Basement Bathroom Addition on an Existing 3-Inch Building Drain
You are roughing in a basement bathroom group and laundry in an existing single-family house. The existing 3-inch building drain runs 42 feet at 1/4 inch per foot to the city sewer, and the 3-inch stack above it already serves a second-floor water closet (3 f.u.) and lavatory (1 f.u.). In the basement the owner wants a water closet (3 f.u.), lavatory (1 f.u.), shower (2 f.u.), laundry tub (2 f.u.), and washing machine (2 f.u.). The building department has flagged the permit for two reasons: the added load, and the fact that the new basement fixtures sit below the level of the next upstream manhole cover. The slab must be cut for the new branch, and the homeowner wants everything concealed in the floor.
1. Compute the total drainage fixture-unit load and decide whether the existing 3-inch building drain and stack are adequate. Show the arithmetic and state what else still has to be checked.
Read the worked answer
Load: existing 3 + 1 = 4 f.u.; new 3 + 1 + 2 + 2 + 2 = 10 f.u.; total = 14 f.u.
Capacity: a 3-inch building drain at 1/4 inch per foot is rated on the order of 20 drainage fixture units, and a 3-inch stack carries well above 14 f.u., so the 14-unit total falls inside the capacity of the existing piping. Confirm the exact table values against the code edition your jurisdiction adopted before you commit.
Non-negotiables that the f.u. math does not cover:
- No drain may be smaller than the minimum required for the fixture it serves: the water closet needs a 3-inch branch, so the basement branch carrying the closet cannot be reduced to 2 inch.
- The branch carrying closet + shower + lavatory (6 f.u.) must be at least 2-1/2 inch, realistically 3 inch.
- 1/4 inch per foot slope on the new work, with the invert checked at the sewer connection so the basement fixtures still drain by gravity over the 42-foot run.
- Venting of the new group is a separate approval item and cannot be assumed from the drain sizing.
2. The new fixtures are below the next upstream manhole cover. What protection does the code require, and where should it be installed?
Read the worked answer
When a fixture drains to a building drain that can be surcharged by the public sewer and the fixture is below the level of the next upstream manhole cover, the code requires a backwater valve on the drainage path serving those fixtures. The reasoning: during a sewer surcharge the basement is the lowest point, and without a valve the sewer will discharge into the basement through the shower or laundry tub.
Where to put it:
- On the branch serving only the basement group, downstream of all the new fixtures and upstream of the connection to the building drain, so the second-floor fixtures above are not dependent on the valve to drain.
- Choose a listed, gravity-operated, normally-open (during forward flow) type. A manually operated or normally closed valve is a nuisance and is not a substitute.
- Provide a cleanout immediately upstream of the valve and keep the valve accessible for service (see next item).
3. The installer wants to bury the backwater valve in the concrete under the new shower to keep it out of sight. Evaluate that proposal.
Read the worked answer
Reject it. A backwater valve has a moving flap that has to be inspected, cleaned, and repaired; the code requires backwater valves and their cleanouts to remain accessible. Encasing it in poured concrete removes the only access.
Acceptable alternatives:
- Set the valve in a listed floor box or valve box with a removable cover set flush with the finished floor, in a location that is not under a fixture or in a traffic path that will be covered by finish flooring.
- Relocate the valve into the adjacent unfinished mechanical space or a wall cavity with an accessible panel.
Also confirm the valve does not create a low spot that will hold water, that it is installed in the orientation its listing requires, and that the cleanout upstream stays accessible. If the owner refuses any visible access point, the design change is to relocate the basement group so it drains above the surcharge level (or pump it), not to bury the valve.