By Del Radomske
Featured in: ILBA Log Building News #51
Overscribing of the lateral groove means that the scribe setting for the groove is larger than the scribe setting for the notch. We might also refer to overscribing as a “shrink-to-fit lateral groove”, or of course we could reverse our thinking and call the whole procedure “underscribing the notch”, which would be an accurate description too.
Overscribing is a technique that puts 100% of the weight onto the notched corner as you build. After shrinkage and settling (a better term for settling is compression) you hope to keep 75% of the weight on the notched corners and 25% of the weight on the lateral, with the result that both the notches and laterals appear to fit tightly. The trick is to know how much to overscribe to achieve this situation after shrinkage and compression has taken place.
The basic structural aspect that ties a log building together is the notched corner systems we employ. It is my opinion that using an equal scribe technique for both the notch and the lateral groove will eventually cause the log building to have a less than acceptable locking comer mechanism, produced by the shrinking and settling process. Overscribing the lateral groove will maintain the structural integrity of the log building for generations to come.
The chinked style of log building is produced by underscribing the notches, which leaves a small to large opening between the logs. This style allows for complete weight bearing at the notches. That’s why chink style buildings, that have had doors and windows installed properly, have tight corner notches.
Now understanding that, if you make a mistake and overscribe the lateral grooves of a log building too much, there would be a slight gap along the length of the grooves for the life of the building. Which can be corrected by applying a caulking between the logs. Even though this mistake was made, that building will have tight fitting notches for the next 200 years or more and the building will always be structurally sound. Can you even call that a mistake? Maybe a slight error in judgment, but certainly not a mistake.
Your ultimate concern during the construction of any log building should be maintaining the locking corner system for the life of the building. In other words, not allowing any significant pressure to be released from the notches.
If you use the equal scribe technique, the pressure will be released from the notches during the shrinkage and settling processes. In other words, gaps will form around the notches and the locking corner mechanism will fail, allowing the logs to take control. Once the logs have control, they can twist in any direction they desire producing gaps along the lateral groove.
In my opinion, the equal scribe technique is a waste of logs, effort, and skill. It doesn’t matter that the building may have been beautifully constructed, the end result is always the same: a poor quality building that is not as structurally sound as it could have been.
Overscribing of the lateral groove can and does correct these faults. No matter what type of notching system you use, overscribing of the groove will create a compression-fit notch that will deliver the correct results.
Dalibor began his presentation by saying, “First, wood doesn’t burn; it is a good insulator.” He clarified that as heat is applied there is thermal degradation that causes wood to give off gases, and it is the gases that are flammable.
Dalibor, working on his PhD at the Technical University of Zvolen, Slovakia, built a furnace that applied heat to one side of 11-inch diameter scribe-fit logs, in which the long grooves averaged 50 mm (2”) wide. The logs were kiln-dried spruce at 19% moisture content.
Fire resistance is defined as the ability of a wall system to withstand exposure to high temperatures without collapse – and so a vertical load was applied to the top of the test walls.
The walls were equipped with thermocouples and monitored as heat was applied to one side – the ISO 384 standard test. For 45 minutes heat was applied to one side – and it reached 700º C (1,300º F). No flames penetrated the wall during the test, and there was no structural failure.
The thermocouples showed there was no measurable increase in temperature on the side of the wall opposite the furnace. In fact, once the heat source was removed, the log walls stopped burning on their own in just two minutes.
Dalibor then showed a dramatic video of the large-scale burn test. A scribe-fit wall panel about 8-feet square was built, and a heat source applied flame to one side of the wall. The wall was preloaded with a fixed load and rigged with thermocouples.
After three hours there was no penetration of flames through the wall. The temperature on the side with the furnace was 1,100º C (2,000º F), and the highest temperature reached on the other side was just 32º C (90º F).