The previous article, which I’m sure you’ve read, looked at the roof geometry of circular buildings, and using simple theoretical models, demonstrated that large 16-17m roundhouses are probably at the technical limits of the design. We shall extend our simple model to include a theoretical longhouse, and compare the two forms to try and understand what advantages this challenging form of roof construction offers, with interesting results.
This time we are also going to look at real examples, feeding some actual numbers into our theoretical model. As usual, truth is stranger than fiction, but two important things emerge about these ancient buildings and the people who built them, which may come as a bit of a surprise to some prehistorians.
Interestingly, there is an exception that proves the rule. At West Plean in Stirlingshire there was a roundhouse where the builder put the supporting posts at the middle of the roof span. This is not unreasonable in a rectangular building, but is technically wrong, both by prehistoric standards, and in terms of contemporary understanding, even in such a small building as West Plean. This serves to remind us that Mr Cockup, and his mate Mr Bodgit, are perennial characters in the narrative of building.
It is worth noting that the diameter of the roof should include the ‘eaves’ (the part of the roof that overhangs the wall), which is the widest and heaviest part of the roof. However, this difficult to detect archaeologically, so for our case study a nominal 1m is added to the wall diameter to give a figure for the roof base.
A modern three bedroom house might have a total floor area of about 150m ² /1600 sqft , so Moel y Gaer is smaller than this, but Little Woodbury is larger on the ground floor alone; including the upper floor, it could be twice this size. The difference a small change in diameter can make to a roundhouse is emphasised again by the figures for theoretical building volume and floor space, Cow Down being twice the size of Pimperne Down, and 3 times the size of Moel y Gaer.
What is so remarkable is that the figures for post loading are almost identical for three of the buildings, and just slightly lower for Pimperne Down, which indicates that the builders were using similar principles of foundation design. We have seen the significant difference, in terms of roof weight, between the largest and smallest buildings, so to find that the actual loading on each post is so similar is good news, but actually what you would expect if you were dealing with specialist architects.
The figures are an estimate derived from the model, and the parameters can be varied or refined to produce different figures. However, what is important is that the same model is used on each. In general terms, they are quite conservative figures, quite comparable with the modern figures we considered in a previous article, and similar to figures for other prehistoric buildings I have studied.
Understanding prehistoric foundation design is particularly useful when we are considering the underlying problem of how to identify buildings from the evidence of postholes. Knowing you are looking for the work of architects who understand the importance of level foundations and utilise a rational system of design is a great help when you are trying to detect structures. Theoretical structural archaeology is a methodology, a way of looking at the evidence, which sees postholes as foundations, and tries to detect and understand the structures they supported, with the broader objective of understanding the development of prehistoric architecture in these terms..
The big story here is that prehistoric timber building was architecture created by competent specialists. Thousands of years before the appearance of building regulations and the man from the council, prehistoric builders had it covered.
Sources & Further Reading:
 G. Bersu, 1940. Excavations at Little Woodbury, Wiltshire. Part 1, the settlement revealed by excavation. Proceedings of the Prehistoric Society, 6, 30 -111.
 K A Steer, 1956. "An Early Iron Age Homestead at West Plean", Proc Soc Antiq Scot vol.89, pp.227-49
 D W Harding, I M Blake, and P J Reynolds, 1993. An Iron Age settlement in Dorsett: Excavation and reconstruction. University of Edinburgh. Department of Archaeology Monograph series No. 1. Also: http://www.butser.org.uk/iaflphd_hcc.html & http://www.butser.org.uk/index_sub.html
 Graeme Guilbert, 1981. Double-ring roundhouses, probable and possible, in prehistoric Britain Proc Prehist Soc 47, 299
 S. C. Hawkes. 1994. Longbridge Deverill Cow Down, Wiltshire, House 3: A Major Round House of the Early Iron Age. Oxford Journ. Archaeol. 13(1), 49-69