Drainage and stormwater defects in Australian residential builds

What it is

Stormwater drainage on a residential build is the network that collects rainwater from roofs, paving, driveways and lawn areas and carries it to a legal point of discharge. In Australia the work has to comply with AS/NZS 3500.3, Plumbing and drainage Part 3: Stormwater drainage, alongside the National Construction Code Volume Three. When that work falls short, the defects show up fast. Water pools next to the slab, sub-floor areas stay damp, paving heaves and downpipes overflow during normal rain events.

Legal point of discharge

Every site needs a defined legal point of discharge. The local council sets this. In most metropolitan areas it is either a connection to a council stormwater pit, a kerb and channel outlet, or in some rural and outer-urban areas a legal absorption trench or rubble pit. The point has to be approved by the council before the slab is poured because it dictates the site falls and the pipe alignment from day one.

A common defect is a build where the stormwater discharges into the sanitary sewer. That is prohibited under AS/NZS 3500.3. Stormwater and sanitary drainage must be fully separated systems. The other variation is a discharge that runs across a neighbour's land without an easement, which shows up later as a civil dispute about nuisance and trespass.

Pipe sizing and falls

Pipe sizes are calculated from the catchment area, the rainfall intensity for the suburb and the average recurrence interval set by the local council. Undersized pipes are the single most common drainage defect on a residential build. The pipe looks fine on a normal day then overflows in a five-year storm event. AS/NZS 3500.3 sets minimum gradients for gravity stormwater pipes. A pipe laid flat or back-graded is a block-level defect.

Inspection openings and pits

AS/NZS 3500.3 requires inspection openings at every change of direction sharper than 45 degrees, at every junction and at the head of every line. The opening has to be accessible from the surface for jet cleaning. Builders sometimes bury inspection openings under paving or topsoil, which removes the maintenance access the standard requires.

Stormwater pits on the property need a removable grate, a sediment sump and an outlet that sits above the floor of the pit so leaves and grit settle out instead of washing into the council main.

Surface drainage and ponding

The finished site has to shed water away from the building. The National Construction Code requires a fall of at least 50 mm over the first one metre away from external walls. Around 80 to 100 mm is better practice in clay soil suburbs. Where lawn or garden beds back-fall toward the slab, the result is water ponding against the footing, lifting the soil moisture profile under the slab and causing differential movement. That is the path to slab heave and cracking in plasterboard internally.

Sub-soil and agricultural drains

On reactive clay sites a sub-soil drain (an ag line) is often installed against the perimeter of the slab to relieve water pressure on the footing. The defect to watch for is an ag line that discharges into the same stormwater system without a proper junction, or worse, terminates blind in the ground.

How defects get raised

Most stormwater defects are picked up on the practical completion inspection by the principal certifying authority, the homeowner's independent inspector or the homeowner themselves. The standard process is a written notice of defects served on the builder. Under most Australian residential building contracts the builder has the defects liability period (commonly 13 weeks but contract-specific) to rectify. In New South Wales the statutory warranties under section 18B of the Home Building Act 1989 continue past that period and cover major defects for six years. Victoria has equivalent warranties under section 8 of the Domestic Building Contracts Act 1995. Queensland operates under the Queensland Building and Construction Commission Act 1991 and the Queensland Home Warranty Scheme.

Hidden drainage defects usually only surface during the first wet season. Document them in writing as soon as they show up. Photos with a date stamp, a rainfall record for that day and a written description of where the water went are enough to start a claim.

Common rectification scope

A typical rectification on a stormwater defect involves opening up the affected line, replacing the undersized or back-graded section, adding the missing inspection opening, and reinstating paving or landscaping. Where the discharge point itself is wrong, the work runs all the way back to the council connection and the cost climbs quickly. Catching the defect at rough-in is far cheaper than catching it after final.