Conflicting Vent Size and an Invert That Leaves Almost No Fall
A two-storey residence is being plumbed from a set of drawings. Sheet P-2, the sanitary plan, shows the 100 mm PVC soil stack continuing through the roof as a 50 mm vent. Sheet P-3, the sanitary isometric, shows the same stack with a 75 mm vent, and the general notes state 'all vents 50 mm unless otherwise noted'. Separately, the site development plan gives the invert of the public sewer connection as 1.05 m below natural ground line (NGL). The ground-floor plan annotates the building drain outlet invert as 1.35 m below finished floor level, and finished floor is 0.45 m above NGL. The sewer connection is 18 m from the building drain outlet. The excavation subcontractor is ready to dig, and the client has verbally asked for an extra floor drain on the covered lanai that appears on no sheet.
1. How do you resolve the 50 mm versus 75 mm vent conflict, and what do you do while it is unresolved?
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Do not pick the one that is easier or the one drawn last. Both are project documents, and the general note is a default that a specific annotation can override - but which specific sheet governs is the designer's call, not the installer's.
- Raise an RFI citing P-2, P-3 and the general note, with the exact stack, the fixture units it serves and the roof penetration size, and ask for the governing size.
- Check the code minimum for the vent stack against the fixture units and the 100 mm drain; a 75 mm vent is not automatically wrong, but the answer must come back in writing.
- Record it. Mark the response on the drawings, keep the RFI in the project file, and reflect the final size in the as-built.
- Do not cut the roof penetration for either size until the answer arrives; the flashing and the sleeve size depend on it.
While the RFI is open, the crew can proceed with the stack below the highest fixture connection if that work is unaffected, but must not install the vent terminal or the roof flashing.
2. Using the levels given, what fall is actually available to the sewer, and what do you recommend?
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Convert everything to one datum. Finished floor is NGL +0.45 m, so:
- Building drain outlet = 1.35 m below FFL = 1.35 − 0.45 = 0.90 m below NGL.
- Public sewer invert = 1.05 m below NGL.
- Available fall = 1.05 − 0.90 = 0.15 m over 18 m, which is about 1:120, roughly 0.8%.
That is flatter than the usual minimum for a 100 mm building drain (in the order of 1%, or 1:100, depending on the governing code), and it is measured to the connection, leaving nothing for construction tolerance. A drain at that grade will hold solids and silt up. Before digging, re-survey the two levels with a level and staff rather than trusting the annotations, and confirm both are on the same datum. If the survey agrees, raise an RFI: options include relocating or lowering the connection, revising the building drain route, or a written variance - none of which the crew can decide on site. Do not 'split the difference' by laying a flat pipe.
3. How should the client's verbal request for an extra lanai floor drain be handled?
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Treat it as a variation, not a favour. Confirm the location and the discharge route in writing, then check the engineering before pricing it:
- A 50 mm floor drain branch needs a trap and a slope of about 1:50 (2%), so the lanai finished level must sit high enough above the connection point; measure it, do not assume.
- The branch needs a vent, or a connection to an existing vented branch within the allowed trap-arm length, and the trap must be accessible for cleaning.
- Check that the extra fixture load does not overload the branch it joins.
Issue a sketch or have the designer revise the drawing, get written approval (and a variation order covering cost and time) before installing, and update the as-built. Installing it on a verbal instruction creates a drain with no drawing, no vent check and no basis for payment - and if it backfalls, the installer owns the problem.