I often get asked if it is okay to use green, or freshly cut timbers, for a timber frame. In fact, most 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 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 several years, I can remember having to dry off my chisel after cleaning a mortise because there was so much moisture in the 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 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 conventional framing, siding, and trim can be used to dry timbers effectively. However, they have to be used at a low temperature for weeks 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.
- Reclaimed wood is dry by its nature – timbers standing for many decades as a built structure are often the driest timbers available. But the reclaimed timbers themselves are often pricey. And it’s a look that may not be for everybody: because the beams are usually irregular, twisted, and non-square, they must be hand-scribed to connect, which adds to the labor cost. But if you like a weathered look, the result is a beautiful, artistically rustic structure.
When using green timbers, you will need to give special consideration to the joinery to make sure you consider the shrinking of the timbers. Depending on the species of timbers you choose and the moisture content 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:
- In 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.
Green timber is more likely to develop checks (splits and cracks from uneven shrinkage in the beam) than dried timbers. But checks add to the frame’s rustic beauty, and they are rarely a structural issue. Checks start to form on the exterior surface of timber when the beam shrinks as it dries. The crack usually stops at the center of the timber- if a crack continues all the way through a beam it would more than likely be a concern as far as the structural integrity.
Different species of wood shrink to different degrees. But they all shrink more than you might expect! Probably the most common species used in the U.S. is Douglas Fir. If you take a green Doug Fir timber and dry it to a moisture content of 8%, it can be expected to shrink almost 5% in size. Western Red Cedar doesn’t shrink quite as much: The same timber in Western Red Cedar timber would shrink a bit less, about 3%. In contrast, White Oak can be expected to shrink almost 6%.
Working with Green Timbers
So, what are some ways to minimize problems when you use green wood in your timber frame project? Number one, use free-of-heart-center timbers instead of the boxed heart. FOHC means that the timber is cut completely outside of the “bullseye” of the growth rings, and since all the growth rings are severed, the timber dries more evenly. Boxed Heart timbers retain moisture in the inner growth rings, and as the outside dries, shrinkage creates surface tension, which causes checks.
Another way to get the most out of your green timbers is to apply a wax-based sealer to the end grain. This forces all drying to occur out the timber’s sides and slows the drying process, which helps to discourage checks. One good product for this is Liquid Wax End Sealer from Heritage Natural Finish. Another product folks use for this purpose is Anchorseal.
There are some schools of thought that consider when the trees are harvested to affect the timbers’ stability during the drying process. The recommendation is to harvest trees in the fall, cut joints over the winter, and raise the frame in early spring. Enclose the frame over the summer so that by the next fall, when you completely enclose and air condition space, the timbers will be more stable due to a slower drying process.
What is Blue-Stained Wood?
Blue-stained logs and timbers used to be considered damaged goods. However, in recent years blue-stained timbers have seen a huge increase in popularity. So what is blue-stained wood? It is lumber (usually a pine species) with microscopic fungi in its sapwood of a tree. Blue-stained wood is often harvested from trees that have died from a bark-beetle infestation. They are harvested while they are still standing. Since the trees being harvested are already dead, this is a very ecologically-conscious type of timber to use.
According to the Southern Forest Products Association, the only impact on the wood is the color, and the fungi do not cause structural issues or pose any kind of health risk. But to many people, the unique coloration adds a beautiful element to a timber frame.
What and How to Store and Dry Your Own Timbers
Living trees contain large quantities of water. After the tree is cut and the wood begins to dry, the moisture that remains in the wood tends to equalize with the surrounding environment’s relative humidity. If you handle, dry, and store your green wood properly, you can minimize moisture content changes that might occur after your frame is built.
Usually, if the frame is fabricated at a similar moisture content that it will face after installation, no significant dimensional changes. I the frame is designed properly, some minor dimensional changes can be tolerated. Most lumber for timber frames should also be dried to an average moisture content of 15%, not to exceed 19%.
- Where to Store Your Timbers: The best place to stack your timbers is away from direct sunlight and out of the weather. A covered structure is best, although it does not need to be climate controlled.
- Create a Proper Base for Your Timbers: Build a foundation for your green timbers with timber already dry. Make sure the base is flat and level. Lay out cross pieces every 12” to 24” (depending on the beams’ size). The cross pieces (called stickers) need to all be the same thickness.
- Pile Your Timbers: Lay your timbers out on the stickers, which should be at right angles to the timbers. Put the biggest timbers or slabs at the bottom of the pile. Keep the weight of the lumber evenly distributed. You want all your stickers to line up vertically with the ones below to support the weight evenly. Once all the lumber is stacked, put some more stickers across the top and add some more weight on top.
- Keep Your Timbers Out of the Weather: You want the drying process to be a slow one. You don’t want direct sun or rain on your lumber. Protect it from wind and rain, and if necessary, protect it from the wind as well. You want to encourage air circulation – so if you lay metal roofing or a tarp over the stack, make sure there is some air space in between.
- Keep an Eye on the Moisture Content: It simply takes time for the wood to dry. Temperature and humidity are the critical elements in determining how slow or fast the process will be. Use a moisture meter to help you monitor the moisture content. Before you begin to cut the frame, it’s a good idea to store the lumber in your shop for a couple of weeks.
There are so many variations in the timbers that are used for framing. That is a part of what makes the craft of timber framing much like an art form. When you are deciding whether to use green or dried timber, new or reclaimed timbers, or timbers with blue stain, the main criterion should be the final aesthetic you want from your frame.
In the end, the choice will be up to you to decide on whether or not your timber is dried or left green. For 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.