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The roof section and juzhe

Source: Liang (1984, 265)

The roof section is calculated by juzhe ('raising and lowering the roof'), a parametric process based on the building type, the building depth, and the type of roof tile. Once these parameters are known, only one roof section is possible.

The procedure is simply to locate the purlins, beginning with the eaves purlins, whose positions have already been determined through the building depth. Then locate the remaining purlins in succession from the top of the roof down.

The procedure for ting tang is as follows:
  1. Locate the ridge purlin (ji tuan) , or 'raise the roof' (ju wu) . The distance h from the top of the eaves purlin to the top of the ridge purlin is b/4 + x, where
    b is the depth of the building, measured between front and back eaves purlins (liaoyan fang) (on centre), and
    x = 8b/100 (for curved tiles) or 5b/100 (for flat tiles).
  2. Locate the remaining purlins, or 'lower the roof' (zhe wu) . Connect the ridge purlin to the eave purlin with a line. Locate the first purlin below the ridge by lowering it from this line by h/10. Connect this purlin to the eave purlin with a new line. Lower the next purlin below this new line by h/20. Lower succeeding purlins from succeeding lines by h/40, etc.
  3. For an illustration, see Liang (1983, 265).

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