Plan Measurement, Invert Arithmetic, and a Grade Beam in the Way
A tenant improvement in a three-story office building. The plumbing plan is drawn at 1/8 inch = 1 foot. On the plan, the new 4-inch PVC building drain under the slab measures 9-1/4 inches from the base of the cast iron soil stack to the point where it exits the foundation wall. The isometric notes 1/8 inch per foot slope and shows the stack base invert 12 inches below the top of slab. The civil drawings put the city sewer stub invert at 18 inches below the first floor, 10 feet outside the foundation wall. The structural plan shows a grade beam with its bottom 3 feet 6 inches below the slab, and the drain route appears to cross beneath it. The 4-inch run continues 12 feet outside the wall to the stub.
1. Convert the scaled dimension, compute the fall at the noted slope, and determine whether the drain can reach the sewer stub by gravity at minimum slope.
Read the worked answer
Scale conversion: 9-1/4 inches measured at 1/8 inch = 1 foot is 9.25 x 8 = 74 feet of horizontal run.
Fall at 1/8 inch per foot: 74 ft x 1/8 in/ft = 9.25 inches, or 9-1/4 inches.
Invert at the foundation wall: 12 in + 9.25 in = 21.25 in, roughly 1 ft 9-1/4 in below the slab.
Compare to the stub: the sewer stub invert is only 18 inches below the first floor. Your pipe would arrive 3-1/4 inches below the stub - the drain is under the sewer it is supposed to discharge into.
Conclusion: gravity drainage fails at minimum slope, and 1/8 inch per foot is already the minimum for 4-inch pipe, so you cannot buy fall by flattening the pipe.
Options to price or ask about:
- Raise the under-slab run to the minimum cover that still works - starting at 6 to 8 inches instead of 12 puts the exit above the stub and still leaves more than 1/8 inch per foot over the 10-foot exterior run.
- Ask civil/utility to verify or lower the stub.
- Re-route to shorten the run.
- A lift station only if gravity truly cannot be achieved - the code treats that as a last resort.
Do not bid any of these as included scope without an RFI answer.
2. The drain appears to pass through a grade beam whose bottom is 3 feet 6 inches below the slab. How do you handle it, and what are the technical constraints on any solution?
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Handle it before the bid, not in the field. The pipe invert runs at roughly 12 to 21 inches below the slab, which is inside the depth of a beam whose bottom is 42 inches down. This is a genuine conflict, not a drafting nuisance.
Process:
- Issue an RFI to the structural engineer of record with the plan sheet, the isometric, the measured location, and the computed inverts.
- Do not price the beam penetration as base scope on a guess, and never field-cut, notch, or core a structural member without written approval - it is a code violation and an insurance problem.
Technical constraints if a sleeve is approved:
- Sleeve diameter sized to let the pipe pass with clearance - typically two pipe sizes larger.
- Slope maintained through the sleeve; the sleeve must not create a belly or a high point.
- Sleeve, penetration location, and reinforcing details all come from the structural engineer, ideally coordinated before concrete placement.
- Corrosion and firestopping at the penetration per the assembly listing.
Alternate outcomes: re-route around the beam within the available fall, or lower the beam. Both change quantities and both go through the same RFI.
3. Estimate the 4-inch PVC material and field labor for the under-slab and exterior run, and state the assumption that must accompany the bid.
Read the worked answer
Pipe quantity: 74 ft under slab + 12 ft exterior = 86 ft linear.
Waste: apply 5 percent: 86 x 1.05 = 90.3 ft. Ordering in 20-foot lengths: 90.3 / 20 = 4.5, so 5 lengths = 100 ft with offcuts. Do not round down - you cannot join 86 feet from four 20-foot lengths once you account for the bends.
Fittings: take them off the isometric rather than guessing - a typical route of this shape carries a long-sweep or two 45-degree bends at each change of direction, a wye for the stack base, and bends at the wall penetration. Count each symbol, list it, and price from the list.
Labor: at the company's unit rate for 4-inch PVC in a slab trench, say 0.45 hours per foot: 86 ft x 0.45 = 38.7 hours. At a loaded rate of $72 per hour that is about $2,786. Hangers at the code interval for the pipe material, with supports at every fitting and every joint at a change of direction.
Assumption to attach to the bid: the invert elevation shown does not reach the sewer stub at minimum slope, and the route conflicts with the grade beam. The bid is based on a revised civil invert supplied by the owner and on the structural engineer approving a sleeve. If neither is provided, the pipe will need to be raised, re-routed, or pumped - all of which are change-order scope, and the bid should say so in one sentence that a project manager can find.