells an excellent end sealer. Store your end sealer where it won’t freeze. If this is not an option then purchase “winter formula” sealer which won’t freeze.

Many hobbyists use other forms of end sealer, such as latex paint, parafin wax, roofing tar, etc. While these may be effective in reducing excessive moisture loss from the ends of the boards, it is best to trim the ends of the boards after drying before they are run though your equipment. Latex paint may cause accelerate dulling of planer and jointer knives, and the residue from roofing tar may be difficult to remove (and transfer to your other boards if not removed quickly from your equipment). Anchorseal does not need to be removed and thus you will usually yield more lumber and have less time spent in post processing.

If you don’t use an end sealer, one of the best ways to minimize the growth of end checks is to sticker your boards within an inch or so of the ends of the stacks.

Kiln Dried versus Air Dried Lumber

Kiln dried lumber has two primary advantages over air dried lumber. These are that the kiln drying process will allow the operator to dry the lumber down to 6% – 8%, the ideal range for interior wood, and also that the kiln operator can heat sterilize the load at the end of the kiln cycle and kill any unwanted pests in the lumber. Kiln drying also allows the operator to safely dry the lumber more quickly than ambient environmental conditions would allow for air drying.

There are four different types of kiln drying processes. 1 – Conventional kilns which use high temperature steam to dry, 2 – Dehumidification (DH) kilns which use low temperature (90 – 120 degree) drying methods, 3 – vacuum kilns, and 4 – solar kilns. The only one of these four drying process that will change the color of black walnut is conventional kilns that use steam. Black walnut dried in solar, DH or vacuum kilns does not lose any color. Solar kilns will usually get hot enough to sterilize lumber in the summertime, but not in the winter.

Lumber shrinks as it dries; typically 5%- 6% in thickness for flat sawn lumber and up to 12% for quartersawn lumber. Width shrinking is opposite of face shrinkage; a flat sawn board will shrink 12% or so in width as it dries, and a quartersawn board will shrink 6%. For this reason a 1” flat sawn board is typically milled green at 1-1/16” – 1-1/8” to allow for shrinkage. Longitudinal shrinking is minor – typically around 1%.

Boards will distort during the drying process due to several factors, including stresses that preexisted in the tree, sapwood stress, poor stickering and knots. For this reason typically a board will require at least 1/8” of face jointing / planing per side in order to remove the rough sawn marks. Boards wider than 8” may require 3/16” – ¼” per side to clean up, and boards wider than 24” even more. A good rule to use when having lumber custom milled for you is to allow 10% for drying related shrinkage, and ¼” per 8” of board width for surfacing both sides. Boards milled within a few inches of the center of the log will cup more in the center than boards milled further away.

When brought into a climate controlled environment (50% RH and 75 degrees F), lumber that has been stored in a non-climate controlled environment will continue to dry as long as it is stickered and has good exposure to ambient air on the faces of the boards. Usually lumber will acclimate within a few weeks. Conversely, lumber that has a low MC% will gain moisture if stored in an environment with a higher RH%.

The average hobbyist can sterilize lumber at home by using a simple foam board built chamber and a space heater. There is an article in Fine Wood Working magazine about this.

Air dried lumber may be susceptible to insect infestations such as termites and powder post beetles. The best method of prevention is to store kiln dried lumber in a controlled environment and away from air dried lumber. Heat is the best sterilization method for lumber. When this is not practical, boric acid based solutions such as Timbor (for green lumber) or Bora-care (for dry lumber) have been used successfully for treatment. Fumigation methods have also been reported to be successful. If you will be drying thick slabs for an extended number of years, it would be a wise investment to treat them green with Timbor or something similar. 

Written by  Scott T Smith more info at his website: www.WPSawmill.com or https://sawmillcreek.org