Understanding Timber Species, Shrinkage & Drying
Here are some of the biggest practical considerations to think about as you get started with your post and beam project.
Q: What is the best species of wood to use for my post and beam project?
Answer: In our neck of the woods (Pacific Northwest) our favorite is Douglas Fir for its strength, and ability to take a multitude of stains. Can handle larger spans. Available in very large sizes for free-of-heart timbers and long beams. A pretty stable wood species with about half the shrinkage of oak.
Cedar is another favorite for its rot resistance although it lacks somewhat in strength. If you’re looking for a weathered look, it grays out in a natural way and doesn’t require a lot of maintenance.
Oak has a very pretty grain pattern and is very strong. The perpendicular to grain strength is the same as its parallel to grain strength. Excellent for bigger spans or high tension loads. Hard to get long lengths and big sizes out of Oak logs. Accepts a multitude of stains.
Q: What are the worst timber species to use?
Answer: Alaskan Spruce: which shrinks endgrain as opposed to perpendicular to grain, which causes serious issues after shrinkage. It’s very easy to chisel, which is a bad sign if you’re looking for structural integrity.
Pine: A weak timber with lots of pitch (sap).
Hemlock: Need super sharp tools to work with and is prone to twisting a lot. Doesn’t have a really pretty grain pattern. Still preferable to pine.
Some people want certain grain patterns, colors and hues. Others are more concerned with getting the biggest Bang for their buck.
Others want wood with a story – such as it reclaimed timbers from an old structure – and yet others want to follow historic precedent. In other words if they want to replicate a colonial saltbox style home they choose the exact wood species commonly used by early post and beam timber framers.
For most people, staining addresses aesthetic concerns. This leaves availability, cost and performance as the primary decision drivers. Wet conditions, heavy tension loads and big spans can play an important role here.
Q: What are the pros and cons of kiln dried versus fresh sawn timbers?
A: Important Note!
This comparison will not include the most common form of kiln dried timber sold on the street today.
I cannot say this “feel good form of taking control” doesn’t do diddly-squat. But I can say the benefits are negligible when it comes to timbers. Legally this type of kiln drying can only be referred to as S-dry. (Surface dry)
Let’s look at how the process works.
Your full sawn 8X12 is put into kiln for 2 weeks or so until the outer 1″ of timber reaches 19% moisture content.
Your 8×12 is resawn and planed to a final dimension of 7 1/4″X 11 1/4″, leaving you with 5/8″ of dry timber.
How much will this benefit you? Regarding shrinkage with bigger timbers dried using this process, your benefit is minuscule.
The beams will be noticeably lighter as the S-dry process removes a lot of free water from the timbers. But only until the trapped water within the wood cells is removed will actual shrinkage occur.
Regarding sap/pitch leaks you get more benefit because your timbers are heated to 170. The sap will not run again until your home/project again reaches 170, or so the theory goes.
Yet because this drying process is superficial with much of the dried outer wood removed, the results have shown improvements but are hardly guaranteed not to leak sap.
KEY POINT: Spend the extra 10 to 20% with your eyes wide open.
With timbers dried using a special radio frequency process you can achieve truly stable wood. And you can get stable wood will glue-lam beams as the S-dry material used to make the glue-lam beams are thin enough to allow trapped sell water to completely exit resulting in stable wood.
Your other options are reclaimed or air dried timbers.
As you can imagine, the biggest benefit to using true kiln dried timbers are the avoidance of sap/pitch as well as avoiding unsightly situation caused by a wood movement. This includes dry wall cracks from settling beams, once perfect joinery opening to display large gaps from timbers which can check, twist, bow, warp, and cup… Scary! But we before we dismiss the jury let’s look at the other side of the coin.
‘Fresh Sawn’ or ‘Green’ timbers will give you the biggest Bang for your buck – provided you:
1) Are OK with special-designed joinery to address shrinkage.
2) Like the age old concept of green wood shrinking tighter around dry pegs.
3) View the imperfections resulting from shrinkage as adding character to the unique rustic vibe.
Reasons for the economic advantage are manyfold. The most obvious savings is the avoidance of material handling, inventory and time spent associated with true kiln drying, air drying, or using reclaimed timbers.
Less obvious is the easy replacement cost and lead time to replace a beam which may get cut too short or does not make quality grade. With long lead times and difficult to source timbers we factor extra “backup” timbers to hedge against costly irritating delays if things don’t go perfectly for one reason or another. So your decision will be based largely on personal preferences regarding a aesthetics and your cost/benefit perspective.
Proceed with confidence
As timber artisans we’ve worked with almost every conceivable option, and have the expertise and knowledge to make the best recommendation for your project.
If you’d like to discuss a post and beam timber frame project, get in touch.