The rings of postholes at Stonehenge [Y, Z, Q, and R holes] are often ignored, or are thought to be redundant stone holes, but it is just one of a group of concentric timber structures known from various periods in British Prehistory. Like Woodhenge, Durrington Walls, Mount Pleasant, and The Sanctuary, Stonehenge was a large timber building. This was tentatively recognised by Tim Darvil in 1996, who called them Class Ei structures.
What I am arguing:
2. Form: These Class Ei structures are based on a system of 5 rows of posts; this is the form of roof evident in the Neolithic. All of the observations below are consistent with an attempt to create a roof based on this model. Each of these round buildings is built like a Neolithic long house turned in a circle. They are annular, with the tighter circles probably having been covered with a central cone. Not all have 5 rings of posts: Stonehenge has 4 plus a circular wall, while Woodhenge and Durrington Walls have additional rings on the inside of the circle.
3. Later examples: These Class Ei structures are known from the late Iron Age, as at Naven Fort, dated to where it has a proto historical context. The pattern is also evident in the layout of the Dark Age ringfort at Lissue. This clearly argues that concentric rings of posts are a technological solution to roofing circular spaces. Stonehenge is simply a particularly unique variation on a type of large building.
4. Proportion: For a roof of this form to ‘work’, it must be proportional and be composed of parallel elements at different heights. Viewed in section, this is broadly the case:
6. Scale: Class Ei buildings are about 50’/15m across the roof; this is the traditional width for large timber buildings in Britain. Much subsequent development in architectural technology was directed towards pushing these limits, and creating space less encumbered with posts, perhaps reaching one climax with Westminster Hall, [68’(20.72m)].
- Some individual timbers may be considerably longer, up to and even over 60’; these can be split from the trunks of larger, more mature trees, just as in medieval buildings.
- Prehistoric builders may have had an advantage in terms of natural forest resources.
- This limit is also reflected in the size of roundhouses.
- Trees of this size/age range are represented in the diameter of postholes.
9. Structural detail: Spans: So it can be supported, the path of each of the horizontal ties across the base of the roof, directly between the corresponding inner and outer posts, will pass next to, but not through, the intervening post. The maximum unsupported timber does not exceed c.25’. At Stonehenge the ties would be supported by the center posts of the Z holes, and then by the Sarsen ring.
12. Woodhenge: This is not a circular building, but it is the exception that proves the rule: it is based on a Lozenge, a difficult and singular shape to roof, yet it still works as a building. This geometry is shared with the Bush Barrow Lozenge and other motifs of this period.
13. Structural geometry: [Interlace Theory]; Interlace properties: By joining the posts of a ring to each other at different intervals, the parallel elements at different heights and angles necessary to facilitate a curving multilevel roof are created. Longer elements are higher on the widening outside surface of the roof, but lower on the inner surface. It also increases the rigidity and strength of the structure by:
14.Assembly: At its most basic, each section of roof can be thought of as five parallel pieces of wood supporting a rafter pair, each element supported by posts in the post rings. They could be assembled by starting from the lowest section of roof; from a pair of posts in the outer ring and a set of parallel elements in the inner ring. The adjacent sections sharing one of the original posts would have to be slightly higher, and so on until a high side is reached.
· The inner ring[s] and roof is a scaled down version [reflection] of the outer roof shape.
The real interest lies in the many other buildings that can be understood in detail, and the new light this can shed into the nature of the prehistoric built environment.
Sources and further reading
 ’Neolithic houses in northwest Europe and beyond’’ (1996) Neolithic Houses in North-West Europe and beyond (Oxbow monograph 57) [Paperback]. T.C. Darvill (Editor), Julian Thomas (Editor) [Fig 6.9]
 Previous Articles on this site:
Bronze Age Architecture; Woodhenge
Interlace Theory; Understanding Woodhange
Debunking the myth of Timber circles
Stonehenge and the Archaeology of the Prehistoric Roof
 Lissue Ringfort; http://www.lisburn.com/books/historical_society/volume6/volume6-4.html