Reconstructing Iron Age Buildings
By Phil Bennett
Phil Bennett, manager of Castell Henllys Iron Age fort in West Wales, explains the fascinating process of re-building ancient roundhouses, starting with little more than a few holes in the ground as a guide.
Reconstructing the past
Visitors to Castell Henllys Iron Age fort in the Pembrokeshire Coast National Park in West Wales look upon a sight that takes them back in time 2,300 years, to late prehistoric Britain. They see a reconstructed Iron Age settlement, consisting of three massive thatched roundhouses, a smithy, and a round thatch-roofed grain store, which is raised off the ground on four posts, with a fence around the front.
Castell Henllys is the only Iron Age fort in Britain to have been (partially) reconstructed. Here buildings have been reconstructed on sites previously excavated by archaeologists, using evidence recovered from the excavation.
Archaeological sites
'Most Iron Age archaeological sites reveal only a small part of what was there originally.'
Most Iron Age (and other) archaeological sites reveal only a small part of what was there originally. This is because organic material such as wood, leather, rope and thatching reed rot away very quickly under most conditions.
So an Iron Age fort might be easy to see today if its earthwork ramparts are still standing, but evidence for the houses, storage buildings and animal pens within would only survive underground, mainly in the form of foundation trenches, post holes, packing stones and sometimes floor surfaces. Even when the walls of roundhouses were made from stone, little more than the foundations survive, usually because building stone would be reused once the roundhouse was abandoned.
The evidence
A little evidence sometimes survives to show us how these buildings were used in the Iron Age.
Again it is usually only robust material that stands the test of time, such as stones and burnt clay from hearths and ovens, artefacts made from clay or stone such as loom weights and the occasional spindle whorl representing weaving activity, querns for grinding grain by hand and pottery sherds.
More delicate artefacts made from wood, leather and textiles rarely survive unless in waterlogged conditions where the lack of oxygen slows down the rate of decay. Metal artefacts corrode but sometimes survive to enter the archaeological record.
Reconstruction
'...we can never be 100% sure that such reconstruction is entirely accurate...'

So how do we go about reconstructing Iron Age buildings? First of all we have to reconcile ourselves to the fact that we can never be 100% sure that such reconstruction is entirely accurate, the archaeological record is usually too patchy to provide all the information we need.
We have to apply a range of alternative information sources to 'plug the gaps' such as geometry, woodland management practices, experimentation and comparisons with cultures alive today who are building structures that appear to match our archaeological record. We also have to make a number of assumptions which, while seemingly practical and reasonable to us, might make the finished article quite different to the original.
Roundhouses
Archaeologists think that Iron Age people, and earlier Bronze Age people, in southern Britain mostly (though not always) lived in circular houses with conical thatched roofs. They are known as roundhouses, and it's a term used to describe buildings relating to particular forms of archaeological evidence.
Often this evidence takes the form of a circular wall gully or foundation trench for the building, sometimes with post or stake holes regularly spaced within it about a meter or so apart.
Occasionally, there are no postholes within the circular trench and sometimes there is no trench, but just a circle of postholes. Iron Age roundhouses varied in size from the quite small to the gigantic - at Castell Henllys the reconstructed roundhouses are mostly between 10m and 13m (20 to 30ft) in diameter.
Wall construction
The normal form of wall construction would be to weave hazel or willow wattle around larger stakes, to form a very strong circular wall with a gap for the doorway. The walls were plastered with daub, which dries to a very hard and durable surface.
Occasionally this daub has been burnt either accidentally or deliberately to harden the wall surface.
When this happened the daub became fired in the same way as pottery, making it robust enough for small pieces to survive in the archaeological record. Pieces like these also retain the shape of the hazel or willow wattle they covered.
Sometimes the ring showing the extent of the roundhouse wall is formed by a row, or rows, of packing stones set so close together that archaeologists presume the wall was made of timber planking. Systematically splitting timber lengths of perhaps oak to a desired width and then making an even plank by using a tool such as a side axe to remove excess timber would have achieved this. Split timber is far stronger than sawn wood and would have been more desirable as a structural material.
Roofs
There is less archaeological evidence for roundhouse roofs, although occasionally worked timber survives from waterlogged archaeological sites, and this might be linked to roof construction. We assume that, because the walls are circular in shape, the roofs were conical, with rafters stretching from the top of the wall to a central apex.

It seems reasonable to suggest that, in the absence of tiles at these sites, the roof covering was thatch of some sort. To thatch a roundhouse roof with wheat straw or, more efficiently, with reed, it is important to get the pitch of the roof right. If the pitch is too great the result is an inefficient
'witch's hat' shape, which is unstable and needlessly wastes resources. If the pitch is too slight the thatch will not shed rainwater or snow fast enough to prevent decay setting in quickly. Around 45 degrees is probably the most efficient pitch, and this gives us the general dimensions of a roundhouse, based on its wall diameter.
'The roundhouses...are capable of withstanding extreme weather conditions when more modern buildings suffer structural damage.'
The occupants of roundhouses would have wanted to use as much as possible of the available interior space and would have built the walls to at least chest height to allow easy access against the incline of the roof. It would also be practical to allow the eaves of the roof to extend a way beyond the wall to protect it from weather.
Sometimes evidence for this has been found in the form of a drip gully, formed by rainwater dripping off the end of the eaves and eroding a small trench in the ground beneath. In other cases the roundhouse builders have deliberately cut a shallow drainage trench to carry rainwater away from the walls.
Some excavated roundhouses have an internal ring of postholes, which some suggest may have housed posts to help support the roof. Where there is no such evidence, we can only assume that
Iron Age builders had some way of achieving a free spanning roof - unless they were propping rafters in such a way as to avoid them being housed in postholes, which would be detected during excavation.
Experimental archaeology has shown that it is comparatively easy to reconstruct a roundhouse with a free spanning roof, so long as an internal ring beam has been constructed about half way between the rafters and the apex to take the pressure of the weight of the roof.
Materials
The building materials required for Iron Age roundhouses called for the careful management of natural resources. For the most part, the timbers used are coppice products. Some trees respond well to being cut down at ground level at the right time of the year, by sending up multiple shoots which quickly grow tall and straight. By coppicing trees in this manner a building resource can be both preserved and increased.
Hazel used for wattling the walls is best coppiced on a seven or eight year rotation, while it might take 25 years for a coppiced oak tree to provide long enough roof rafters. Reed beds, which would have supplied the roofing thatch, also need careful and regular management; alternatively, part of a wheat crop might have been used.
The roundhouses reconstructed at Castell Henllys are very efficient buildings, and are capable of

withstanding extreme weather conditions when more modern buildings nearby suffer structural damage. Like any house they need regular minor repairs and maintenance, but as a result of experimental work and observation there is good reason to believe that well built Iron Age roundhouses might have stood for over one hundred years.
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About the author
Phil Bennett is the manager of Castell Henllys Iron Age fort, in Pembrokeshire Coast National
Park, where visitors can see roundhouses built on original Iron Age foundations.