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Monday, 6 May 2019

What was the diameter of the biggest round house?

This illustration of a round house structure does not seem to be very sensible.  With no central supporting post (and not even a truncated post to lock the tops of the rafters together) all of the weight of the roof would be pushing outwards on the rather flimsy walls, and the building would probably collapse.  Too much lateral pressure.  The three "ring beams", lashed into place, would probably be under enormous tension,  and would be insufficient to maintain the integrity of the structure.

I have been having a discussion on a Facebook site on the differing characteristics of ring cairns, circular banked enclosures, Iron Age round house remains, and  medieval cattle pounds.

We have one of the latter in Newport, characterised by massive stone walls about 2m high and almost 2m wide.  It''s well preserved, as you might expect for something less than a thousand years old  and used up to maybe 100 years ago. There is a single entrance with an iron gate still in good repair.  Internally, the diameter is about 10m -- but there is effectively no limit to the size of pounds like this, since there is no roof to support -- a pound could be 30m or more across, the the builders had enough energy and enough building stones for the completion of the task.

But when you want to built something with a heavy conical thatched roof on top, real engineering problems appear.  What we were discussing on the Facebook page was this -- how big can a hut be before the weight of the roof becomes unsupportable?

There are lots of "circular features" in the prehistoric record for Preseli, and the Dyfed Archaeological Trust field workers are often quite circumspect about their origins:

It seems to me that features with a diameter of between 5m and 20m are sometimes considered to be the remnants of huts -- and referred to as " hut circles" -- while larger features (with a diameter of over 30m) are often deemed too large to have been roofed, and are therefore referred to as animal enclosures.  So is there an effective limit of about 20m as the maximum diameter for a genuine Iron Age roundhouse?

I know nothing at all about structural engineering, but here are a few images culled from the web:

From a BBC animation on "how to build a round house".  Note that the walls are quite high, with the wall plate about 2m above the ground.  The central supporting post is used in the early stages of construction, to support the rafters, and is sawn off beneath the lowest ring beam when the structure is complete, to allow free movement in the living space beneath.

In this example, a ring of supporting posts surmounted by lintels takes much of the weight of the roof, with the outer walls also taking weight.  This leaves an interior somewhat cluttered with heavy wooden posts.

The interior of the biggest round house still standing at Castell Henllys -- with ample living or communal space uncluttered by either a central column or any other supporting posts.

This is an interesting animation: 

One of the old round houses at Castell Henllys was taken down a few years ago -- not sure whether that was because the thatch had rotted to the point where is was leaking at an unacceptable level, or whether the structure was collapsing.  Somebody up at the site told me the other day that the wall posts embedded into the ground were rotting -- that probably meant that they were no longer able to withstand the outward pressure exerted by the weight of the roof.

Now the building is being erected again, no doubt with lessons having been learned! Note that the wall pillars are quite sturdy and short, with a maximum exposed length of about a metre (the building is on a slight slope, and the wall plate is carefully levelled).  So when all of the rafters are in place the wall plate will take the weight of the roof.  The rafters will be locked at the top of the cone by the centre post, which is currently supported on a scaffolding platform (that will be removed once the structure is complete).  There may be no need for "ring beams" like the ones shown at the top of this post) since the preferred method here for stopping the collapse of the cone is a series of crossing purlins like these:


The crossing purlins on the largest standing round house at Castell Henllys, tied to the main rafters that make up the cone, and fixed with dowels into the truncated centre post.  Here there is one ring beam, but it is rather lightweight, so the crossing purlins are doing most of the work -- under great tension -- of holding up the roof.

Back to our original question -- was there a limit of about 20m to the diameter of a conical structure made like this in the days before modern carpentry techniques were invented?  It would be good to get some expert opinions.  Also, does it matter what shape the cone is or was?  My instinct tells me that a steep cone is easier to support than a flattish one -- since in the latter case there would be enormous lateral pressure that would force out the wall plate and cause the roof to collapse.

All very intriguing.......

The interior of the big Castell Henllys hut -- very cosy by the fire, as long as the roof does not collapse on you.....


Jon Morris said...

Some carpentry techniques would have had to have been invented. But we don't know which ones. The more techniques you know about, the bigger you can go.

The first obvious constraint is the size of tree available. The things they've referred to as 'purlins' are 'ties'. If the tree you're using as a tie is 20m max length, then 20m is the max diameter for a tied structure with no advanced joint detailing (there are ways to extend this, but depends what access you have to knowledge about joining techniques). Purlins support a roof structure in bending and are almost always close to the roof structure. Ties are primarily for tension or compression.

As you point out, the first illustration is a bit daft and would become either become unsafe pretty quickly or collapse soon after the build. The second illustration has a central prop so would work but would become unstable if the perimeter posts were not deeply buried. Either way, susceptible to rot. The third, (a photo), is fine but will suffer from very early rot of the main purlin-props at the perimeter.

Chances are that a few of the above would have been tried out at some point in the past. When problems discovered, a different method would then have been chosen for most subsequent structures.

The cairn is interesting because it indicates that someone had already found out about the rot problem and so was building raised footings so that the upper structure lasted longer. To have this amount of effort spent on the base means that someone probably spent a lot of thought and effort on the upper structure. On that basis alone, probably a 'ring beam' at the perimeter of the roof (which could be made of smaller jointed timber pieces) supporting cantilevered purlins at the perimeter and leading up to a central point on the roof. Would probably need a secondary ring about half way down to get the best result.

BRIAN JOHN said...

Thank you Jon -- very interesting. Sorry for the confusion over terms -- partly my fault!

So do you think some Iron Age huts could have been more than 20m diameter?

Jon Morris said...

Could have been if anyone had bothered Brian. Interesting thing is that you probably wouldn't be able to see the evidence that it had been done (the simplest, and therefore probably the best, method would leave little or no trace). What you would almost certainly see is a perimeter of stone.

I wrote about that in the novel as it happens: Described a retro-fit structure that wouldn't leave a trace (it made for an easier storyline development rather than because there's any evidence at all of it actually happening).

Summary is that you have to know what range of things could have happened, but wouldn't leave any evidence, before you can interpret foundations: Where a foundation arrangement has what appears to be unnecessary features, the assumption (that makes those features look unnecessary) will almost certainly turn out to be wrong. If people were building rationally, they would not spend time building unnecessary features into ordinary buildings.

BRIAN JOHN said...

A stone bank around the circumference would make sense, I suppose, but there would also be post-holes -- therefore making it a bit more interesting and enigmatic for those who come along 2,000 years afterwards.....

Jon Morris said...

Depends on the roof weight to large degree. If lightweight, post-holes needed. If heavier, they're not really required (other than to confuse archaeologists in the future): Any ring structure that can't be shifted by wind would only need them for temporary works (if at all).

Circular structures are inherently stable. Internal posts rot at lower rate if placed on plinths. Timber plinths can be replaced when rot sets in. You wouldn't see this in the archaeological record.

So maybe to postholes, But maybe not. They're needed more in non-circular structures.