I often get asked if it is okay to use green, or freshly cut timbers, for a timber frame. In fact, the majority of the frames that go up today are cut and raised without going into a kiln or going through the process of being air-dried. Before the advent of kilns to dry the wood, all of the timber frames (for thousands of years) have been green or air-dried, with most of those being green timbers. So, which is best: green vs. dried timbers?
The engineers consider anything over 19% moisture content to be green timbers. Having been in the shop for a number of years, I can remember having to dry off my chisel after cleaning a mortise because there was so much moisture in a timber. It is common to use green wood for a couple of reasons such as cost and extended lead times. If dried timber is preferred, the process can be done in several different ways and can produce a really stable wood if done correctly.
- Air-drying is by far the cheapest and easiest way to dry timbers. You are not using energy except for what the sun gives you. However, the catch is that on average, it takes about a year per inch to dry. So if you have an 8×8 post it would take 4 years to dry all the way through; and, depending on where you are, it may not be able to get below the 19% range.
- Traditional kilns that dry the majority of the convention framing, siding and trim can be used to dry timbers effectively. However, they have to be used at a low temperature for weeks on end to really do an effective job. Unfortunately, this uses quite a bit of energy and therefore costs money. If done too fast, it may cause the outside to dry too fast leaving the inside with too much moisture.
- Radio frequency kilns are the best choice for kiln drying timbers. There are only a couple of them around and therefore drying timber this way is very expensive. They work like a microwave oven, which basically dries the timbers from the inside out.
When using green timbers, you will need to give special consideration to the joinery to make sure you take into account the shrinking of the timbers. Depending on the species of timbers you choose, and the moisture content of it when it was cut, you may have up to ¼” shrinkage in some places. Some of the joinery used in timber framing relies on one part bearing on another part to transfer loads, such as a beam resting in a mortise in a post, or a rafter landing on a plate. Thinking through the process the timbers will go through as they dry in your home or porch will help you make good joinery decisions. A couple of suggestions:
- On post and beam situations, move the pegs closer down to the bearing surface between the post and beam. This will allow the beam to stay closer to the bottom of the housing in the post over time.
- Where a housed rafter dives into a plate, cut an extra 1/8” off the bird’s mouth so the rafter will always bear on the housing and not the rafter tail.
In the end, the choice will be up to you to decide on whether or not your timber is dried or left green. For both of my personal timber frame projects, I chose to go with green timbers and have not regretted the decision. Timber frames have certainly stood the test of time before the advent of the kilns.