How Do You Roof a Room Without an Arch? Lean Each Ring Inward

The method builds a circular wall in courses, with each course set slightly inward of the one beneath it, so the wall leans progressively over the space until the rings close at the top or leave a hole small enough for a single capstone. Each stone is held by the weight of everything above and behind it, which presses down on its inner end and prevents it tipping, and the structure resolves into compression all the way round with no outward push at any point. That is why no mortar is needed and why the technique long predates the true arch, which requires the stones to be cut as wedges and the whole ring to be completed before it stands.

The Mycenaean examples follow a consistent procedure:

• •Cut into a hillside, or build a mound to be cut into
• •Excavate a circular pit and lay a level stone floor
• •Build the corbelled chamber upward, backfilling behind as it rises
• •Cut a long straight passage in to the doorway
• •Set a massive lintel over the door, the largest stone in the work
• •Cover the finished chamber with earth and block the passage

The largest surviving example, built around 1250 before the common era and traditionally called the Treasury of Atreus, encloses a chamber roughly fourteen and a half metres across and thirteen high, which held the record for the widest and tallest unsupported span anywhere for more than a thousand years until Roman concrete domes surpassed it. The lintel over its doorway weighs an estimated hundred and twenty tonnes, and how it was raised is still argued about. The interior was decorated with bronze fittings, and rows of holes in the stonework show where they were attached. The tombs were robbed in antiquity, so almost nothing is known of what they contained, apart from a few that escaped.

Almost every large example was emptied long before archaeology existed, which is the central frustration of the subject, but the exceptions are informative. A small number in outlying areas escaped, and the material from those shows burials accompanied by weapons, jewellery of gold and amber, imported objects from Egypt and the Near East, and the remains of animals and of feasting in the passage, which indicates ceremonies conducted at the time of burial and probably repeated afterwards. The chambers were used for multiple burials over generations, with earlier remains pushed aside to make room. Imported goods in them are among the best evidence for how far Mycenaean trade reached.

Building this way is arrived at independently wherever there is workable stone and no tradition of the arch, which makes it a useful illustration of how technique follows material. Neolithic tombs in Ireland roof their chambers the same way, and one at Newgrange has stayed watertight for over five thousand years. Stone huts in the west of Ireland use it, as do field shelters across the Mediterranean and southern Europe, built by farmers from cleared stones with no mortar at all. It appears in the Near East, in Africa and in the Americas. The limit is always the same, since the chamber cannot be much wider than the wall can lean before the stones tip.

Stacking rings of stone that each overhang the one below closes a roof by compression alone, with no wedge-cut stones and no mortar, which is why it appears wherever stone is worked and the arch is unknown. The largest Mycenaean example spans fourteen and a half metres and held the record for the widest unsupported span for a thousand years. Neolithic Irish tombs and Mediterranean field huts use the same method.