Drying Time for 6/4 Red Oak

Here's a quick technical look at the factors affecting kiln drying 6/4 Red Oak, including kiln type and initial moisture content. Air drying may be more cost-effective. July 16, 2012

We will be loading a kiln with 6/4 northern red oak soon. My guess is the load will be around 80% MC going in. How long will it take to dry?

Forum Responses
(Sawing and Drying Forum)
From contributor R:
If, in the past, you have dried 4/4 red oak of the same area of moisture content, take how long it took to dry (in days) and multiply that times the thickness to get an estimated time for the 6/4 to dry. For example, if it has taken 60 days in the past to dry your 4/4 red oak, then take your 60 X 1.5 (for 6/4 thickness) and you should expect that in or around 90 days your lumber will be near ready.

A quick formula that I use is to take my initial M.C.% minus my final M.C.% and divide that by my targeted loss per day. Let's say we use the 80% from your estimate and your final M.C.% is 8%. 80 - 8 = 72 and we want to target losing .25% to .5% moisture per day. We can look forward to having that particular lumber in the kilns for roughly - at a .25% loss per day = 288 days or .5% loss per day = 144 days.

Hope this helps you get an estimate on drying your 6/4 and does not perturb you by seeing the amount of time you could be looking at.

From the original questioner:
Thank you. I'm trying to determine my drying cost for the 6/4. I'm hoping that this time of year we can get a little air time on it to minimize kiln time.

From Professor Gene Wengert, forum technical advisor:

The multiplier for 6/4 is 1.7 and 8/4, 2.5. Kiln time for 4/4 red oak at 80% MC is around 45 days.

From contributor K:
First off, not too many kiln operators take 45 days to dry 4/4 red oak. That is about 10 days over the average, unless you're drying infected or southern. To tell you how to dry heavy oak is not as simple as following a schedule. The book schedules are a rough guideline only. Every single load has its own circumstances - the kiln, the lumber source, the MC range, etc. It is going to take trial and error. Watch the lumber closely. Weigh the samples. Don't always believe your controller. Use thermometers and a hygrometer. Watch your airflow. Get the message. I have been drying heavy oak for 30 years and it is not cut into a stone tablet anywhere. Use the book for guidelines. Focus on the lumber and by all means talk to someone you know has experience and listen. The room for error is practically nonexistent with heavy oak.

From Professor Gene Wengert, forum technical advisor:
Drying time is determined by several factors. One key is the wettest incoming MC, and not just the average of 80% MC. Another is the equipment. I suggest that it is very hard to find a DH kiln, for example, that can dry under 45 days for dead green. This time also includes equalizing and conditioning. Bacterial wet wood, southern oak (this post is about northern) and plump 4/4 also affect time.

Finally, when drying green, air flow is a big factor in drying time. A modern kiln can dry much faster than an older kiln because of better air flow. Another reason for faster drying in a modern kiln is because of computer controls with ramping.

So, we can find faster drying. Note that drying 4/4 green in the kiln is very uncommon. Why take 5 weeks to dry one load when you can dry 4 loads of air dried or shed dried stock? When you sell the wood, you get the same price increase, so with AD you make roughly 4 times more money. Of course, poor kiln drying or poor air dying will be expensive, so follow the guidelines in the books.