how long should oaks dry before building timber roof for cob home

The subject pretty much says it all. I've read through a number of books on cob building but have not found an answer to this question. The timbers in question are post oaks about 6-8 inches in diameter and have not yet been cut down.

I have heard that logs can to
take up to a year per inch dry. Is drying even neccesary considering the roof will rest on the cob walls?

My concern is that as the logs dry naturally that they will rip the deadmen out of the cob walls or atleast put stress on the walls causing cracking or worse.

Any help appreciated, thanks!

Hi Anne,

The answer to your question, like many other natural building questions is; it depends.

You can use 1 month old green wood and get a very nice structure, but there will be some checking and the joints must be properly designed or they can open up.

Temple builders in Japan often age their wood for years in order to get those perfectly smooth posts and beams that they are famous for.

Feel free to post more questions as your build progresses.

Welcome to Permies

Like most wood, Oak will check if not dried properly. And as you mention green wood in construction is a recipe for disaster. Here is my suggestion if you must hurry the process. When the posts are cut, take wood good or shellac and coat the ends and all the cuts. Seal them up from drying to fast. The loss of moisture at different rates is what causes checking. The cut ends that expose capillaries obviously are going to dry the fastest. Don't let that happen.

Now take the sealed logs and build a box around them. Use scrap plywood or MDF, whatever can be had inexpensively. Enclose the wood and put in a light bulb (remember incandescent light bulbs?) Have one every 4 feet. a 60watt is all that is needed. Let this create a 'hot box' or wood dryer. It will significantly cut the dry time from years to months. Keep the wood over 100 degrees F, which if the box is sealed off, should not be difficult. If outdoors perhaps insulate the box.

To get an idea, here is an article on a "Solar Kiln" for force drying fresh cut dimensional lumber. One need not be so elaborate with glazing and fans, but it will speed the process.

Solar Kiln

Hello Anne....welcome...

As Bill has alluded to, there are some "rare" examples of large timber frame builders that do age and dry...some...of its principal timbers in their frames, such as the 大黒柱 (Daikoku-Bashira-central pillar or spirit post) as an example. It should be noted that this is indeed an uncommon practice and usually only for "spiritual" architecture. Then only species like 檜 (Hinoki) which is a very stable wood to begin with is the primary material. It is not a common practice (nor practical) to age (dry) timbers for 50 to 200 years from trees that may already been growing for 1500 to 2000 years to begin with. The "Temple Carpenters" don't just buy trees...they often buy entire mountains and the forest on them...

Green woodworking has been the most common form of crafting this medium probably since the first human made something of wood. The entire concept of "aging wood" (best done under water) and drying it...is a relatively recent concept and only in the last 60 years has it become an "obsession," with modern woodworkers that it...."has to be done." Air dried wood has been used in some limited application historically in finer styles of furniture making, yet even here the dominate form of woodworking even today (if perceived from a global perspective) is still dominated by what would be called "green woodworking," be it a timber frame, or a table and chairs.

Timber frames, both historically and today are still dominated by the simple reality that "big wood" can not be effectively left to "dry." Some companies "advertise" kiln dried wood, that all to often ends of more "case hardened" than properly dried. Properly dried wood, when it is done, we see employ the methods of "air drying" over time very slowly. The later is done in the shade, and over time, with the best quality having been aged in water from a few years to even 10 millenia... I would say, having worked both, I still prefer fresh, green, hand riven wood over even air dried. "Water aged" wood that is hand riven and work is simply the best of the best....

Regards,

j

(Sorry Jack, though your process sounds interesting, it is not necessary to dry wood to use it...though I agree completely with "end sealing" and slowing the process way down. We "wax" our ends and joints to slow drying down on many frames, and also to lubricate the joints for easier assembly. As for checking, this is a natural process in wood to behave this way, it seldom (if ever) weakens it, and there are a number of traditinal methods for arresting it or mitigating its affects. One is called "kerfing wedging" and is a common practice in Asian timber framing.)