Conflicting Slope and Vent Size on a Dormitory Drainage Drawing
During a four-storey dormitory fit-out, the drainage crew is about to install a 14 m horizontal 100 mm diameter soil branch that serves six water closets and connects to a 100 mm soil stack. Plan sheet P-201 shows the branch at a 2.0% slope. Section detail P-301 shows the same branch at 1.0% and calls up a 50 mm vent, while a plan note says provide a 75 mm vent through roof. The stack invert at the connection is +2.850 m. The ceiling slab soffit is +3.300 m, and structural drawing S-05 shows a 300 mm deep beam crossing the corridor 12 m from the stack, with its bottom at +3.150 m. The pipe outside diameter is 110 mm. The foreman says the detail is just a typical and wants to build at 2.0% to be safe. The engineer is off-site until the next day.
1. Calculate the upstream invert and pipe crown at the beam for both 2.0% and 1.0%, and determine whether either slope clears the beam.
Read the worked answer
At the stack, invert = +2.850 m. Total fall = slope x 14 m.
- At 2.0%: fall = 0.020 x 14 = 0.280 m. Upstream invert = 2.850 + 0.280 = +3.130 m. Crown = 3.130 + 0.110 = +3.240 m.
- At 1.0%: fall = 0.140 m. Upstream invert = 2.990 m. Crown = +3.100 m.
The beam is 12 m from the stack, so only 2 m of fall remains upstream of it.
- 2.0%: invert at beam = 2.850 + (0.020 x 12) = +3.090 m; crown = +3.200 m. Beam soffit is +3.150 m, so the crown is 50 mm above the beam: clash.
- 1.0%: invert at beam = 2.850 + (0.010 x 12) = +2.970 m; crown = +3.080 m. This is 70 mm below the beam: clear.
So the 2.0% line cannot pass the beam without a structural sleeve or rerouting. The 1.0% line fits geometrically, but that does not prove it is acceptable hydraulically or code-compliant.
2. How would you resolve the conflicting vent size and slope information before installing? Include what you would do in the next 24 hours.
Read the worked answer
Do not let the crew choose a slope from a drawing conflict. The 2.0% plan note and the 1.0% section, plus the 50/75 mm vent discrepancy, are design conflicts.
Next 24 hours:
- Stop work on the affected branch; install only portions that are unaffected and can be confirmed.
- Mark up the drawings with actual dimensions, beam soffit, stack invert and pipe outside diameter; take photos and a sketch.
- Raise a formal request for information to the engineer: which slope governs, what vent size is required, and whether a sleeve or reroute is needed.
- Ask for the fixture-unit calculation and the code basis for the vent and slope.
- Record any verbal advice and have it confirmed in writing before proceeding.
A safer 2.0% slope is not safer if it hits the beam. A 1.0% slope is not automatically acceptable just because it fits. The engineer and the authority having jurisdiction must resolve it.
3. If the engineer later approves a 1.0% slope, what else must be checked before the branch is covered?
Read the worked answer
If 1.0% is approved, check: fixture units and whether the code permits 1% for that diameter; self-cleansing velocity at full and partial flow; vent size and connection so the branch is not wet-vented improperly; trap arm lengths and trap seal protection; support spacing so the pipe cannot sag below 1%; and the actual invert at each connection. Also confirm that the 75 mm vent note, not the 50 mm detail, governs, and that the vent can be routed through the roof without a clash.
Before closing the ceiling, perform a water test or smoke test on the branch and record the results. If any of these cannot be confirmed, do not install; the geometric fit alone is not approval.