NZ Certifying Drainlayer common mistakes
NZ Certifying Drainlayer · Understand the traps and how to avoid them.
Answers questions about who may legally carry out drainlaying by citing the Building Act 2004 and describing a single licence type, without distinguishing a registered drainlayer from a certifying drainlayer or naming the Plumbers, Gasfitters, and Drainlayers Act 2006 as the source of licensing and supervision requirements.
Candidates revise from generic building-code summaries and council consent checklists where 'licensed' appears as a single undefined term, so the two statutory frameworks get blended into one.
What to do instead
Build a two-column summary: (a) Plumbers, Gasfitters, and Drainlayers Act 2006 - licensing classes, what each class may do, supervision and suspension/discipline; (b) Building Act 2004 - consenting, inspections, compliance certificates, certifying roles. Practise answering 'which Act and which section' before each answer.
States that a drain must fall at a single remembered gradient (typically 1:60) no matter what the pipe diameter, drain type or receiving system is, and quotes that figure in every design calculation.
One number gets drilled in during study and is easy to recall under time pressure, whereas the actual figure is selected from a table that varies with pipe size, and candidates never practise looking it up.
What to do instead
Memorise the gradient table as a small grid indexed by pipe diameter and application, not as a single figure, and get into the habit of writing the source (Acceptable Solution clause or standard table number) beside every gradient you quote in an answer.
Calculates the fall by multiplying the gradient by the length of pipe barrel laid (the sloped distance) rather than the horizontal distance, and separately mixes up ratio gradients with percentage gradients when converting.
Trench lengths are talked about on site as 'metres of pipe', so the sloped length feels like the natural input, and the ratio/percentage conversion is treated as trivial and never rehearsed.
What to do instead
Do a set of worked examples in a level book: reduced levels at each node, horizontal chainage, fall = chainage x gradient, then check cover at every node. Convert ratio to percentage and back in the same exercise until both directions are automatic.
Sizes the trench and specifies the bedding using only the pipe outside diameter, omitting the working space either side of the pipe, the surround/overlay depth, and the extra excavation required at joints, so the designed levels and pipe support do not match the drawing.
Answers are written from the pipe catalogue rather than from a trench detail, and the working-space allowance is invisible unless you have actually set out a trench.
What to do instead
Learn one standard trench cross-section and reproduce it from memory with every dimension labelled - width as a function of pipe OD plus side allowance, bedding depth, surround height, overlay, select fill. Then apply it to two or three worked depths and soil types.
Chooses the jointing method from habit or from what was on the last job - solvent cement where an elastomeric ring joint is required, or a push-fit ring on a pipe that must be welded/electrofusion jointed - and describes the joint without mentioning the lubricant, insertion depth mark or the restraint needed at changes of direction.
Jointing is learned by repetition on one or two materials, and the exam asks about materials the candidate rarely handles, so the familiar method gets applied by default.
What to do instead
Make a material-to-joint matrix (uPVC, PE, concrete, cast iron, earthenware) with columns for joint type, preparation, lubricant/adhesive, insertion depth, restraint and testing implication. Recite it until each cell can be justified by the reason, not just the name.
Describes backfilling a flexible pipe trench as a single lift, or with a heavy plate compactor working directly over the pipe, and omits the granular surround being brought up evenly on both sides before compaction.
Backfill is treated as an earthworks afterthought rather than part of the pipe's structural support, so the sequence that stops deflection is left out of the answer entirely.
What to do instead
Write the backfill sequence as numbered steps with the compacted layer thickness and the plant restriction for each zone (haunching, side support, surround, overlay, backfill), and state the deflection limit that the sequence is protecting against.
Gives one remembered test figure - one head, one duration, one pass criterion - for every situation, and does not say whether the test is a water test or an air test, where the head is measured from, or what make-up water is acceptable.
The test section feels like a single fact to memorise, and candidates never distinguish drain testing from stack/plumbing testing or from the pre-cover inspection.
What to do instead
Learn testing as a procedure with named variables: test medium, head and where it is measured from, stabilisation period, duration, permissible loss/make-up, and who witnesses it. Practise writing the procedure out for a 20 m drain run and for a two-storey stack separately.
Sizes an on-site wastewater system from the number of bedrooms alone, skipping the soil category/permeability assessment, the depth to the water table or restrictive layer, and the effluent application area calculation, and treats septic tank and secondary treatment options as interchangeable.
Bedroom count is the only input the candidate can recall with certainty, so the rest of the sizing chain is replaced with a plausible-sounding general statement.
What to do instead
Build a sizing flow: design flow from occupancy, then treatment level required, then soil/land application area from the permeability class and site constraints, then the disposal method. Learn it as a chain where each link must be justified by a site test result, and practise with two contrasting soil categories.
Produces as-built drawings and the completion documentation by copying the design invert levels and pipe sizes rather than the levels actually recorded on site, and does not reconcile them against the test results and inspection records before handover.
The design set is clean and available, while site records are scribbled in a notebook, so the design set gets submitted as if it were as-built.
What to do instead
Adopt the discipline of recording reduced levels at each node as the line is laid and transferring those numbers, not the design numbers, into the as-built. Before answering a documentation question, list the full handover pack - as-builts, test records, inspection sign-offs, certification - and check each item against what was actually done.
Answers a trench safety question with 'shore or batter the sides' only, omitting the depth that triggers the controls, the need to locate existing services before digging, the means of escape for a person in the trench, and the confined-space/gas-testing controls when entering or connecting to a live manhole.
Shoring is the one control that comes to mind for 'trench', so the answer stops there; the other hazards are not attached to the word 'trench' in the candidate's memory.
What to do instead
Learn a per-task hazard template: identify the task, then services, then collapse/engress, then atmosphere, then plant and traffic, then emergency rescue. Run the template over three tasks - deep trench, live manhole connection, night work in a road reserve - and write each control against its trigger.