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)
Vitruvius praises the Greeks for binding the upright joints “by interchanging the direction of the stones as they lie in the courses”, and describes brick walls laid with whole bricks and half-bricks in alternate courses so that the joints always break. A continuous vertical joint is a crack waiting to happen; lapping each unit over two below spreads loads and ties the wall together.
Later brick bonds are elaborations of this single rule. In English bond whole courses of headers (bricks laid across the wall) alternate with courses of stretchers (laid along it); in Flemish bond headers and stretchers alternate within each course, the headers centred on the stretchers below.
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)
Build an arch of wedge-shaped stones (voussoirs) whose joints all point towards the centre of the curve: each stone can then fall only by forcing its neighbours apart, and so the ring holds itself up once the last stone, the keystone, is in place.
joints ⟂ intrados (radial to the centre)
An arch pushes outward on its supports as well as down. For the same load and span the horizontal push grows as the rise falls: halve the rise and you roughly double the thrust.
H ≈ w·s² ⁄ 8f (uniform load w per metre, span s, rise f)
Trace the path of the resultant force through the stones from crown to foundations. If a line can be found that stays everywhere within the masonry, the arch will stand; where it touches an edge, a hinge will open, and four hinges make it collapse.
Cite this page Chris Paton, “Break joints”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/break-joints. Text, drawings and models licensed under CC BY-NC 4.0.