Rules · Walls · Materials · Structure
Build walls of squared stone in regular courses with the vertical joints of each course falling over the middle of the stones below: either with all courses of one height (isodomum) or with courses of alternating heights (pseudisodomum).
Vitruvius admires the Greek way of walling because every stone is bedded level and bonded to its neighbours, so the wall settles evenly and holds together “to eternity”. He contrasts it with the Roman habit of finishing only the faces and filling the middle with loose rubble.
Isodomic masonry, with its even rhythm of equal courses, was the Greek ideal for temple walls. Pseudisodomic work (tall courses alternating with thin ones, or a steady diminution of course heights upward) used stone more economically and gave a livelier surface. Both were laid without mortar, the joints so fine that a blade will not enter.
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)
In a bonded masonry wall the courses above an opening corbel over it of their own accord, so the lintel carries roughly the triangle of wall with sides rising at 60° from its ends; everything above that triangle arches onto the jambs.
load ≈ equilateral triangle on the span: W ≈ 0.43 · L² · t · ρ g
Weight for weight, timber is many times stronger in bending than stone; long spans in trabeated buildings are therefore roofed with timber beams resting on stone walls and columns.
bending strength ÷ density: oak ≈ 70 · limestone ≈ 3 (kPa per kg ⁄ m³)
Set the outer columns so that they incline slightly inward, toward the cella wall, the middle columns of the fronts plumb on their own axes and the corner columns leaning both ways.
Parthenon: lean ≈ 6–7 cm over 10.43 m (≈ 1 : 150); the axes would meet some 2 km above the ground
In a thick wall, do not build two separate skins with loose filling between them: lay stones lengthwise into the thickness at intervals, and set single stones right through from face to face, so that the wall acts as one mass.
Never let a vertical joint stand over another in the course below: interchange the direction of the stones, lengthwise and across the wall, so that every joint is covered and every stone rests on two.
overlap ≥ ¼ of a unit (brick: a quarter-brick lap)
Cite this page Chris Paton, “Isodomic and pseudisodomic walls”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/isodomum. Text, drawings and models licensed under CC BY-NC 4.0.