Available fall short on a consented 100 mm drain
A building consent has been granted for a 12 m extension of a 100 mm uPVC foul drain serving a new bathroom group. The existing drain invert at the connection point is fixed. The new upstream invert can be raised only 150 mm because of the slab underside, and site measurement gives 180 mm of available fall over the 12 m run. The approved plan shows 1:60 grade and 100 mm pipe. The builder wants to "just lay it flat and silicone the joints". The drain will pass under a proposed 3 m wide concrete patio.
1. Calculate the grade the site allows and state whether it complies with the approved plan. Show your reasoning.
Read the worked answer
Grade = fall ÷ length = 180 mm ÷ 12 000 mm = 0.015 = 1.5%, i.e. 1:66.7.
The approved plan requires 1:60, which is 1.67% (1:60). The available 1:66.7 is flatter than 1:60, so the run as measured does not comply. The shortfall is small but real: over 12 m, 1:60 needs 200 mm of fall, and only 180 mm is available - a 20 mm deficit.
Do not treat the deficit as a tolerance. A flatter-than-permitted foul drain reduces self-cleansing velocity and increases blockage risk. The correct first step is to confirm the minimum grade in the edition of AS/NZS 3500.2 cited in the consent and then redesign or re-measure before any pipe is cut.
2. List the compliant options you would put to the designer, with the trade-offs of each.
Read the worked answer
Options to evaluate:
- Recover the missing 20 mm of fall - lower the downstream invert by 20 mm if the connection depth and existing drain permit, or raise the upstream invert by 20 mm if the slab and gully trap allow. Cheapest option if physically possible; verify cover and connection levels.
- Shorten the run - relocate the upstream fixture or introduce an inspection opening to reduce the pipe length to about 10.8 m, which gives 1:60 at 180 mm fall. Check the fixture connection and access requirements.
- Upsize to DN150 - the minimum grade for DN150 is flatter (commonly 1:100), so 1:66.7 would comply on grade. Check capacity, velocity, connection to the existing DN100, and bedding/cover implications. Upsizing is not a substitute for checking the whole system.
- Redesign the alignment - a different route that gains fall, subject to clearance from the patio and other services.
Any change to the consented grade or pipe size is a variation: obtain amended plans/approval before proceeding. Separately, building over a drain is not solved by pipe grade - the patio over the drain needs the designer's and territorial authority's approval, with appropriate protection, access and documentation.
3. What records would you expect the certifying drainlayer to hold at completion?
Read the worked answer
The file should show:
- the approved plan and any approved variation/RFI for the changed grade, alignment or pipe size;
- the as-built invert levels and grades for the run, with the calculation showing compliance;
- pipe and fitting product evidence, bedding details and inspection records before backfill;
- the test result (water or air) for the run, with the test method and acceptance criteria;
- the territorial authority inspection sign-off and any conditions;
- the record of work/producer statement and certification for the drainage work, plus maintenance or access information for the patio section.
If the grade cannot be made compliant, the correct outcome is not to certify it - escalate to the designer and territorial authority.