Face a concrete wall either with small irregular stones set at random (opus incertum), which is strong though not handsome, or with small square-faced stones set diagonally in a net (opus reticulatum), which is handsome but, its joints running every way, more likely to crack.
Vitruvius calls reticulatum the method “now used by everybody”. The stones are small pyramids of tufa, their square bases forming the face and their points driven into the concrete core; their diagonal joints form a regular net. He prefers incertum for strength, because its stones lie in rough courses and overlap, while the continuous diagonal joints of reticulatum give cracks an easy path. Both, he insists, need plenty of mortar, or the porous stones will suck it dry.
Reticulatum was normally finished at the corners with quoins of brick or small blocks and laced through with courses of brick: the combination called opus mixtum. Its decorative net was usually hidden under stucco.
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
In a row of arches the thrusts of neighbouring arches cancel over each intermediate pier, which may therefore be slender; at the ends of the row nothing balances them, so the outermost piers must be made wider or buttressed.
Over a doorway or window spanned by a lintel, turn an arch in the wall above so that the weight of the masonry is carried round the opening onto the jambs, leaving the lintel to carry only the small triangle of wall beneath the arch.
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)
Build a great concrete dome thickest and heaviest at its haunches and thinnest and lightest toward the crown, grading the aggregate from dense stone at the base to light volcanic pumice at the top.
Pantheon: thickness ≈ 6 m at the springing to ≈ 1.5 m at the oculus; aggregate travertine and tuff → brick → pumice and scoria
Cite this page Chris Paton, “Opus reticulatum and opus incertum”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/reticulatum-and-incertum. Text, drawings and models licensed under CC BY-NC 4.0.