The quick rule
Divide the building length in inches by the spacing, round up, then add one.
| Building length | 12 in | 16 in | 19.2 in | 24 in |
|---|---|---|---|---|
| 20 ft | 21 | 16 | 14 | 11 |
| 30 ft | 31 | 24 | 20 | 16 |
| 40 ft | 41 | 31 | 26 | 21 |
| 50 ft | 51 | 39 | 32 | 26 |
Use the calculator for your own length. Ask the supplier whether gable-end trusses and hip girders are included. The quantity calculator also gives lumber totals.