Take the section of the standard timber, the cai, as the module of the building: it is 15 fen high and 10 fen wide, and every member is dimensioned in fen.
1 cai = 15 fen (height) × 10 fen (width)
Rules · Modules · Proportion
Choose the grade of cai by the rank of the building
There are eight grades of cai, from 9 × 6 cun down to 4½ × 3 cun; use the first for great halls of nine or eleven bays and the smallest for pavilions and ceiling coffers.
grades 1–8: 9×6, 8¼×5½, 7½×5, 7.2×4.8, 6.6×4.4, 6×4, 5¼×3½, 4½×3 cun
The grade fixes the absolute size of the fen and therefore of everything built from it. At grade 1 a fen is 0.6 cun (about 1.9 cm with a Song foot of roughly 31 cm) and a cai is some 28 cm deep; at grade 8 a fen is 0.3 cun. A large hall is therefore not merely a small one scaled up by arbitrary amounts: it is the same design drawn with a larger module, chosen from a fixed list that also expresses its rank.
The generator converts fen to metres through the selected grade; the Song foot varied between about 30 and 32 cm, so the metric sizes are approximate.
Related rules
Between two stacked arms leave a spacer of 6 fen, the zhi; a cai plus a zhi, 21 fen, is a “full timber” (zucai) and the vertical step of each tier of a bracket set.
zucai = 15 + 6 = 21 fen
Build the bracket set (puzuo) of blocks (dou) and arms (gong) crossing at right angles, each tier projecting no more than 30 fen beyond the one below; a set of n projecting tiers is called an (n + 3)-puzuo, from four to eight.
projection per tier ≤ 30 fen · rise per tier = 21 fen · puzuo = tiers + 3
For a great hall, measure the distance between the front and back eave purlins and raise the ridge purlin above them by one third of it; for lesser halls, by about a quarter.
rise = ⅓ × depth between eave purlins (halls); ≈ ¼ (+ a small addition) for ting-tang halls
Make the central bay (mingjian) the widest and let the bays diminish toward the ends; keep the eave columns no taller than the width of their bay.
Project the eaves from the centre of the eave purlin by 3½ chi for rafters of 3 cun diameter and 4 to 4½ chi for rafters of 5 cun; beyond them add flying rafters projecting six-tenths as far again.
eave projection ≈ 3½–4½ chi · flying rafters = 0.6 × eave projection
Cite this page Chris Paton, “Choose the grade of cai by the rank of the building”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/cai-grades. Text, drawings and models licensed under CC BY-NC 4.0.