Plumbing defects in new homes — the most common issues and how to spot them

Plumbing defects are among the most expensive problems in a new home — not because they’re always complex to fix, but because water is uniquely destructive. A small leak behind a wall, a drain with inadequate fall, or a failed shower seal can cause thousands of dollars of damage to structure, insulation, and finishes before anyone knows something is wrong.

The challenge is that many plumbing defects only become visible during or after occupancy. A drain with poor fall might work fine with low usage but back up within weeks of a family moving in. A pinhole leak behind a vanity might take months to show as a water stain. Knowing what to look for — and checking methodically at handover — gives you the best chance of catching these issues while they’re still your builder’s problem.

Why plumbing defects matter beyond the inconvenience

A dripping tap is annoying. A slow drain is frustrating. But plumbing defects that go unaddressed can cause:

• Mould growth behind walls and under floors — which has health implications and is expensive to remediate
• Structural damage to timber framing, plywood substrates, and engineered flooring from sustained moisture
• Waterproofing membrane failure if leaks bypass the membrane and saturate the substrate
• Corrosion of nearby metalwork and fixings
• Insurance complications — some policies exclude damage from slow leaks if there’s evidence the issue was pre-existing

Early detection and prompt written notice to your builder is the only way to keep this from becoming your financial problem.

The most common plumbing defects in new builds

1. Inadequate fall on drainage pipes

Every horizontal drain run must have a minimum fall (slope) to allow waste to flow freely — typically 1:40 to 1:60 for most household drains. If the fall is insufficient or the pipe runs level in sections, you’ll notice slow drainage from sinks, basins, or showers, or intermittent blockages that recur despite clearing.

Diagnosing this definitively requires a plumber with a drain camera or level, but you can identify it as a problem simply by noticing that multiple drains in the same area are consistently slow without any blockage.

2. Leaking joints and connections

Pipe joints in new construction can fail due to inadequate jointing compound, poor compression fitting technique, or thermal movement that wasn’t accounted for. Leaking joints in accessible areas (under sinks, at toilet connections) are usually noticed quickly. Leaks in wall cavities or under slabs can take months to surface.

Check under every vanity and sink at handover. Look for staining on the cabinet floor, swelling of the MDF cabinet base, or moisture on the wall behind the pipes. These are signs of a slow leak that has been dripping for some time.

3. Water hammer

Water hammer is the loud banging or thudding noise that pipes make when water flow is suddenly stopped — typically when a tap is turned off sharply or when a washing machine valve closes. In severe cases, it can actually loosen pipe fittings over time.

It’s caused by a lack of air chambers or pressure-limiting devices in the plumbing system, pipes that aren’t adequately secured to framing, or water pressure that’s too high. Test for it by turning taps off quickly. If you hear a significant bang in the walls, flag it in your defect notice.

4. Inadequate hot water pressure

Low hot water pressure at fixtures — especially showers — is a common complaint in new builds. It’s caused by undersized hot water supply pipes, a water heater with insufficient capacity for the home’s size, or an incorrect pressure-limiting valve setting.

Test every shower and bath at the practical completion inspection. A barely-functional dribble from a shower in a new home should not be accepted as normal.

5. Incorrectly positioned or unsupported fixtures

Taps, showerheads, and hand-held shower brackets that are installed at the wrong height, not adequately supported within the wall, or not in the locations specified in the contract are a common defect in wet areas.

Once tiles are laid, repositioning a fixture requires cutting out and retiling — significantly more expensive than adjusting it during construction. If something isn’t where it should be, flag it immediately.

6. Missing or faulty isolation valves

Every toilet, basin, and sink should have its own isolation valve — a small tap that lets you shut off the water to that fixture without turning off the mains. In new builds, these are sometimes missed, incorrectly installed, or installed but seized (won’t turn).

Check every isolation valve at the practical completion inspection. Turn them partially to confirm they move. A valve that won’t operate provides no protection if a fixture fails after you’ve moved in.

7. Shower recess waterproofing failures

This sits at the intersection of plumbing and waterproofing. The shower base membrane, wall tile membrane, and the transitions between them are common failure points. A leaking shower causes damage to the wall framing and floor structure below and is one of the most frequently litigated defects in new home construction.

Water ingress from a shower won’t usually be visible at a single inspection. What you can check is whether the grout and silicon at the junctions (floor-to-wall, wall-to-screen) are continuous and properly applied, whether the screen seals against the tiles effectively, and whether the shower base has an appropriate fall to the drain.

8. Toilet cistern running continuously

A running toilet cistern is easily identified by a slight sound of water from the cistern even when not recently flushed, or by a very slight movement in the water in the bowl. It’s typically caused by an incorrectly adjusted or faulty flush valve. It’s a simple fix but wastes significant water if left unaddressed.

How to check for plumbing defects at handover

Work through the property systematically at your practical completion inspection:

1. Turn on every tap — hot and cold. Check pressure, check for drips
2. Test every shower — hot water available, acceptable pressure, drain clearing at adequate speed
3. Flush every toilet — fully flushes, cistern stops running within 60 seconds
4. Check under every vanity — no moisture, no staining, no swelling cabinet floors
5. Open the hot water system access — note the make, model, and capacity; check for any active drips from connections
6. Run the dishwasher — confirm water supply and drainage working
7. Check the laundry taps — hot and cold operational, drainage clear
8. Look at all exposed pipe runs — no unsupported sections, no contact with other materials that might cause corrosion

Photograph anything that looks out of place and include it in your defect notice.

How to give notice of a plumbing defect

Plumbing defects should be notified in writing with:

• Location of the issue (e.g., “en suite bathroom, shower drain”)
• Description of what you observed (e.g., “shower drain takes approximately 90 seconds to clear standing water after 30 seconds of normal flow”)
• When you first noticed it
• Any photographs showing the issue

Avoid vague descriptions like “low water pressure” or “slow drain” without context. The more specific your notice, the easier it is for your builder to dispatch the right trade and the harder it is for them to dispute the issue.

Key takeaways

• Plumbing defects can be invisible until they cause significant water damage — checking methodically at handover is the only way to catch them early.
• Inadequate drain fall, leaking joints, water hammer, and waterproofing failures are the most common and most damaging plumbing defects in new builds.
• Check under every vanity, test every shower and tap, and flush every toilet at your practical completion inspection.
• Isolation valves are easy to miss but important — confirm every fixture has one and that it operates.
• Give written defect notices with specific descriptions and photographs — vague reports are easier for builders to deprioritise.
• A running toilet, minor drip, or slow drain that’s your builder’s problem at handover becomes your problem and your expense the moment you sign off without documenting it.