Geoff Carter on round houses

The largest of the "old" round houses at Castell Henllys

A number of articles suggest that the most common size for Iron Age / Celtic roundhouses was 10 - 13 m. There are apparently some as large as 15m in diameter — and one or two records of buildings 17m across — but nothing as large as 20m. So my intuition about this being a maximum size seems more or less correct……...

On googling about, I found this fascinating blog. I have no idea his accurate Geoff Carter's calculations are, but his articles are fascinating!

Theoretical structural archaeology
Geoff Carter
19 FEBRUARY, 2009
22. Iron Age Graphs; an important discovery

http://structuralarchaeology.blogspot.com/2009/02/22-iron-age-graphs-important-discovery.html

"The roofs of prehistoric roundhouses in Britain are assumed to have been conical; as a result, their surface area, and therefore their weight, increases disproportionately with roof diameter. To put this graph in perspective, roundhouses are said to average about 10m in diameter, and 17m would be considered about the limit. [2]
So when we compare an average roundhouse with a large one, although only 70% larger in diameter, the latter’s roof is 3 times the size, with 3 times the weight and thrust of the former. So large roundhouses are much larger buildings than the average circular building, and more structurally challenging to construct.”

The surface area of a 10m roundhouse roof would be c 120 sq m — but for a 20m roundhouse with a 53 deg pitch the surface area of the roof would be almost 600 sq m. (He also considers a 45 deg pitch, but considers that the steeper angle was most probably used……)

"A steeper angle of the thatch is better at shedding water, and this increases the life of the roof surface. An additional complication is junctions between roofs, such as where a porch joins the main roof. These ‘valleys’, which channel water, are slacker than the main pitch. A steeper main roof pitch helps ensure that valleys are at the minimum pitch of 45 degrees.”

Regarding roof weight, there is a similar steepening curve — roof weight would be about 10 tonnes for a 10m roundhouse, but almost 50 tonnes for a 20m roundhouse. Then we come to the added weight of water, ice and snow. "It clear that a fall of ‘heavy snow’ lying on a roof can double its weight, increasing our large round house roof (around 15m diameter) to over 50 tonnes, and that even ice can add several tonnes to the weight of the roof. We have not found any 20m-diameter roundhouses, and a snow-loaded roof weighing close to 100 tonnes may be one of the reasons, since an extra few meters in width can double the weight.”

Then we come to the length and weight of ties and rafters. A 15m roundhouse would need rafters 12m long, but a 20m roundhouse would have required rafters 16m long — almost impossible to obtain, even in the days of pristine forests — and probably very ifficult to hande because of the extreemely large girth of the tree where cut close to the ground. So in reality, even if there was a good supply of tall slim oak trees, there was a practical limit in which trees with a base diameter over 30 cms would probably not be used. Trees about 60 years old, from a rather dense forest, would be the best ones to use….

http://structuralarchaeology.blogspot.com/2009/02/23uncovered-prehistoric-building.html

http://structuralarchaeology.blogspot.com/2008/10/8-who-would-live-in-house-like-this.html

So, to round off.  There does indeed seem to be a practical upper limit of about 20m on the diameter of large "self-supporting" round houses, in that anything larger would be liable to collapse without the addition of many large internal posts. Too much extra effort, with hardly any benefits.  

Ring cairns and circular enclosures in Pembrokeshire, as elsewhere, were probably animal enclosures.  The small rings of boulders (which look like tumbledown walls) on Carningli and Preseli may well have been circular hut sites with stone walls instead of wattle and daub walls.