Building Sewer Gravity Connection at Fixed Inverts
A contractor is replacing a collapsed 4-inch cast-iron building sewer for a 1950s two-story house. The basement floor is 5.0 ft below street grade at the curb. The existing building sewer leaves the foundation 3.2 ft below the basement floor and runs 85 ft to the city main. The city main invert is 9.5 ft below street grade. The owner wants to reuse the existing trench line and connect by gravity. The contractor proposes 4-inch PVC at 1/8 in/ft, the minimum slope commonly allowed for 4-inch sewer. The local inspector is concerned about whether the connection can be made by gravity and about depth of cover. The lot has a 6-inch public sidewalk 12 ft from the building, and the basement is finished with a bathroom and laundry that discharge to the sewer. Soil is firm clay, and no overhead obstructions are present.
1. Calculate the proposed invert at the city main and determine whether the 4-inch PVC at 1/8 in/ft can connect by gravity. Show the conversions and state the shortfall.
Read the worked answer
Set street grade = 0.00 ft.
- Basement floor = -5.00 ft.
- Foundation invert = -5.00 - 3.20 = -8.20 ft.
- Fall over 85 ft at 1/8 in/ft: 85 x 0.125 in = 10.625 in = 0.885 ft.
- Proposed invert at the main = -8.20 - 0.885 = -9.085 ft.
- City main invert = -9.50 ft.
Shortfall = 9.50 - 9.085 = 0.415 ft, or about 5.0 in. The proposed sewer is roughly 5 in too high, so it cannot connect by gravity at 1/8 in/ft with the given inverts.
2. What uniform slope is required over the 85 ft run? Express the answer in in/ft and percent, and evaluate whether it is practical and code-compliant.
Read the worked answer
Required fall = 9.50 - 8.20 = 1.30 ft over 85 ft.
- Slope = 1.30 / 85 = 0.01529 ft/ft.
- In in/ft: 0.01529 x 12 = 0.1835 in/ft, approximately 3/16 in/ft (0.1875).
- Percent = 1.53%.
This is steeper than the 1/8 in/ft minimum but still modest. A 3/16 in/ft slope is practical for 4-in PVC and should reach the main if the survey is accurate. Check velocity, bedding, and cover, and confirm the main invert by survey before excavation. If the main is actually higher than reported, a gravity connection may not be possible.
3. Recommend a design and permitting path if the owner rejects a pump. Include inspection and testing steps.
Read the worked answer
Re-survey and benchmark the city main invert and the basement floor. Design the replacement at about 3/16 in/ft, using long-radius fittings, proper bedding, and cleanouts at the building and at required intervals. Obtain a plumbing permit and call for inspection of the open trench and piping before backfill. Test the building sewer by water or air test as required, then backfill and restore the sidewalk. If the required slope cannot be achieved, the owner's no-pump preference cannot be met; propose a listed grinder or ejector pump with a sealed basin, vent, check valve, alarm, and dedicated circuit. Do not reduce pipe size or install an undocumented flat line.