Relocating a Water Closet onto an Old 4-Inch Building Drain
A contractor is remodeling the lower level of a 1955 split-level in Grand Rapids and must relocate the water closet about 6 feet. The new 3-inch branch will run 6 feet to the existing 4-inch cast-iron building drain, which runs 40 feet to the city sewer at 1/8 inch per foot. The installer plans to cut a 3-inch by 4-inch sanitary tee into the back of the horizontal building drain and to pitch the new branch at 1/8 inch per foot. While opening the ceiling he finds that the last 8 feet of the building drain have settled and now fall 1 inch the wrong way. The homeowner intends to finish the ceiling, and the installer proposes to bury the existing 4-inch cleanout above the new finished surface.
1. Is the 3-inch by 4-inch sanitary tee installed on the back of the horizontal building drain an acceptable way to make this connection? What should be used instead, and is the 1/8 inch per foot pitch on the new branch acceptable?
Read the worked answer
Fitting: not acceptable.
A sanitary tee on its back turns the flow 90 degrees in the horizontal plane. The pattern of a sanitary tee is designed for the vertical-to-horizontal direction, not horizontal-to-horizontal. In that orientation the branch enters at the same level as the flow line, producing turbulence and a low-velocity pocket where solids drop out - a chronic stoppage point.
Changes of direction in horizontal drainage piping must be made with drainage-pattern fittings: a wye, or a combination wye and 1/8 bend. For this job use a 3-inch by 4-inch wye with a 1/8 bend (or a combination wye and 1/8 bend) so the branch turns smoothly in the direction of flow.
Slope: acceptable.
- 2-1/2 inch and smaller horizontal drainage pipe: minimum 1/4 inch per foot.
- 3 inch and larger: 1/8 inch per foot is permitted.
The 6-foot branch is a 3-inch pipe, so 1/8 inch per foot is legal. Remember, though, that the branch is also the water closet's trap arm; a 3-inch trap arm may run up to 12 feet before it reaches its vent, so 6 feet is fine - provided the required vent is installed within that distance.
2. The 8-foot section that has settled and now falls 1 inch the wrong way - what is required, and how does slope apply to a 4-inch building drain?
Read the worked answer
It cannot be left as is.
A 1-inch backfall over 8 feet creates a sump in the line. Water stands there, solids settle, and the line will back up repeatedly no matter how well the fixture branches above it are installed. The building drain must be uniformly sloped from the building toward the sewer.
Code basis and correction:
- Horizontal drainage piping must be installed in uniform alignment at a uniform slope. The minimum for 4-inch pipe is 1/8 inch per foot (1 percent); 1/4 inch per foot is also permitted and is easier to hold in practice.
- Re-support or replace the settled 8-foot section so it falls continuously toward the sewer at not less than 1/8 inch per foot. Adding hangers/pipe supports at the proper spacing and re-bedding the trench or re-hanging the pipe is generally cheaper than re-pitching the whole run.
- Verify by test (water test or ball test) after the repair that there is no remaining belly.
- A backfall also invalidates the 'slope only' assumption used to check capacity: if the pipe must be re-pitched steeper, confirm the new invert at the sewer connection still works with the depth available.
3. The installer wants to conceal the existing 4-inch cleanout above the new finished ceiling. Is that permitted? Where are cleanouts required on this job?
Read the worked answer
No - cleanouts must remain accessible.
A cleanout is useless if it cannot be reached with a cable or a jetting hose. Concealing one above a hard-finished ceiling is a violation regardless of how unlikely a stoppage seems.
Acceptable options:
- Relocate the cleanout to a wall or an accessible location.
- Provide a listed access panel/cover of adequate size directly below the cleanout, identified and removable without damaging the finish.
Cleanouts required on this project:
- At the base of each stack (the stack serving the water closet).
- At the junction of the building drain and building sewer.
- Where the drainage piping makes a change of direction of more than 135 degrees.
- At the upper end of a horizontal drainage run where required by the code (e.g., the end of the branch serving the relocated water closet), and at intervals along long horizontal runs.
Since the new branch is 6 feet long and there is a change of direction where it enters the building drain, plan for a cleanout at the upstream end of that branch even if the fitting is a wye.