Single-family house: two full baths (1.6 gpf toilets, lav, tub), a kitchen sink with disposer and dishwasher, and a washer.

No. A 3-inch building drain at 1/8 in/ft slope is not permitted for this load.

DFU Calculation (IPC Table 709.1 / UPC Table 702.1):

• 2 × Water Closets (1.6 gpf) @ 3 DFU = 6 DFU
• 2 × Lavatories @ 1 DFU = 2 DFU
• 2 × Bathtubs @ 2 DFU = 4 DFU
• Kitchen Sink w/ Disposer @ 2 DFU = 2 DFU
• Dishwasher (discharges to sink tailpiece) = 0 DFU (included in sink)
• Clothes Washer Standpipe @ 2 DFU = 2 DFU
• Total = 16 DFU

Code Capacity Check:

• IPC Table 710.1(1): 3-inch pipe at 1/8 in/ft slope → Max 14 DFU. (At 1/4 in/ft → 35 DFU).
• UPC Table 703.2: 3-inch pipe at 1/8 in/ft slope → Max 12 DFU (Horizontal). (At 1/4 in/ft → 35 DFU).

Result: Your 16 DFU exceeds the 14 DFU (IPC) or 12 DFU (UPC) limit for 3-inch at 1/8 in/ft.

Solutions:

1. Increase slope to 1/4 in/ft (Capacity jumps to 35 DFU).
2. Upsize to 4-inch pipe at 1/8 in/ft (IPC: 216 DFU; UPC: 180 DFU).
3. Verify actual slope: If the installed pitch is steeper than 1/8 in/ft (e.g., 3/16 in/ft), interpolate capacity per code footnotes, but 1/4 in/ft is the standard fix.

Yes, the building drain can be 3 inches at 1/8 in/ft slope — provided the total DFU load does not exceed the maximum allowable for a 3-inch drain.

Step 1: Calculate Total DFU (Drainage Fixture Units)

Using IPC Table 709.1 (or UPC Table 702.1 — values are equivalent for residential):

| Fixture | Quantity | DFU per Fixture | Total DFU | |----------------------------|----------|------------------|-----------| | Toilet (1.6 gpf) | 2 | 3.0 | 6.0 | | Lavatory (bath sink) | 2 | 1.0 | 2.0 | | Bathtub | 2 | 2.0 | 4.0 | | Kitchen sink | 1 | 2.0 | 2.0 | | Garbage disposer | 1 | 3.0 | 3.0 | | Dishwasher | 1 | 2.0 | 2.0 | | Clothes washer | 1 | 3.0 | 3.0 | | TOTAL | | | 22.0 DFU |

Step 2: Check Maximum DFU for 3" Building Drain

Per IPC Table 710.1(2) (Building Drains and Sewers — Horizontal):

| Pipe Size | Minimum Slope | Max DFU (Horizontal) | |-----------|---------------|-----------------------| | 3" | 1/8" / ft | 35 DFU |

> ✅ 22 DFU ≤ 35 DFU → Within limit

Step 3: Verify Slope

• 1/8 in/ft is the minimum allowable slope for a 3" horizontal drain per IPC 704.1.
• This slope is acceptable and commonly used for 3" drains in residential applications.

Conclusion

✅ Yes, a 3-inch building drain at 1/8 in/ft slope is code-compliant for this fixture load (22 DFU). No upsizing to 4" is required. Ensure proper venting (IPC Chapter 9) and cleanouts (IPC 708) are provided.

Note: Always verify local amendments — some jurisdictions may adopt UPC or have stricter rules, but DFU values and slope minimums are nearly identical between IPC and UPC for residential.