Master Plumber Bidding a Commercial Retrofit With Conflicting Specs
You hold a Master Plumber license and are bidding a 12-fixture commercial restroom retrofit in an older building. The architect's drawings show 4-inch cast iron below-grade drainage with a 1/8-inch-per-foot slope on a 60-foot run to the building drain. The mechanical spec calls for a 3-inch PVC branch with a 1/4-inch-per-foot slope, and the two documents conflict at the connection point. The building has an existing 4-inch building drain at the basement slab. The owner wants to reuse the existing vent stack, which is 2-inch and serves three water closets. Your estimator asks whether you can value-engineer the drainage to 3-inch throughout to save money and still pass inspection.
1. How should you resolve the conflicting slope and pipe-size direction before pricing the job?
Read the worked answer
Do not silently pick one. The conflict is a design issue that must be resolved in writing (RFI) before bid, because drainage sizing and slope are governed by code minimums and the adopted plumbing code, not by whichever is cheaper.
Reasoning:
- A 60-foot run at 1/8 inch per foot drops 7.5 inches; at 1/4 inch per foot it drops 15 inches. That difference affects whether the branch can connect to the existing 4-inch building drain at the slab without re-cutting or adding a lift.
- Slope minimums depend on pipe size and the code edition adopted by the local authority; smaller pipe generally requires steeper minimum slope.
- The RFI should ask the architect/engineer to confirm the governing slope, pipe material at the transition, and the invert elevation at the connection.
Price the job to the resolved design, and carry an allowance for the transition fitting. Bidding the cheaper interpretation and hoping it passes shifts the risk to you as the master licensee.
2. The owner wants to reuse the 2-inch vent stack serving three water closets. What must you verify, and what is your answer?
Read the worked answer
Verify against the adopted code's vent sizing tables and the fixture-unit load, not against the owner's preference.
- Three water closets are a substantial drainage/vent load; a 2-inch stack is generally undersized for multiple water closets and must be checked against the code's stack and vent tables for the building height and fixture units.
- Reusing an existing stack is only acceptable if it is sized, installed, and in condition to meet current requirements; a retrofit often triggers upgrade obligations.
- Confirm the stack's condition (corrosion, illegal connections, proper termination above roof) and whether the new fixtures add load.
Answer to the owner: reuse only if the code tables confirm adequacy and the stack passes inspection; otherwise the bid must include a correctly sized replacement. Put this in writing so the decision is documented.
3. What should your written bid include to protect your master license on this job?
Read the worked answer
Include:
- Explicit code edition and local amendments the work will comply with.
- Clarification that conflicting documents will be resolved by RFI, with the governing slope/size stated once resolved.
- Exclusions/allowances for the transition at the existing building drain and any slab work.
- A statement that existing vent reuse is contingent on code compliance and inspection.
- Permit responsibility (who pulls it) and inspection milestones.
- A change-order clause for field conditions (e.g., deteriorated cast iron, unknown invert elevation).
This matters because as the master licensee you are accountable for code compliance; a vague bid that adopts an unverified owner preference creates both financial and licensing exposure.