Beam clash and unvented branch on a six-storey apartment
A six-storey apartment project in Rajagiriya is at second-fix stage. The approved plumbing plan shows a 110 mm uPVC soil stack serving third-floor bathrooms rising inside a 150 mm blockwork wall. At second-floor ceiling level a 300 mm × 450 mm reinforced concrete beam crosses the same wall. The drawing shows no sleeve, offset or structural opening. The site supervisor instructs the plumber to 'core a 150 mm hole through the beam - it is only a few bars'. The same drawing shows a 4.5 m horizontal 110 mm WC branch falling at 1:80 to the stack, with no vent pipe or air admittance valve. The building is 18 m high and has 18 bathrooms on the stack. The structural engineer is on site only on Fridays; today is Tuesday. The client wants first-fix inspection on Thursday.
1. What checks and calculations should be made before accepting the soil stack route and the 1:80 branch?
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First, do not treat the plumbing drawing as approval to cut structure. Compare the plumbing plan with the structural drawings and beam reinforcement; a 300×450 beam at second-floor ceiling cannot be cored without the structural engineer's design, because a 150 mm hole may cut main bars or shear reinforcement. Check whether the wall is loadbearing and whether an offset can be made with two 45° bends above or below the beam, or whether the stack can be boxed out.
For the branch, calculate the fall: 4.5 m at 1:80 gives 56 mm. If the project specification or the relevant standard requires a steeper minimum for a 110 mm WC branch - commonly 1:60, which would need 75 mm - the drawing is 19 mm short and will pond. Check the branch length against the maximum unvented length, the trap seal, and whether a vent or air admittance valve is required. With 18 bathrooms on an 18 m stack, check stack capacity and venting. Record all checks and raise them before installation, not at inspection.
2. Draft the immediate action and the technical query you would raise, including what you would not do.
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Immediate action: stop first-fix work at the beam and branch; keep the area clear; do not core, cut or drill the beam. Mark the drawing with the clash and take dimensions and photographs. Inform the site supervisor in writing that a structural alteration cannot be made on a verbal instruction.
Raise an RFI to the consultant and structural engineer stating: drawing number and revision, grid location, beam size, proposed 110 mm stack route, the requested 150 mm core, and the branch calculation (4.5 m at 1:80 = 56 mm fall; required minimum and shortfall). Ask for either an approved structural opening with reinforcement details, or an alternative route, and for the venting requirement or AAV approval. State that work is suspended in that area until a written response. Copy the main contractor and client's representative. Record the delay and any labour and plant stood down. Do not proceed on a 'few bars' assurance, and do not install the branch at 1:80 unless the designer confirms it in writing.