18.2 Analyzing a DWV Isometric for Code Compliance
Key Takeaways
- IPC Table 706.3 permits a sanitary tee only for horizontal-to-vertical changes of direction, so a sanitary tee at a stack base is a defect.
- IPC 905.4 requires a dry vent to rise vertically to 6 inches above the flood level rim of the highest trap being vented before offsetting.
- IPC 912.1 limits horizontal wet venting to fixtures within two bathroom groups on the same floor level, with additional fixtures discharging downstream of the wet vent.
- IPC 911.3 prohibits a water closet as the upper fixture where common-vented fixture drains connect at different levels.
- IPC 904.4 requires a vent stack to connect within 10 times the stack diameter where it connects to the building drain, while IPC 704.3 requires branches to connect not less than that distance downstream.
The eleven defects that account for most items
Isometric Analysis items are built from a short list of recurring errors. Learn to spot them on sight.
| # | Defect on the drawing | Section violated | What it should be |
|---|---|---|---|
| 1 | Sanitary tee turning a stack into a horizontal building drain | Table 706.3 | Long sweep or combination wye and eighth bend |
| 2 | Quarter bend turning a 4-inch line vertical to horizontal | Table 706.3 footnote a | Permitted only on 2-inch or smaller fixture drains |
| 3 | Trap arm too long — 1-1/2 inch trap with an 8-foot arm | Table 909.1 | 6 feet maximum for a 1-1/2 inch trap |
| 4 | Vent turns horizontal below the fixture rim | IPC 905.4 | Rise to 6 inches above the flood level rim first |
| 5 | Vent connects at or below the centerline of a horizontal drain | IPC 905.3 | Connect above the centerline |
| 6 | Crown vent taken immediately at the weir | IPC 909.3 | Not within two pipe diameters of the weir |
| 7 | Kitchen sink on a wet-vented bathroom branch | IPC 912.1 | Only bathroom group fixtures; others discharge downstream |
| 8 | Common vent with a water closet as the upper fixture at different levels | IPC 911.3 | The upper fixture shall not be a water closet |
| 9 | Three fixtures on one "common vent" | IPC 911.1 | A common vent serves two traps |
| 10 | Five-branch-interval stack with no vent stack | IPC 904.2 | Vent stack required at five or more branch intervals |
| 11 | Branch tied in 18 inches downstream of a 4-inch stack base | IPC 704.3 | Not less than 10 x 4 = 40 inches |
Reading a defective isometric
The drawing shows a single-story commercial restroom:
- A 4-inch building drain running from left to right, with a cleanout at the upstream end.
- A 3-inch horizontal branch serving, upstream to downstream: LAV, LAV, WC, WC, WC, WC.
- A 2-inch vent rising from the branch between the third and fourth water closet.
- The branch ties into a 4-inch soil stack serving two floors above.
- Each lavatory has a 1-1/4 inch trap with a 7-foot trap arm to the branch.
- The vent turns horizontal at 36 inches above the floor and runs to the stack vent.
- The lavatory flood level rims are at 34 inches above the floor.
Working the defects
Defect A — circuit vent connection location. Six fixtures on a horizontal branch is within the eight-fixture limit of IPC 914.1. But IPC 914.2 requires the circuit vent connection to be between the two most upstream fixture drains — here, between the two lavatories. The drawing puts it between the third and fourth water closet. Violation.
Defect B — missing relief vent. The branch receives four water closets and connects to a stack that receives discharge from upper horizontal branches. IPC 914.4 requires a relief vent, connecting between the stack and the most downstream fixture drain, carrying not more than 4 dfu if it doubles as a fixture drain. Not shown. Violation.
Defect C — trap arm length. A 1-1/4 inch trap permits a maximum developed length of 5 feet to the vent fitting (Table 909.1). The drawing shows 7 feet. Violation. Fixes: move the vent, enlarge the trap and arm to 1-1/2 inch (6 feet) or 2 inch (8 feet), or individually vent each lavatory.
Defect D — horizontal vent height. The vent turns horizontal at 36 inches, and the lavatory flood level rims are at 34 inches. IPC 905.4 requires the rise to reach 6 inches above the flood level rim — 40 inches — before offsetting. Thirty-six inches is only 2 inches above the rim. Violation.
Defect E — branch sizing. Four public 1.6 gpf water closets at 4 dfu + two lavatories at 1 dfu = 18 dfu. Table 710.1(2) horizontal branch: 3-inch = 20 dfu, so the branch complies numerically. But IPC 914.3 requires the entire length of the vent section of the horizontal branch to be sized for the total drainage discharge to the branch, and caps its slope at 1 unit vertical in 12 horizontal. Verify both on the drawing.
Defect F — vent size. The circuit vent is drawn at 2 inches. IPC 906.2 requires not less than one-half the required diameter of the drain served: half of 3 inches is 1-1/2 inches, so a 2-inch vent complies — provided its developed length does not exceed 40 feet, which would force it up one nominal size.
A second worked isometric: a residential stack
The drawing shows:
- A 3-inch stack rising four stories, one branch interval per floor.
- Each floor has a bathroom group (5 dfu) and a kitchen sink (2 dfu) on a 2-inch branch.
- The stack continues above the top branch as a 3-inch stack vent through the roof.
- No separate vent stack.
- The kitchen sink ties into the bathroom branch upstream of the water closet, with the branch wet vented from a lavatory individual vent.
Working it
| Check | Result |
|---|---|
| Branch intervals | Four floors = 4 branch intervals. Fewer than five, so no vent stack required (904.2) |
| Stack load | 4 x (5 + 2) = 28 dfu. Table 710.1(2), greater than three branch intervals: 3-inch = 72 dfu. Complies |
| Single-interval load | 7 dfu into one interval; 3-inch allows 20. Complies |
| Branch size | 5 + 2 = 7 dfu on a 2-inch branch. Table 710.1(2) horizontal branch: 2-inch = 6 dfu. VIOLATION — a 2-1/2 inch branch carries 12 dfu, and because the branch serves a water closet the practical size is 3 inches |
| Wet vent composition | The kitchen sink is not a bathroom group fixture. IPC 912.1 permits only bathroom group fixtures on the wet-vented branch, and requires additional fixtures to discharge downstream of the wet vent. VIOLATION |
| Dry vent location | An individual vent on the lavatory is a valid dry vent for a horizontal wet vent (912.2.1), provided not more than one wet-vented fixture drain is upstream of it |
| Building drain | Total 28 dfu; Table 710.1(1) at 1/8 in/ft: 3-inch = 36 dfu, but footnote a requires 3 inches minimum for a water closet. Complies at 3 inches, though 4 inches is the usual build |
Two defects, both fixable on paper: enlarge the branch, and move the kitchen sink downstream of the wet vent with its own vent.
Building your own checklist
When an item shows you a drawing, run this sequence:
- Fittings — every change of direction against Table 706.3.
- Trap arms — every one against Table 909.1, plus 909.2 fall and 909.3 crown-vent clearance.
- Vent elevations — 905.3 above centerline, 905.4 six inches above the rim, 905.5 vent-to-vent connections.
- Venting method — name it; if it is a wet vent, verify the two-bathroom-group and same-floor limits and that no foreign fixture is on the branch.
- Loads — accumulate dfu and check both the horizontal branch column and the stack columns of Table 710.1(2), plus Table 710.1(1) for the building drain.
- Stack requirements — vent stack at five branch intervals, relief vents past ten, offsets per 711 and 907.
- Cleanouts — 100-foot intervals, direction changes, sewer junction.
- Slope — Table 704.1 by size, with the 1/4 inch per foot override upstream of a grease interceptor.
- Distances at stack bases — branches not less than 10 diameters downstream (704.3); vent stacks within 10 diameters (904.4); pump discharge not less than 10 diameters through a wye into the top (712.3.5).
Nine passes, and every defect in the list at the top of this section is caught.
An isometric shows a sanitary tee used to turn a 4-inch soil stack into a horizontal building drain. What is wrong?
A lavatory with a 1-1/4 inch trap is drawn with a 7-foot developed length from the trap weir to the vent fitting. Which fix is NOT acceptable?
A circuit-vented horizontal branch serves five water closets and connects to a stack receiving discharge from upper branches. What additional vent is required?
An isometric shows a kitchen sink connecting to a wet-vented bathroom branch upstream of the water closet. What does IPC 912.1 require?