The photograph below is a picture of a garden fence at my parents home. It divides a portion of their garden from their neighbours’. The fence is supported from a series of four square timber posts embedded in the ground. Horizontal timber battens are fixed to the timber posts both at the head and base of each post. Timber boarding is then fixed alternately either side of the battens.
In one sense the fence looks unremarkable, but looks can be deceiving. On closer inspection it is evident that something is wrong. A vertical split has begun to appear at the head of the two middle posts. For the curious mind the question arises as to why this has happened and why only to the middle posts?
For those who have read my prior post titled ‘On Cladding Garden Sheds’ you may have an idea what the cause may be. For those who haven’t its worth re-capping. That said, while part of the mechanism is similar, ultimately the load-path is actually different.
‘When a tree is felled its moisture content could well be 100%. As it starts to dry out free water will evaporate from its cells until it reaches somewhere between 25 and 35%. Beyond this point water is lost from the cell walls of the timber fibres themselves. This causes the timber to shrink’.
It seems self evident that on warm days the timbers forming the fence begin to dry out causing all of the timbers to shrink. The battens are fixed rigidly to the posts supporting the fence; in the case of the two middle post there is one either side. If both of the battens shrink at the same rate then they will pull on the middle posts in equal, but opposite directions causing a split to form in each.
Conversely the two outside posts have a timber batten fixed to one side only. They are therefore pulled from one side only. Since there is no counter pull these posts are free to flex. It follows that there are no splits in the two outside posts.
If the mechanism described above is correct the next obvious question is; why there is much less evidence of splits at the base of the fence; there are perhaps hairline splits at most? If all the battens are fixed in the same way why does the base not match the head of the fence?
There are several possible answers to this question. The true answer may be a combination of each.
Firstly, the base of the fence is closer to the ground and therefore when it rains timbers in this location will receive splash back. Secondly, when it rains water will tend to run down the fence under gravity and will collect near the base of the fence. These factors would make the timber at the bottom likely to be more wet than those at the top.
A third factor would be that sunlight would reach the top of the fence for a longer period of the day and the base would tend to be in shadow for longer. Fourthly, there is dwarf wall built on the neighbours side of the fence located close to the base of the fence. This side of the fence would definitely be in shade for longer. When the sun shines from the neighbours side of the fence this effect would be exaggerated. Together these two effects mean that timber at the base of the fence would dry out more slowly.
Taken together it is self-evident that the timbers at the base of the fence are on average wetter than those at the top thus providing less potential for shrinkage. It follows that the forces experienced by the timber posts are less at the base.
It is also possible that since the bottom of the fence is close to the point at which the posts are embedded in the ground, the posts are harder to split due to their confinement by the ground.
The question now arises how could the fence builder have prevented the posts from splitting. The obvious answer would have been to double up the internal fence posts to create a movement joint, but of course that would cost more money. The moral of the story is that being cheaper carries its own price.
Postscript
For the most observant it is also interesting to note that the nails fixing the battens are corroded and stains are starting to appear in the timber. Conversely the nails connecting the timber boarding to the battens are not corroding.
One can see that the nail heads vary in size suggesting that they are of a different type. It would appear that the smaller nails used to connect the vertical boards are of the stainless steel variety while those connecting the battens to the posts are not.
No comments:
Post a Comment