Existing Building Drain Slope and a New Water Closet
A tenant fit-out adds a new water closet and lavatory in a former office suite. The nearest connection is an existing 100 mm (4 in.) cast-iron building drain running under a concrete slab. A laser level shows the drain falls about 1.0 m over a 40 m run to the building sewer - roughly 2.5 percent. The contractor proposes connecting the new 100 mm branch with a wye and running 6 m of branch at 1 percent to reach the stack. A cleanout exists at the base of the stack but there is none at the new branch. The slab is post-tensioned, and cutting is tightly restricted.
1. Assess the proposed branch slope and cleanout arrangement.
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Branch slope. A 100 mm (4 in.) drain branch must be laid at the code-prescribed slope - commonly 1/4 unit vertical per unit horizontal (about 2 percent) for 100 mm pipe, with 1/8 unit (about 1 percent) permitted only for larger sizes under stated conditions. The proposed 1 percent slope on a 100 mm branch is below the ordinary minimum and is not acceptable for a 100 mm line. The fix is to raise the branch or shorten the run so the required slope is achieved; a 6 m run at 2 percent needs 120 mm of fall, which may be feasible if the fixture can be set slightly higher or the connection point moved.
Existing drain. The 2.5 percent slope on the existing 100 mm building drain is steeper than minimum, which is fine - steeper slopes are permitted for drainage (the concern with excessive slope is solids separation, but 2.5 percent is not excessive). No correction needed there.
Cleanout. A cleanout is required at the upper end of a horizontal drain branch and at changes of direction, and the new branch has none. The existing cleanout at the base of the stack does not serve the new branch. A cleanout must be added at the upstream end of the new branch, accessible without breaking the slab (e.g., in the wall or a accessible floor plate).
2. Given the post-tensioned slab restriction, propose a practical compliant solution.
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Because slab cutting is restricted, the solution must work within the existing geometry:
- Raise the fixture connection. Set the water closet on a raised floor or platform, or use a rear-discharge (back-outlet) water closet with a carrier, so the branch can be run at the required 2 percent within the available depth. A back-outlet carrier often solves exactly this problem in slab-on-grade and post-tensioned retrofits.
- Shorten the branch. Move the stack connection closer - for example, connect into a nearby vertical stack rather than running 6 m - so the required fall is achievable without lowering the connection.
- Use an accessible cleanout. Install the cleanout at the upstream end of the branch in an accessible wall cavity or a removable access panel, avoiding slab penetration. If a slab opening is unavoidable, locate it away from tensioning tendons and coordinate with the structural engineer.
- Verify venting. The new branch must be vented - a wet vent or individual vent as permitted - and the vent must not be compromised by the raised floor arrangement.
Document the slope achieved (measure and record), the cleanout location and accessibility, and structural clearance. This satisfies the slope rule, the cleanout rule, and the no-cut constraint simultaneously.