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 · Proportion
Alae, tablinum and fauces from the atrium
Proportion the rooms round the atrium from it: the alae from its length (a third for atria of 30–40 feet, a quarter for 50–60), the tablinum from its width (two-thirds for a 20-foot atrium, half for 30–40 feet, two-fifths for 40–60), and the entrance passage from the tablinum.
alae = L⁄3 … L⁄5; tablinum = ⅔ W (20 ft), ½ W (30–40 ft), ⅖ W (40–60 ft); fauces = ⅔ or ½ tablinum
Vitruvius’s rules scale down as the house grows, because rooms proportioned for a small atrium would be monstrous in a large one, and those for a large atrium unusable in a small one. This “sliding” proportion is an early statement of a principle every architect learns: some dimensions follow the size of the building and some follow the size of the human body.
The tablinum, on the axis opposite the entrance, held the family records and opened toward the peristyle garden beyond, so that a visitor standing in the fauces could look through the whole depth of the house.
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
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
Architecture depends on order (ordinatio), arrangement (dispositio), eurythmy, symmetry, propriety (decor) and economy (distributio).
A column is narrower at the top than at the bottom, but the taller the column the less it should be diminished, because distance already makes its top look smaller.
upper Ø : lower Ø = 5⁄6 (≤15 ft) · 11⁄13 (15–20 ft) · 6⁄7 (20–30 ft) · 13⁄15 (30–40 ft) · 7⁄8 (40–50 ft)
Cite this page Chris Paton, “Alae, tablinum and fauces from the atrium”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/atrium-rooms. Text, drawings and models licensed under CC BY-NC 4.0.