We build houses ultimately to provide shelter. Regardless of the size or scale of any building or the climate you build in, it is designed pimarily to keep the weather out (be it rain, wind, cold, heat or even bugs and animals) from the environment we have created on the inside.
Log homes are primarily built using green timber, that is, the trees have been recently felled and have not quite reached an equilibrium moisture level, nor have they been naturally or artificially dried ( as in a kiln ). The costs of drying wood to that degree are prohibitive and it's not generally practical. Air drying can take up to 4 years to reach equilibrium and kilns are not generally designed for whole logs. I suppose when the pioneers built using whatever materials were around, they didn't have time to wait 4 years before their timber dried and they could start building their houses. So, in some cases, building with dry timber wasn't even an option. There are also certain disadvantages to using dry timber such as having to deal with warped, twisted, cracked and brittle logs all of which is a function of the drying process.
Using green timber with appropriate techniques, the generally unavoidable characteristics of drying logs can be minimised or even used to advantage. Green logs are easier to cut with a chainsaw, are easier on tools because the moisture acts as a natural lubricant and are generally straighter and more flexible when wet. Of course green timber logs are much heavier as they are laden with water. However, the traditional techniques for log building have been built around the assumption of using green timber.
How do we build a weathertight shell using green timber but have it remain weather tight even after drying, twisting and shrinkage? The way in which the logs are joined at the places where they meet is of the utmost importance and is the key. The ways in which logs have been joined to provide weathertightness is an area which has been at the center of log bulding for centuries. Many techniques abound - many of which have become best practices refined over centuries and handed down from generation to generation. Which is not to say that there has been a convergence on any specific technique or that there isn't still room for improvement. Many regional differences have developed arising from differences in the available materials and/or tools and/or skills.
In the last few decades, with better communication and sharing of techniques as well as scientific study, there have been advances made in the understanding of wood movement. Let's begin with some common facts about working with green wood:
Properties of green timber
1. Wood, as it dries, shrinks tangentially to the grain more so than along the length of the grain ( in fact shrinkage along the grain is so small as to be ignored ). In a round log this means that the outside rings of the log will shrink more than the inside rings ( as there is more cross grain on the outside ). This invariably leads to checking in the log. This is a natural occurrence and is not an indication of a faulty log. Checking will develop in every log home, in every log on every part of the log. Some checks will be facing up, some facing down, there is no way to predict where the checking will occur. If a log is left out on a hot sunny day, you can even hear the surface checking as it occurs, as a series of small pops and snaps.
2. A major check will occur along the length of the log. This check will always occur from the pith to the closest edge of the log. Check the end of any round log and you will see this occur with certainty. Sometimes the check will wrap around the log along the length of it, but this is only because the edge closest to the pith changes along the length of the log. Knowing this, the location of this check can actually be controlled - a groove cut into the wood towards the pith will artificially create the edge closest to the pith and therefore force the checking to be inside this groove. We'll see later how this can be used to advantage in log building.
3. As a green log dries it shrinks. Anywhere from 4-5% is the normal allowance. It will depend on the species of wood used and how 'dry' the logs you are using are( ie: how close your green logs are in moisture content from the equilibrium moisture content for your area ).
So, how we do take all these things into account in order to stack logs together and ensure they are tight from the day they are built until the process of drying completes in about 4-5 years?
Building a shrinking house
Firstly, because of shrinkage, you're going to have to build your house about 6% taller to arrive at the final correct height. Keep in mind that doorways, window opening and and location of joist beams all need to be adjusted to ensure that they end up where you want them after the 6% shrinkage. You'll also have to compensate for parts of the house which will not shrink, such as stud walls, sheetrock, cabinetry, stonework against the house, upright posts etc. There are ways of dealing with all of these but not the subject here. Enough for now to say that you have to think about this and design this into your construction.
Whenever logs are laid on top of each other, as in a wall, they will connect in only one of two ways - along the 'long groove', where the two lengths of the log join each other and the 'notch', where the log crosses another log (generally at right angles though this is not necessary). Both of these joints need to be tight when the building is built but they also need to remain tight even after 4-5 years of shrinkage has occurred.
To join two logs together lengthwise in a scribed joint ie: where the logs perfectly fit each other as if they grew together, the logs are temporarily stacked above each other, roughly 4" apart and the two logs are scribed at the same time with a single scriber. That is, a scriber with two pens ( or pencils) is run along the length of both logs, held perfectly level ( using the built in levels on the scribers). This is done on both sides of the logs. When the log is removed from the building, a scribe line remains on the removed log as well as along the top of the log still on the bulding.
The current best practice is that on the log remaining on the bulding, a line 1" in from the scribe line is marked out and a deep V groove is cut along these lines into the log by running the chainsaw on eiter side of the lines at a sharp angle. The cut should come within inches of the center of the log. Marks can be made on the chainsaw blade to indicate the proper depth. Obviously near the ends of the log as you approach the notch of the other log, the deep V has to exit and it forms a boat shape. This long groove will ensure that the check in this log will be upwards towards the log above it. On the log above it (which is now upside down on the ground) we cut two shallow cuts at a fairly steep angle along both sides of the scribe line, then another two cuts at right angles to it, so that we end up with a W shape. The centre part of the W must clear the deep V groove so occassionally some fine tuning must be done to ensure that the logs do not 'hang up' - ie: that there is sufficient clearance when the logs are put back together.
The W shape plays an important role in conjunction with our long V groove. Remember that the long V groove has forced our log to check inside the V groove. When it does, the log tends to splay apart. Now when it does, rather than creating an opening just anywhere, it will actually press tighter against the scribe line of the log above it. Therefore, now the joint will tighten as the log shrinks and checks, rather than opening up in inappropriate places.
On the notches, the shape of the notches are designed to stay closed as it shrinks. Here , the notch in the log above is basically an upside down V sitting on two sloped cuts in the log below - imagine the legs of a horse rider sitting on a saddle. The notch purposely is made to sit tight against the saddle on its edges but space is left inside the notches so as the log shrinks in height it is able to slide down and the notch remains tight.
For those paying attention, you might wonder how the notch can slide down when the two logs are already tightly scribed to each other along the long groove. In fact to allow for shrinkage at the notches, the long grooves are not in fact tight to each other at first. This is because the notches are 'underscribed' by a small amount. That is the scribe setting used to create the tight long grooves is reduced down a bit when scribing the notches caused the logs to be initially 'hung up' at the notches by the same amount. The exact amount depends on which course of logs are being placed and other factors ( which is to say that it is not an exact science yet) but in general about 1/2" on the bottom course and diminishing to about 1/8" near the top. The remaining 1/8" is closed from compression - ie: the weight of the logs and the roof itself will tend to compress the wood to closure.
These techniques are still being perfected but in general it has been found that these give very satisfactory results. Certainly better than techniques used in the past as the following photo of an historic log cabin I took in the US will attest. Note that in this photo a different technique was used called chinking which intentionally leaves gaps between the logs which are then filled with lime plaster or mud - less than ideal but which has also managed to survive for significant amounts of time, though the weathertightness of these homes to today's standards may have been questionable.
I hope that you can see that if you work with the wood, then even properties which we would nomally consider undesirable can be put to great benefit and a building's effectiveness may actually improve over time.