A spiral grain log is one where the natural grain of a logs outermost layer twists around its circumference. A log can be either straight grain, left hand spiral grain, or right hand spiral grain. In the picture below log A has straight grain, and log B has a left hand spiral grain:
You really shouldn’t use spiral grain logs when building a notched log home. That’s because spiral grain can cause logs to twist in the wall, which results in the notches ‘lifting up’ on one side. You end up with an unsightly gap at the corner notch of your log home. Aside from looks, the gap also reduces the bearing surface of the notch, and it reduces the weather tightness of the notch, et cetera.
The conventional wisdom says that left hand spiral grain logs are generally more of a problem than those with a right hand grain… but logs with a right hand spiral can often still move within the wall of a notched log home. To really minimize potential problems it’s best to stick with straight grain logs.
Unlike notched log homes, with the butt and pass method it largely doesn’t matter whether a log has a spiral grain. The way butt and pass homes are built just makes it nearly impossible for a spiral grain log to twist after being installed in the log wall. Although if you do have logs with severe spiral grain, then it is best to season them before use.
So it’s possible to use use left or right hand spiral grain logs when building a butt and pass log home. That offers a distinct advantage over notched log homes, because you can use a wider range of logs (your logs can be easier to find, and less expensive). It also simplifies and accelerates the construction process, and it also reduces future maintenance issues.
To recap: for a notched log home you should ideally use straight grained logs, but depending on some variables having some right hand spiral grain logs can still be ok. By contrast, with butt and pass log homes you can use logs with straight grain, right hand grain, or even left hand spiral grain and it typically won’t matter. We’ve never seen a spiral grained log cause problems in the walls of a properly built butt and pass log home.
- It’s very easy to tell which way the spiral grain runs. Just place your right arm on the log (lengthwise with the grain), near the butt of a log and pointed toward the top end. Pretend your hand is in a mitten, and that it’s wide open on a flat surface (where your fingers are together and pointed in one direction, and your thumb is pointed in the other direction). Look at the grain of the log near your hand; if it follows your fingers then the log has a right hand spiral. If the grain follows your thumb, then the log has a left hand spiral.
- Most softwoods will have their grain change directions over time as they grow. They will start life with a left hand spiral, and then during the maturation process the grain slope usually switches to right hand.
- During that transition from left to right hand spiral grain, there will be a few years where the tree has straight grain. So you could say that every mature tree has left hand spiral grain, straight grain, and right hand spiral grain… but for our purposes we only focus on the orientation of the grain in the outer part of the tree or log. It’s when a log has straight grain that it is really ideal for log home building, due to its inherent stability.
- A log can be straight grained near the base, but spiral grained near the top. That’s because the base has gone through additional growth (added rings) that the top never experienced. This can be a bit confusing, when you can still end up with a log that is straight grain for 20′ and spiral grain for 20′. It really complicates the building process – if you’re building a notched log home.
- It would not be advisable to use spiral grain logs for girders, joists, ridge poles, purlins or rafters, regardless of the style of log home you’re building. In those instances the twisting of the log can cause problems.
- You can mostly ignore the spiral grain issue by building a Skip style Butt and Pass log home. It makes things a lot simpler and easier.