It is with mixed feelings that I find myself having to describe the collapse of our Mesolithic house which managed to stand for a somewhat shorter period of time (three months) than we had hoped . We think that the collapse was caused by failings in our understanding of the materials and the execution of the construction, not the design itself and hope to revisit some of these problems in a future rebuild. The collapse has also given us lots of really useful information about these structures, and in this sense the experiment has still been succesful. But firstly, I should explain what happened.
Following the ‘completion’ of the house in late July, we did a first batch of running repairs. This mainly involved patching areas where the turf had slipped and placing a thatch cone over the apex of the building. This was undertaken in early Sept. Following this, we began to monitor the structure and soon spotted that some poles were showing signs of strain.
The first pole (pole 1) to split was a very thin birch, which bent inwards and cracked. This had supported parts of the door and porch, and was a bad choice of materials on our part. We repaired this section by ‘tying in’ the bent pole with bands of wattle. In late Sept we noted a more substantial crack in another pole on the same side of the building. Both breaks were 2 – 3 m from the apex of the structure.
On 21st October we spotted a substantial crack in one of the main timbers that formed our original tripod, again on the same side of the building as the other timbers. This time around, repair was not an option and we shifted our focus to monitoring collapse. The bowing inwards of the whole of that side of the building is very evident in the photographs.
By 23rd October this pole had snapped clean through. We sampled the timber, which appears to have suffered a straightforward mechanical failure: there is no evidence of rot. (see below for further discussion). Following this, the final collapse during a week of very heavy winds and rain was very rapid.
The collapse of the house has caused mixed emotions. Whilst we would have loved the building to have stood for longer we are already learning a great deal from the processes of collapse. Firstly, once a structural element of the building breaks, collapse is rapid and systematic: once a certain point is passed, all of the timbers break and would not be reusable. Secondly, we are already getting a sense of how artefacts incorporated into the roofing turves spread, in some places appearing at some distance from any visibly collapsed turf. We will continue to monitor this which is providing very valuable information about the spatial distribution of arcaheological materials.
Of course, we want to know why the house collapsed. At this stage we see four possible explanations, many of which are connected
- the bad weather led to overloading of the structure through waterlogging and wind damage
- the birch trees were not strong enough. We used birch saplings which had started their life in a nursery. At the break, the tree lacks many rings and is not as structurally robust as an older, slow grown birch may hav been.
- related to this, failures on our behalves in terms of skills, understandings of the materials and poor execution weakened the house. We will explore this in more detail in the coming weeks but this explanation might include: not understanding how much curvature and twist could be in a ‘straight’ pole; not managing to keep a consistent circumference because of this curvature; failing to place sufficient wattling in place.
- finally, it is possible that the design was simply not appropriate for a turf roof and that buildings of this type were covered in thatch, skins or possibly had internal supports for which we have no archaeological evidence.
We cannot fully differentiate these explanations, but we feel that a combination of the second and third are most likely: the failing of the building ultimately reflects our lack of skill and understanding of the materials. We are currently considering how to remedy these problems in a rebuild for early 2014.
There is much more to consider about the collapse of the house. This was always an experimental archaeology project and in these senses it has been successful – we know much more about these buildings, their potential forms and how they behave than we did when we started. If it is back to the drawing board it is a much more informed and knowledgable way! At the same time aspects of the experiment continue, as we monitor the collapse of the house and try to understand how and why the different materials behave. We look forward to keeping you updated with these developments.