Jǔzhé: raising the roof
The Song rule for the curved roof section: raise the ridge by a third of the depth, then lower each purlin from the straight line by 1/10, 1/20, 1/40… of the rise.
The rules it illustrates
Raise the ridge by a third of the depth. 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.
Depress each purlin: 1/10, 1/20, 1/40…. Draw a straight line from the ridge purlin to the eave purlin and lower the first purlin below the ridge from it by one tenth of the rise; from that point draw a new line to the eave purlin and lower the next purlin by one twentieth; continue, halving each time.
Qing jǔjià: five-tenths at the eaves, nine at the ridge. Give each rafter span a fixed slope, increasing from the eaves to the ridge: half-pitch (5 jǔ) for the eave span, then about 6½ or 7, 7½, and 9 tenths for the span below the ridge.
Where it appears in the book
In the chapters The Roof.