WET WALL STACK
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.
How to use this calculator
Enter the total DFU on the stack (sum of every fixture connecting to it). Set the number of branch intervals (one floor of fixtures = one branch interval, typically 8 ft of vertical run). Most residential stacks have 1-3 intervals.
Enter the largest single branch DFU — the most heavily-loaded branch entering the stack at one level (a typical full bath = 7 DFU, two baths sharing the floor = 14 DFU). The calculator returns the minimum vertical stack diameter satisfying both the total-stack DFU limit and the per-branch-interval cap. Stacks taller than 3 branch intervals use the stricter "tall stack" column from IPC Table 710.1.
Worked example
For a 2-story home with one bath per floor sharing a single stack: total DFU = 12 (6 per bath × 2 floors). Intervals: 2. Largest single branch: 6 DFU.
Looking up: 3" stack handles 48 DFU on stack ≤3 stories AND 20 DFU per branch interval. Both limits satisfied. Final: 3".
For a 4-story apartment building with 4 baths per floor sharing one stack: total DFU = 4 baths × 6 DFU × 4 floors = 96 DFU. Intervals: 4 (tall stack — stricter limits). Largest branch: 24 DFU (all 4 baths on one floor).
Looking up tall stack column: 4" handles 180 DFU total and 90 DFU per branch interval. Both satisfied. Final: 4".
For a 6-story commercial building with 200+ DFU stack and 50+ DFU per branch interval: 5" stack handles 390 DFU total and 200 per branch interval. Final: 5".
In cold climates, the stack vent through the roof must be at least 3" diameter regardless of stack size — to prevent frost closure during winter.
Common mistakes & waste factors
Confusing branch interval with floor count. A branch interval is the vertical distance between fixture entry points, typically 8 ft. A 3-story house with fixtures on each floor = 3 intervals. A 3-story house with fixtures only on the bottom floor = 1 interval (and a much smaller stack required).
Forgetting the tall-stack penalty. Stacks over 3 intervals (4+ stories) use stricter limits. Using the residential ≤3-story column on a tall stack undersizes by 25-30%.
Missing the per-branch-interval cap. A stack might handle the total DFU at one size but fail the per-branch-interval limit at the same size. Both must pass. The calculator checks both.
Forgetting the stack vent. The stack continues up through the roof as a vent. Cold-climate codes require ≥3" through-roof regardless of stack size to prevent frost closure (ice forming inside the vent).
Rules of thumb
IPC Table 710.1 stack columns: ≤3 stories uses the standard column; >3 stories uses the stricter tall-stack column.
3" stack: 48 DFU total ≤3 stories, 30 DFU tall stack, 20 DFU per branch interval.
4" stack: 240 DFU ≤3 stories, 180 DFU tall stack, 90 DFU per branch interval.
Branch interval ≈ one floor (8 ft of vertical stack with fixture entries).
Stack vent through roof: same diameter as stack, minimum 3" in cold climates to prevent frost closure.
All branches connecting to a stack must enter via wye fittings (not tee), and at least 6 inches above the floor for proper offset.