BUILDING DRAIN SIZE
BUILDING DRAIN SIZE
building drain DFU + slope
RESULT
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FILL IN ABOVE
IPC Table 710.1, building drain column. Not the same as horizontal branch sizing. Estimate only — verify with a licensed plumber and local plumbing code/inspector before purchase or installation. Not a substitute for engineered drawings.
About this calculator
The building drain is the lowest horizontal pipe inside the building, where every branch eventually empties before the line transitions to the building sewer at 5 feet outside the foundation. Its size is set by IPC Table 710.1 using a different DFU column from horizontal branches — the building drain is allowed higher loads at the same diameter because it always runs at full slope. This calculator returns the minimum building drain size for the total DFU load at both ⅛"/ft and ¼"/ft slopes, since slope choice affects capacity. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
Common questions
How is building drain sizing different from horizontal branch sizing?
Same DFU concept, different limits. The IPC Table 710.1 has separate columns for horizontal branches and building drains (and stacks). Building drains carry higher DFU loads at the same diameter because they always run at full slope and don't experience the start-stop flow patterns that branches see. A 3" horizontal branch is limited to 20 DFU; the same 3" building drain at ¼"/ft handles 50 DFU.
What slope should I use for the building drain?
Default to ¼"/ft for any pipe size. ⅛"/ft is allowed only for 3" and larger pipe and is used when ceiling space is tight (basement runs under floor joists). Going steeper than ½"/ft causes scour problems where water outruns solids, leaving them behind to build up on pipe walls. The sweet spot is ¼" for most residential.
Where does the building drain end and the building sewer begin?
Per IPC: 5 feet outside the building foundation. Inside that 5 ft = building drain (subject to building drain sizing tables). Outside = building sewer (different, more permissive sizing). The transition matters for permit and inspection scope; the building drain is part of the rough plumbing inspection, the sewer is its own permit.