WET WALL STACK
WET WALL STACK
stack DFU + branch interval limit
RESULT
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FILL IN ABOVE
IPC Table 710.1 (stack columns). Stacks ≤3 stories use the higher per-branch-interval column; taller stacks use the stricter total-stack column. 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
A wet wall is the framed cavity that carries the drain stack, vent, and hot/cold supply lines through a multi-story building, usually directly behind the bathroom. The soil stack inside that wall is sized for the total DFU load it carries (every fixture above and at its level) and limited per branch interval (the vertical span between fixture connections). This calculator returns the minimum vertical stack diameter from IPC Table 710.1, accounting for both total stack DFU and the per-branch-interval cap. Pair with vent sizing for the stack vent extending above the roof. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
Common questions
What is a wet wall in plumbing?
A wet wall is a framed cavity (typically 6" or wider studs instead of standard 4") that carries vertical plumbing — supply lines, drain stack, and vent — through a multi-story house. Bathrooms are usually backed up against a wet wall so a single chase serves both floors. Stacking bathrooms vertically over a single wet wall cuts plumbing labor and material by 30–50% compared to spreading them across the floor plan.
What is a branch interval?
A branch interval is the vertical distance between two horizontal branches connecting to a stack — typically one floor, about 8 ft. The IPC limits how much DFU can connect to a stack at a single branch interval, separately from the total stack DFU. A 3" stack can carry 48 DFU total but only 20 DFU per branch interval — meaning you can't hang all the load on one floor. Spreading fixtures across multiple intervals is what stacks are designed for.
Does a stack vent need to be the same size as the stack?
IPC 906 sizes vents independently from the drain they serve, but practical residential rule: stack vents extending through the roof are usually the same diameter as the stack to avoid a reduction. Cold-climate codes also require minimum 3" through the roof regardless of stack size to prevent frost closure (ice buildup choking off the vent in winter).