NZ Certifying Drainlayer cheat sheet

Drainage work for a building is building work under the Building Act 2004 and needs a building consent unless it falls within an exemption in Schedule 1.

Relevant Building Code clauses: G13 Foul Water, G14 Industrial Liquid Waste, E1 Surface Water, and G12 for backflow prevention on water supplies.

Core technical standards: AS/NZS 3500.2 (sanitary plumbing and drainage), AS/NZS 3500.3 (stormwater drainage), AS/NZS 1547 (on-site domestic wastewater).

Drainlayers must be registered and licensed under the Plumbers, Gasfitters and Drainlayers Act 2006; licensing is by class, and work must stay within that class.

A certifying drainlayer is authorised to certify specified drainage work and issue the associated certification instead of the work being inspected by the building consent authority.

Discharge to a public sewer is controlled by the territorial authority's trade waste bylaw; a written trade waste consent is required where the bylaw applies.

Connection to, or work near, council-owned public drains requires council approval; the public drain is not the drainlayer's asset to alter.

The Health and Safety at Work Act 2015 applies to every drainage site, including subcontractors, and duties fall on all PCBUs.

AS/NZS 3500.2 is the reference standard cited in compliance documents for gradients, venting, support and testing of sanitary drainage.

Grade as a percentage = (fall / length) x 100; a 1:60 ratio equals 1.67% and a 1:100 ratio equals 1.0%.

Typical NZ minimum self-cleansing grades: DN100 at 1:60 and DN150 at 1:100; flatter grades require justification and often more cleaning access.

Self-cleansing velocity is about 0.6-0.75 m/s at peak dry weather flow; velocities above roughly 3 m/s risk scour and pipe abrasion.

Capacity uses Q = V x A; Manning's roughness n is about 0.011 for uPVC and 0.013 for concrete, with Colebrook-White used for full-bore checks.

Stack, drain and vent sizes are selected from discharge unit (DU) tables in AS/NZS 3500.2, not by counting fixtures alone.

Trap seal depth is a minimum 50 mm and must be protected by venting so siphonage and back-pressure do not break the seal.

Unvented branch and drain lengths are limited by the tables in AS/NZS 3500.2; exceeding them requires additional venting or relief.

The hydraulic grade line must stay below the overflow level of the lowest fixture served; surcharge of an upstream gully trap is a design failure.

Foul water and stormwater are designed as separate systems wherever practicable, and E1 requires secondary flow paths for stormwater.

Levels are referenced to NZVD2016 or an approved local datum; never mix datums on one job, and confirm invert levels against the drawings before trenching.

Soils are classified to AS 1726 / NZGS guidelines; expansive clays, peat, uncontrolled fill and rock each change bedding, pipe class and cover requirements.

Groundwater standing above the invert requires dewatering, and empty pipes or tanks must be checked for flotation.

Trench width is commonly pipe outside diameter plus 300 mm with a 600 mm practical minimum, giving room for haunching and compaction.

Typical minimum cover is 300 mm under gardens and landscaping, increased (commonly 600 mm or more) under driveways and vehicle loading.

Excavations deeper than 1.5 m must be shored, benched or battered, and inspected daily by a competent person.

Trench bottom is trimmed true to grade, free of loose spoil and soft spots; over-excavation is backfilled with approved granular material, not spoil.

Percolation (falling head) testing of the disposal area is required before sizing on-site effluent application beds.

Spoil, contaminated soil and asbestos-containing material are classified and disposed of to an approved facility with records kept.

uPVC sewer pipe to AS/NZS 1260 is supplied in stiffness classes SN4, SN8 and SN16; class is selected for depth, soil and traffic loading.

Polyethylene pipe to AS/NZS 4130 (PE80 or PE100) is pressure rated in PN classes and joined by butt fusion or electrofusion.

Concrete pipe to AS/NZS 4058 uses rubber ring roll-on joints and pipe classes selected for crush load and cover.

Solvent cement jointing of uPVC: cut square, chamfer, clean, apply primer and cement, push fully home with a quarter turn, and hold until set.

Rubber ring joints are lubricated only with the manufacturer's approved lubricant; grease, oil and detergent damage the ring.

Bedding and embedment follow AS/NZS 3500.2 and AS 3725: a granular bed plus haunch and side support, not just support under the barrel.

Flexible pipes carry load through soil-structure interaction, so the quality of the side support and compaction matters more than the pipe alone.

Fittings and adaptors must match the pipe material and stiffness or pressure class, or be an approved transition fitting.

Above-ground pipework needs adequate supports, gradient and provision for thermal expansion, with materials suited to the location.

Pipes are laid with sockets facing upstream so spigots enter downstream, and laying usually proceeds from the outlet upwards where access allows.

Invert levels and line are checked against laser or dumpy level readings at each structure and at maximum intervals along the run.

Inspection points are required at the foot of stacks, at changes of direction greater than 45 degrees, on long runs and near the boundary.

Horizontal drains use swept junctions and bends; sharp 90-degree bends are not acceptable in horizontal drainage.

Backfill is placed in even layers and compacted symmetrically on both sides of the pipe to control deflection and prevent lateral movement.

Thrust blocks or restrained joints are required at bends and tees on pressure mains before testing and commissioning.

Vent pipes terminate above roof level and away from openings to AS/NZS 3500.2 so foul air does not enter the building.

Drains maintain specified horizontal and vertical separation from water mains, power and gas services, with crossings shown on the drawings.

Rainwater downpipes discharge through a gully trap or charged connection to the stormwater system as detailed on the approved plans.

Typical council inspection holds are pre-cover (before backfill), pre-line (before lining), and final, plus any conditions listed in the building consent.

A water test plugs the low end and fills the drain to the specified head, commonly at least 1.5 m above the highest point of the section under test, held for the prescribed period.

An air test applies low-pressure air to the sealed section and passes only if the pressure drop over the specified time is within the allowable limit.

Flexible pipe is checked after backfill with a mandrel or deflectometer to confirm deflection has not exceeded the permitted percentage.

CCTV inspection documents alignment, joint gaps, debris, root entry and infiltration; the record is retained and supplied with the completion documents.

Test certificates and records of work are signed by the licensed or certifying drainlayer and form part of the documentation for code compliance.

Before commissioning, lines are flushed and cleaned, every trap is confirmed to hold its seal, and venting is confirmed continuous to atmosphere.

Pumped systems are commissioned with float settings, alarms and duty/standby changeover tested and recorded.

Trade waste consents set discharge limits for pH (commonly about 6-10), temperature, fats/oils/grease, suspended solids and heavy metals.

Grease interceptors are sized from fixture capacity and retention time, and require a documented maintenance and pump-out regime.

AS/NZS 1547 covers on-site domestic wastewater management, including treatment system selection, application area sizing and setbacks.

Effluent application areas are sized from soil category and hydraulic loading rate, with setbacks from bores, boundaries, waterways and buildings.

Septic tanks are two-compartment, retain sludge and scum, and need periodic desludging and inspection of baffles and outlets.

Secondary and aerated treatment systems need power, a servicing contract and council approval as a condition of consent.

Chemical or high-strength trade waste may need pre-treatment such as neutralising, settling or bunded storage before discharge.

Nothing from a trade waste, washdown or vehicle area may discharge to stormwater; only to sewer under consent or to an approved collection system.

Special systems include silt traps, petrol/oil interceptors, cooling tower bleed disposal and backflow prevention on the incoming supply.

The plan view gives layout and offsets; the longitudinal section gives chainages, invert levels and grades between structures.

Depth of burial = surface level minus invert level, both taken from the same datum.

Grade is drawn as a ratio (1:60) or a percentage; convert with % = 100 / ratio denominator.

At 1:100 scale, 1 mm on the drawing equals 100 mm on the ground; always confirm the stated scale and do not scale off reduced copies.

The legend, north point, drawing number, revision and approval stamp identify the current consented version; superseded revisions must not be built from.

Completion documentation commonly includes as-built plans, test certificates, records of work, producer statements and maintenance information.

Any departure from the consented plans is recorded as a variation and referred back to the building consent authority where compliance is affected.

Consent conditions list the inspections and documents required before a code compliance certificate can be issued.

Under the Health and Safety at Work Act 2015 a PCBU must eliminate risks so far as reasonably practicable, and must consult with workers on site hazards.

Excavations of 1.5 m or deeper require shoring, benching or battering, with a competent person's inspection before each shift and after rain.

Before excavation, service plans are obtained and interpreted, a locator is used, and hand digging is used near marked services.

Confined space entry requires a permit, atmospheric testing, a standby person outside, continuous monitoring and a rescue plan.

Notifiable events, including serious harm and dangerous incidents, are reported to WorkSafe New Zealand as soon as practicable.

Common contract forms are NZS 3910 for construction works and NZS 3915 for smaller building and drainage contracts.

A licensed drainlayer must supervise trainees and take responsibility for the work meeting standards and for the record of work issued.

Site records kept include the daily diary, delivery dockets, inspection sign-offs, test results and any approved variations.

Waste and spoil are managed and disposed of to approved facilities, with documentation retained for contaminated or hazardous material.