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)
Rules · Plans · Spacing · Proportion
The peristyle garden
Make the peristyle a third longer across than it is deep, its columns as high as the colonnades are wide, and the spaces between them not less than three nor more than four diameters.
width = 4⁄3 depth; column height = colonnade width; intercolumniation 3–4 D
The peristyle was the private garden court of the house, surrounded by covered walks off which opened the dining rooms and summer rooms. Its columns are spaced more widely than a temple’s because they carry only a light timber roof; if they are Doric, Vitruvius adds, they must follow the triglyph scheme of the order.
The house of Vitruvius’s rules is idealised, but excavated houses at Pompeii and Herculaneum (the House of the Faun, the House of the Vettii) show the same sequence of fauces, atrium, tablinum and peristyle along one axis.
Related rules
Edge the beds before the portico with box, clip box into figures of animals set facing one another, and in the garden beyond cut it into many shapes, even into the letters of the owner’s name.
Make the corner columns thicker than the rest by a fiftieth of their diameter, because, seen against the open sky, they look thinner than they are.
D corner = D × (1 + 1⁄50); Parthenon: 1.95 m against 1.905 m
A Doric frieze cannot obey all three of its rules at a corner: triglyphs over the column axes, a triglyph turning the corner, and metopes as broad as they are high. One of them must give way.
displacement of the corner triglyph = e − t⁄2 (e = half-depth of the architrave, t = triglyph width)
Keep every triglyph centred over a column and every intercolumniation equal, and finish the frieze at the corner with a half-metope half a module wide.
corner semi-metope = ½ M (Vitruvius); equal intercolumniations throughout
Divide the height from floor to ceiling into three and a half parts and give two and a half to the opening; make a Doric door five and a half twelfths as wide as it is high (an Ionic one two-fifths), and narrow it slightly at the top.
h = 2½⁄3½ H; Doric w = 5½⁄12 h, Ionic w = h ÷ 2½; top narrowed by ⅓, ¼ or ⅛ of the jamb as h ≤ 16, ≤ 25, ≤ 30 ft
Cite this page Chris Paton, “The peristyle garden”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/peristyle-proportions. Text, drawings and models licensed under CC BY-NC 4.0.