wood drying rate

[dan sherman]

I have 275 BF of SYP stacked & stickered in the garage that I'm going to use to make bench tops. The temperature in the garages is about 60 degrees Fahrenheit, and the relative humidity is less than 40% (thanks to the dehumidifier).

Are there any formulas out there that give drying time based on starting MC, RH, & temp?

[Tom Veatch]

Don't know of any formula that would be trustworthy unless the temperature and humidity were maintained fairly constant for the entire period. That's pretty hard to do over the length of time necessary to air dry wood. The rule of thumb that I've most often seen is to allow about 1 year per inch of thickness.

I've also seen recommendations to periodically weigh a sample of the wood and record the weight. When the weight stops changing, it's a dry as it's going to get under those conditions.

Don't forget to seal the ends of the boards to minimize end checks.

[Scott T Smith]

Tom, although widely quoted, the "1 year per inch of thickness" rule is not accurate. Your advice regarding taking sample boards and weighing them is sound though.

If stacked and stickered in a relatively warm and dry (80 degrees F, 60% RH or less) location with good air movement, most 4/4 wood species will dry from green down to 20% MC in less than 90 days.

Factors affecting drying include the wood thickness and the air velocity.

Dan - at 60 degrees F and 40% RH your wood will eventually equalize to approximately 7.7%.

If the wood is 4/4 SYP, and you can maintain an airflow of approximately 300 FPM across it, you'll have a daily drying rate index of 0.3, and I would estimate that your boards will be below 15% MC in around 45 - 60 days, depending upon the initial MC%.

If you can increase the temp in the garage to 100 degrees, and maintain 40% RH and 300 FPM airflow across your stacks, you're drying rate index will increase to 1.1 and you should be down to 10% - 12% MC in 15 days or so.

What thickness are your stickers and boards?

Also, at the end of a kiln cycle SYP is typically run up to 160 degrees F to both sterilize the load as well as crystalize the pitch in the boards. The high temp treatment helps prevent the sap from leaching out over time. If you air dry, will the potential for sap leaching pose a problem?

Scott

[dan sherman]

What thickness are your stickers and boards?

Hi Scott,

The stickers are 1-1/2" thick. I think I should clarify my earlier statement. the wood I purchased was 2X (6/4) kiln dried #1 construction SYP, so it's already been dried to under 19%. After I got the wood home, I took the moisture meeter to all of them, and most of them fell between 13% & 15%. If your interested, I, did a blog post about what I found that can be read here (http://www.dans-hobbies.com/2009/03/09/the-start-of-workbench-tops/).

As I read your post, it sounds like your saying that the drying rate is linear (referring to the .3), is that so? I was expecting the drying rate to be exponentialy decreasing.

[David Freed]

Construction lumber is dried extremely fast (15 to 20 percent per day) in high temp kilns (over 200*), so the pitch is already set. I can safely say that there is no possible way that you can finish drying it too fast in your garage. You can put as much heat and air to it as you want.

[Jack Porter]

Hi Scott,

The stickers are 1-1/2" thick. I think I should clarify my earlier statement. the wood I purchased was 2X (6/4) kiln dried #1 construction SYP, so it's already been dried to under 19%. After I got the wood home, I took the moisture meeter to all of them, and most of them fell between 13% & 15%. If your interested, I, did a blog post about what I found that can be read here (http://www.dans-hobbies.com/2009/03/09/the-start-of-workbench-tops/).

As I read your post, it sounds like your saying that the drying rate is linear (referring to the .3), is that so? I was expecting the drying rate to be exponentialy decreasing.

Dan, there are wood acclimation tables based on species, RH, and member thickness. The tables typcally go up to a wood MC of 15% with varying RH, you might be able to use those as a basis to predict expected drying rates.

[Scott T Smith]

Are there any formulas out there that give drying time based on starting MC, RH, & temp?

Dan, you need to also factor in the air velocity across the lumber being dried. It's actually the air movement across the boards that is responsible for the majority the drying process, not just the temp and RH.

From what I've seen in 5 years of kiln operation, the drying rate will vary depending upon species as well as how the boards are sawn. For instance, quartersawn lumber usually takes 15% or so longer to dry than rift sawn. Also, some species are subject to bacterial infections, which slow down the drying rate.