Drainage Stack and Building Drain Sizing by Fixture Units
Drainage sizing is a bookkeeping exercise in fixture units — provided you total them at the right point in the system and pick the slope before you read the table.
Build the DFU load
Assign each fixture its drainage fixture unit value, then total upward: individual fixture branch → horizontal branch → each storey’s contribution to the stack → whole stack → building drain.
| Fixture | DFU | Min. trap / branch |
|---|---|---|
| Water closet (1.6 gpf) | 3 | 75 mm |
| Lavatory | 1 | 32–40 mm |
| Shower | 2 | 50 mm |
| Kitchen sink | 2 | 40–50 mm |
| Floor drain | 2 | 50–75 mm |
Read the table at a chosen slope
Horizontal drains are sized by DFU and slope. Steeper slope carries more DFU for the same diameter:
| Pipe size | Max DFU @ 1% (1:100) | Max DFU @ 2% (1:50) |
|---|---|---|
| 75 mm | 20 | 27 |
| 100 mm | 180 | 216 |
| 150 mm | 700 | 840 |
Use 2% (1:50) as the default residential slope for pipe up to 75 mm; 1% (1:100) is acceptable for 100 mm and larger where headroom is tight.
Stack rules
- A stack is sized for the total DFU it carries, but also limited by the DFU permitted on any one branch interval.
- An offset of more than 45° from vertical is sized and vented as a building drain through the offset.
- The base of the stack and the first 600 mm of horizontal drain see the highest flow — do not reduce size there.
Building drain and sewer
Total every stack and every ground-floor fixture into the building drain DFU, pick the slope the site fall allows, and size from the table. The building sewer to the street or septic system is sized the same way, usually one size up for maintenance access.
EngEst Pro’s Plumbing Calculator totals DFU by branch, stack, and building drain from your fixture schedule and returns pipe sizes at your chosen slope — then lists PVC drainage pipe, fittings, and cleanouts in the BOM.
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