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Air Drying Lumber

Posted: May 16, 2014

How do you “create” useable lumber from your logs and how does moisture affects wood. Advice on drying lumber for your next “home grown” project.

By Scott Weikert, Forest Resources Extension Educator, Penn State Extension

Many forest landowners use lumber from their own property. What a great way to show a love of the land: to have cared for trees, used good forestry practices, and to have lumber from your land. Unfortunately, many people do not understand how to “create” useable lumber from their logs and how moisture affects wood. Let’s explore how to dry lumber for your next “home grown” project.

Understanding equilibrium moisture content (EMC) is the first step in successfully drying lumber. EMC is the moisture content (MC) at which wood neither gains nor loses moisture when surrounded by air at a given relative humidity (RH) and temperature. Every RH and temperature has an associated EMC. For example, at 80 degrees and 75% RH the EMC is 14.3%. Under these conditions, lumber left for a long enough time would dry to 14.3% MC. Once it reaches EMC the wood will not lose or gain moisture until either temperature or RH changes. If the RH increases to 80% and the temperature remains at 80, the wood will gain moisture until it reaches 15.5%, EMC. So, wood will gain or lose moisture until it is balance with the MC of the air. In Harrisburg, for example, the average outdoor EMC is 12.3%, which logically varies from day to day and month to month. As a comparison, the average outdoor EMC for Erie is 13.8%.

By extension, if outdoor conditions have an EMC so do inside conditions. Usually conditions inside a home have a lower EMC. If the RH in a home is maintained at 40% with a 70 degree temperature the EMC will be 7.7%. In the wintertime when homes are heated, the RH is often much lower which in-turn lowers the EMC. With the temperature at 70 degrees and 20% RH the EMC will be 4.4%.

What does all this talk about EMC talk have to do with drying your lumber? While there are several ways that EMC relates to drying lumber, most important to you is the final use of the lumber – where are going to use the lumber. Are you building a shed, or fine furniture? What are the average EMC conditions where you are using the lumber?

As wood dries below 30% MC, it begins to shrink. As a general rule, lumber is dried to shrink it before it is used to build a product. Otherwise there will be issues: for example, broken glue joints, warp, sticky drawers, unsightly gaps. If lumber is air dried to a 13% MC and then crafted into an end table inside a home with EMC is 5%, the wood will continue to dry and shrink. Some changes in MC will always happen; however, a small change of plus or minus 2% should not cause problems. When MC changes more than that, problems begin to surface. The more MC changes the more noticeable are the effects. Bottom line, air dried wood is not dry enough for interior use. Finishes will not stop moisture gain. They slow it down somewhat, but it will still occur. If you intend to use lumber where the EMC is between 12 to 15% (such as in an unheated shed) then air drying is probably good enough.

A few key points to remember about air drying lumber. Keep it well supported in packs with stickers about ¾” thick between the courses of lumber. Stickers will allow air to flow between the courses and facilitate drying. Place stickers no more than two feet apart; closer is better. Keep each sticker course perfectly aligned with the course above and below it to distribute the weight evenly through the pack to help prevent warp. Protect the top of the pack from rain and sun and put as much weight on top of the lumber as possible. This weight helps alleviate cupping.

If whitewoods such as maple are air dried, try to start the process in the winter or early spring when RH is typically lower and there is more air flow. Whitewoods are prone to fungal stain in summer’s warm humid conditions. If you are drying check prone species, such as oak, consider placing it in an area with less air flow to help minimize the surface checking.

If you want lumber for interior use, the ideal thing is to have it kiln dried to between 6 and 7% MC. If that is not an option, stack air-dried lumber inside where conditions are similar to where you plan to use the wood. How long it needs to be held inside depends on many factors including the species. Remember, if lumber is not stored properly it will gain moisture.

A good information book from the USDA Forest Products Lab is Air Drying of Lumber. Read or download it at http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr117.pdf