Setting Out a Tiled Bathroom With a Misplaced Drain Stub
On a hotel refurbishment, a 2.40 m by 1.80 m bathroom has already been tiled: 20 mm of plaster and tile on each wall and 10 mm floor tile. The 1:50 plan shows the water closet centerline 400 mm from the right structural wall and 700 mm from the back structural wall; the lavatory centerline is 400 mm from the left structural wall and 500 mm from the back wall. A 110 mm water closet drain stub was cast in the slab with its center 450 mm from the right structural wall and 650 mm from the back structural wall. The plan does not give finished-wall dimensions. The foreman says to just center the fixtures on the tiles, but the owner has already rejected one bathroom because the water closet was visibly off-center. The tiler needs the centerlines today, and the wall tiles are already fixed, so any chasing must be minimal.
1. Compute the finished internal dimensions and the plan centerline offsets measured from the finished wall surfaces. Compare these with the existing drain stub and state the discrepancy.
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Finished internal width = 2.400 - 2(0.020) = 2.360 m. Finished depth = 1.800 - 2(0.020) = 1.760 m.
Plan offsets from finished walls:
- Water closet: 400 + 20 = 420 mm from the right finished wall; 700 + 20 = 720 mm from the back finished wall.
- Lavatory: 400 + 20 = 420 mm from the left finished wall; 500 + 20 = 520 mm from the back finished wall.
- Existing stub: 450 - 20 = 430 mm from the right finished wall; 650 - 20 = 630 mm from the back finished wall.
The stub is only 10 mm off the plan line from the right wall, which is within normal flange tolerance. It is 720 - 630 = 90 mm off in the back-wall direction. A 90 mm offset is too large to hide by centering on the tiles and is beyond common offset-flange limits. The layout must be resolved before drilling or breaking tile.
2. Describe how you would establish and verify the two centerlines on the tiled walls and floor, including how to handle the right wall that is 15 mm out of plumb over 1.5 m.
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Establish two datums: the finished floor and one finished side wall. Do not use the ceiling or an unfixed tile edge.
- Confirm the room is square by measuring diagonals and using a 3-4-5 triangle from the back/right corner. Record the actual wall-to-wall dimensions at floor and at 1 m height.
- Set a laser or plumb line 420 mm from the right finished wall for the water closet centerline. Mark it on low-tack tape at floor and at 300 mm above floor, not with a permanent marker on the tile.
- Set the back-wall line at 720 mm from the finished back wall, then check the intersection with the side line using the 3-4-5 method.
- For the lavatory, set 420 mm from the left finished wall and 520 mm from the back wall. Verify both centerlines are parallel to the walls and that the fixtures do not foul each other.
- The right wall is 15 mm out of plumb over 1.5 m. Use the average line at fixture height, and choose a bracket or carrier with slotted adjustment. Do not force the fixture to follow the wall; the fixture must be level.
- Record the final dimensions and photograph the taped lines before drilling.
3. If the water closet stub cannot be relocated, what options do you have and what information do you need before choosing one?
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Options, in order of least damage:
- Use an approved offset closet flange if the manufacturer permits at least 90 mm offset and the flange height still sits correctly on the finished floor. Confirm in writing; many standard flanges will not allow this.
- Relocate the stub by breaking out a local area of slab, rerouting the 110 mm branch with approved bends, and re-waterproofing. This needs structural and waterproofing approval.
- If the ceiling below is accessible, reroute the drain below the slab.
- Do not reduce the drain size or pack the gap with mortar.
Before choosing, obtain: the engineer's approval, the flange manufacturer's offset limit, the finished floor and flange height, the trapway clearance, and confirmation that the waterproofing can be restored. If none is available, the fixture cannot be finally set.