Rules · Structure · Plans · Materials
In a megalithic chamber the room can be no wider than the largest slab the builders can raise over it; the upright stones are set to suit the capstone, not the other way round.
Dolmens show the post-and-lintel principle in its rawest form: a few orthostats, a single roof slab, and a mound of earth or stones heaped over the whole. Capstones weighing tens of tonnes were levered up on timber cribbing or dragged up the rising mound. Where a larger room was wanted, builders either multiplied the slabs along a passage or abandoned the lintel altogether for corbelling, as in the domed chamber of Maeshowe in Orkney.
In a corbelled vault each course may project only a little beyond the one below, and each overhanging half must have its centre of gravity behind the inner face of its wall; load the tails of the stones with heavy hearting until the capstone is set.
stable if Σ W·x (behind the springing edge) > Σ W·x (in front)
A beam is squeezed along its top and stretched along its bottom; stone resists squeezing well but stretching badly, so a stone lintel must be deep in proportion to its span and cannot be long. Doubling the span quadruples the stress from its own weight.
σ ≈ ¾ ρ g L² ⁄ d (self-weight) · + ≈ 0.43 ρ g L³ ⁄ d² (a wall above)
At Stonehenge each lintel is pegged to its uprights by a mortise-and-tenon joint and locked to its neighbours by tongue-and-groove joints, and each is curved to follow the circle: the joints of a timber frame reproduced in dressed sarsen.
Orient the principal axis of the monument on the rising or setting of the sun at a solstice, so that the architecture frames it.
Build a round chamber of horizontal courses, each ring projecting a little beyond the one below, dress the inner face to a smooth curve, close the top with a capstone, and heap earth over the whole.
Enclose the gathering place with a circular bank of chalk and dig the ditch on the inside of it, so that the earthwork marks off a sacred interior rather than defending it; leave causeways of undug ground for the entrances.
Cite this page Chris Paton, “The chamber is as wide as the capstone is long”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/dolmen-capstone. Text, drawings and models licensed under CC BY-NC 4.0.