1. Building depth is measured in rafters, as mentioned above. A ting tang may be 4, 6, 8, or 10 rafters deep.
2. Bay depth. A rafter may not exceed 150 fen in (horizontally projected) length. The fen is a modular unit.

For a ting tang, a bay is 2 rafters deep (not more than 300 fen deep). However, the depth of the bays may not be demarcated by columns. Our sample building, for example, is 2 bays deep, but there are no interior columns to indicate the two bays. (See illustration.)

The depth of our sample building is 4 rafters x 150 fen = 600 fen. At the 6th grade, 1 fen = 12.8 mm; thus the building is 7680 mm deep.