For the answer to this technical question, we turned to structural engineer Brian Sielaff, founder and CEO of Tamarack Grove Engineering, headquartered in Meridian, Idaho.
There are various ways to answer this question based upon what manufacturer it is and how they mill and produce their log components.
Milled logs for log home construction are often referred to by a nominal diameter (such as 12 inches), but their actual dimensions will differ due to the milling and drying process. Here’s how to understand the true or “actual” dimension of a “nominal” diameter milled log:
- Nominal vs. Actual: The nominal size refers to the rough-cut diameter before the log is milled and dried. The actual dimension is the size after this processing.
- Milling: Milling reduces the starting size of the logs. Logs can be lathed/milled to a specific diameter, like 8, 10 or 12 inches and so on.
- Shrinkage: Wood shrinks as it dries, further reducing its size. The amount of shrinkage depends on factors like species, initial moisture content (green vs. kiln-dried) and how it is cut (flat-sawn vs. quartersawn).
- Tolerances: There are published standard tolerances for dimensional lumber, which might provide a general idea of size variances, but they relate to linear dimensions of boards, not necessarily the diameter of milled logs. A 1⁄16- or ¼-inch tolerance may be acceptable in some cases.
- Log Specifics: For milled logs, particularly in the context of log home packages, the actual diameter after milling and drying will be slightly less than the nominal. Some manufacturers might state the exact actual diameters of their specific products, so it is always best to verify with the supplier.
- Typical Example: For general dimensional lumber, there’s a widely known difference between nominal and actual sizes. For instance, a nominal 1-by-12 piece of lumber is typically ¾” thick and 11¼” wide after milling. While this isn’t a direct equivalent to a log’s diameter, it illustrates the reduction from nominal to actual size due to drying and milling processes.
In summary, a 12-inch nominal diameter milled log will likely have a final diameter that’s smaller than 12 inches due to the milling and drying processes. The exact reduction can vary based on the specific milling practices, wood species and moisture content, so it is always recommended to check the manufacturer’s or supplier’s specifications. Prior to any ordering or paying, you should ask the manufacturer precise questions about what their processes are and what a 12-inch-nominal diameter actually gets you in terms of a finished size, and what impact, if any, the difference will have in terms of height and clearances in your home’s design.
