Four-Inch Building Drain That Will Not Fit the Available Fall
You are the plumbing contractor for a tenant fit-out in an existing 1970s warehouse in Orlando. The plans show a new men's and women's restroom group in the front corner of the building. The only available connection is an existing 4 in. cast iron building drain stub whose invert was field-measured 26 in. below the finished slab. The new water closets will set with their outlet inverts 14 in. below the slab. The horizontal run from the last water closet to the stub is 88 ft because the route must dogleg around a column footing. The engineer's general note says all horizontal drainage at 1/4 in. per foot. The slab is post-tensioned and cannot be lowered or cut.
1. Does the route work as drawn? Show the arithmetic and state your conclusion.
Read the worked answer
Required fall at the drawn slope: 88 ft × 0.25 in./ft = 22 in.
Available fall: stub invert at 26 in. below slab minus the water closet outlet invert at 14 in. below slab = 12 in.
Then subtract the fitting allowance. A 4 in. closet bend plus the wye at the stub consume roughly 1-1/2 to 2 in. of elevation even with long-sweeps, and the pipe must be held in a straight grade between hangers with no dips.
Net usable fall: about 10 to 10-1/2 in.
Conclusion: the route cannot be built as drawn. Twenty-two inches required against roughly ten available means the drain would run uphill into the stub. This is a 100 percent call-back and a sewage-on-the-floor event, not a nuisance.
2. What alternatives do you have, and which do you recommend? Cite your basis.
Read the worked answer
The 1/4 in. per ft note is a design standard, not a code minimum at this pipe size. Code permits 1/8 in. per ft (1 percent) for 4 in. and larger horizontal drainage; 1/4 in. per ft applies to 2-1/2 in. and smaller.
At 1/8 in. per ft: 88 ft × 0.125 = 11 in. required, against roughly 10 to 10-1/2 in. usable. That is inside the tolerance of a hand measurement.
Recommend, in this order:
- Re-measure the invert with a laser and a rod, and confirm the stub's true depth. If it is 2 in. deeper than the drawing, 1/8 in. per ft works. Submit an RFI asking for written approval of 1 percent slope with the arithmetic attached, and require the pipe to be set on grade by laser, inspected, and photographed before backfill.
- If the grade still will not fit, move the restroom core 10-15 ft closer to the stub. At 1/8 in. per ft every foot removed buys 1.5 in. of fall. Relocation costs less than a failed inspection.
- Raising fixtures on a platform is a last resort: it creates an accessibility problem at the water closets and a trip hazard in a tenant space.
- A grinder or lift station adds a maintenance liability, needs power and access, and is not a drainage solution.
Do not solve this by cutting or core-drilling the post-tensioned slab, and do not accept a flat or reverse-grade run to make the schedule.
3. What must you verify about the existing stub and downstream line before you rely on that 26 in. dimension?
Read the worked answer
- The invert itself. The number in the report is a field measurement with a tape, not a survey. Laser it and take a photograph with the rod visible.
- What the line actually is. Confirm it is a sanitary building drain and not a storm drain, a roof leader, or an abandoned line. Camera it.
- Condition. A 1970s cast iron line with root intrusion, scale, or a partially collapsed bell will not carry an added restroom group, and you will own the backup.
- Capacity and venting. Add the fixture units from the new restroom group to everything already connected downstream and check the existing building drain and the building sewer. Confirm the line is vented and that the vent is not the reason it was abandoned.
- Elevation at the connection point. The fitting you need may not exist in service-weight cast iron; plan a transition coupling and account for its length in the run.
- Post-tensioning layout. Get the structural engineer's stamped plan before any coring or cutting, and mark the tendon paths on the slab.