Time to Air-Dry Thick Lumber

Many variables affect drying time. Here's a discussion about the typical drying-time window. November 16, 2011

I read that when air drying 8/4 lumber, the rule-of-thumb is one year per inch plus a year... 3 years to dry 8/4 stock. Is there a rule-of-thumb for air drying stock thicker than 8/4? How long to dry a 12" x 12" timber? Would it ever actually dry?

Forum Responses
(Sawing and Drying Forum)
From contributor B:
As an engineer I would caution that rules of thumb can be very misleading unless you know what they are based upon and what important variables can affect the outcome. They are useful and dangerous if taken at face value.

From the original questioner:
As an engineer, do you have any conclusive data on air drying wood greater than 8/4 thickness?

From Professor Gene Wengert, forum technical advisor:
That rule of thumb is not even close to the truth. For 4/4, oftentimes it is well air dried within 60 days; 8/4 in about 180 days of good weather. Air drying longer than needed can increase checking, warp, discoloration, etc. In fact, get air dried lumber into a shed to prevent these losses, as rain and sun cause loss.

From the original questioner:
Thanks for the answer but I'm even more confused now. In one of your co-written publications, I ran across the statement that in some circles, air drying time was estimated to be equal to the square of the stock thickness. The theory was not refuted nor remarked on but evidently there was some difference of opinion among those in the industry. Also, I'd still like to know if 12 x 12 timbers can be air dried to EMC.

From contributor S:
Over the years I have read a few references such as this one. I have customers with timber frame homes who tell me the frames shrink overtime, leaving me to believe the builders do not wait for the timbers to air dry.

Estimates of Air Drying Times of Several Hardwoods and Softwoods

From contributor B:
Fir will dry faster than black walnut, so a rule of thumb is difficult to go by without knowing which type of wood you are trying to dry. An inch a year is indeed a rule many people use, but it is also scoffed at by others. What you believe ends up being up to you. I would tend to believe a moisture meter (purchased at any decent wood seller biz) if you have concerns. Lastly, of course a 12x12 will dry, but it will take longer. I wouldn't trust anything that big to be dry unless done in a kiln or having sat in a good drying environment for 2-3 years if it is fir or pine, maybe twice as long if it is a dense hardwood.

From Professor Gene Wengert, forum technical advisor:
As you will note in the publication linked to above, 60 days for 4/4 is good for warm weather. Generally, 8/8 takes 2.5 times longer than 4/4. 12" x 12" will take forever.

From contributor E:
White pine 10 x 10's stopped splitting in my timber frame after a couple of years. The same was true for red oak 6 x 8's. Maybe one reason the relationship between thickness and dry time doesn't stand up is that the thickness is effectively reduced when the timbers split and allow air exposure to the depths of the timbers.

From contributor T:
A year per inch is a rule of thumb that has a lot of variables and is based on hardwoods. Very little has been mentioned here about the humidity in your area. This has more to do with it than the temperature.

Something misleading about logs and timbers as per kiln drying... Yes, they are put into kilns; yes, it dries them some, but not to the degree we associate with most building materials. This process is actually to kill the creepy crawlies inside them but also helps some with stabilization to a degree.

The above is why even kiln dried timbers still shrink for years. I've read there are a few companies that reclaim standing dead timbers that can get the drying/kiln process closer to being accurate as we know in the building trade.

A lot are cut in a greener/wetter MC with joints designed to accommodate the shrinking. From my research, the companies that will honestly tell you that to kiln dry most logs/timbers used in homes to standard builders' MC specs (as studs), it would take 6+ months in a kiln, which is not feasible costwise.

This is from what I've read and observed over my 27 years of experience in the remodeling/rebuilding trade. I'll accept something different if the documentation is there to prove it.