Some tales about Log Cabins

Log Cabin Mythology!

...or tall tales

We hear on the grapevine all sorts of tales and salespersons yarns originating from our competitors. We will use this venue to air our counter argument to some of these topics.

Have you heard the one about grain direction?

One company insists that it is important to have the circular growth rings of the log ends all aligned in the same direction in the wall stack. They claim that the small radius growth rings indicate the centre of the tree where the wood is older and therefore harder. Thus the harder wood should all face outwards from the walls to give the best weather protection.

We will concur that the smaller radius growth ring will indicate the centre of the tree and therefore the oldest wood. All good quality timber is cut from the central area of the tree known as the heartwood. If the wood is Northern Scandinavian Pine then the tree will have been slow growing and will have attained an age of at least 70 years. For typical 45 or 56 mm thick wall logs there will be miniscule and negligible change in hardness from one side of the log to the other. However there may be a significant change in hardness if the heartwood is compared to the young new growth around the outer surface of the tree.

Firstly, if the company is genuinely concerned about hardness variation across their log width then it might indicate that the logs are not cut wholly from heartwood but instead from younger smaller trees and that the logs contain both hard heartwood and soft young wood.

It is well known practice in woodworking that it is always best to orientate the grain direction of two mating timbers in opposing directions in an attempt to neutralise the forces that cause warping. Thus if all the logs are stacked in a wall all orientated the same way it is more prone to twisting or curvature than a wall stacked with logs in random orientation. This is why all other companies do it this way but one chooses to be different.

We would also suggest that the driving force is really a manufacturing difficulty that they are trying to overcome, perhaps they have to deal with non flat or non straight raw stock and by feeding the stock unidirectional into the machine they get some dimensional consistency but if so, it is probably that the resulting logs cannot be assembled in reverse direction.

Fancy having to look at the grain direction of each log during assembly! There are enough pieces in the jig saw without having to sort into grain direction as well!

We are suspicious that their so called unique selling point is really a cloak for some deficiency earlier on in their process and which may lead to warped walls.

Have you heard about the log with two tongues?

Several companies will show off that their logs are machined with double tongue and groove and therefore twice as good as single tongue and groove!

This could be debated at length and fortunately we do both types so we are not biased one way or the other, but it is worth exploring a little further.

In general the actual tongues and grooves in the double system are less robust than the single system There are three situations where the tongue and groove is important:

During construction, the tongue and groove engagement will have an influence on the control of log straightness. If an occasional log is a little out of straight over the long length then we find that the controlling effect of a deep wide bodied single tongue is far stronger than shallow narrow bodied twin tongues. If the twist is severe then the weak tongues or grooves can break away.

During movements in the cabin as logs swell and contract, it is possible that two logs may try to come out of engagement. The difference between double or single tongue & groove is irrelevant in this situation; it is the depth of engagement that is critical. A shallow tongue will be out of engagement long before a deep engagement tongue.

During normal use when the logs are closed tightly together under the weight of the superstructure. The tongue and groove has little significance in the draft proof ability, because the remaining top and bottom faces of the logs are tightly in contact with each other and create an adequate seal.

In conclusion it is the depth and strength of the individual tongue that is important rather than quantity.

We use a very deep tongue that is wide at the base on all logs up to 45 mm thickness. Our 56 mm log has adequate width to allow us to use a double tongue but still with deep engagement and wide base width. This is far superior to some of the double pimples we see in the marketplace these days.

Airflow under cabin

One cabin manufacturer claims that if you stand your log cabin on a concrete base then the floor will rot. Their argument is that if you elevate it off the ground on pillars then air will flow underneath and keep it dry.

This is only true if the air is not moist! We find that it is more likely that the air underneath the elevated building will be very damp at times of the year from the ground conditions under the cabin and that there is an even greater risk of subjecting the under boarding to dampness. If the under boarding material is not totally impervious to moisture then rotting will take place much sooner. In fact the same company use chipboard as the under boarding and sometimes its purpose is to contain the Rock wool type insulation material under the top floor. So not only does the under boarding rot but then the insulation degrades in the moist environment also.

The use of a concrete base is a tried and tested method for all manner of building foundations.

It is good practice for it to be laid onto a plastic membrane to prevent it from curing too quickly. This also controls rising damp through the concrete after curing. The overall size of base should be dimensioned so as not to create too much of a rain trap shelf around the perimeter.

Timbers in contact with the concrete base should be pressure treated to preserve them. If the cabin floor is not to be insulated then a small airflow is permissible across the dry top surface of concrete and the underside of the cabin. If the floor is to be insulated then a small void under the insulation is good practice but otherwise the perimeter sole plate of the cabin can be sealed to the concrete base to prevent ingress of moisture from the sides of the cabin joist network and entering below the cabin.

There are occasions when the cabin does need to be elevated off the ground, and in these situations we would always choose marine ply or similar as the under boarding to combat the rising damp issue.

Thicker walls make the cabin better insulated

Many suppliers will claim that their thicker logs will mean better log cabin insulation.

We will agree that a thicker log is a better thermal insulator than a thinner log. However this is only part of the story. You should be concerned with the overall performance of the cabin as a whole. It is necessary to appreciate that the primary heat loss zones are the roof, glazing and floor. There is absolutely no advantage in making the wall logs thicker and thicker if the weak links (roof, glazing and floor) are not capped as a priority.

Also it is possible that low grade, open grain wood used to produce some thicker logs are not in fact as good insulators as the high density Northern Scandinavian Pine at less thickness.

A customer recently summed this up by saying, “it’s not much use putting on an extra coat in the cold if you’re not wearing socks and a hat!”.

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