Testing your home's drainage before selling is a smart move — drainage issues are one of the most common problems flagged in pre-purchase building inspections, and they can delay or derail a sale. The good news is there are several straightforward checks you can carry out yourself, though some work must be left to licensed professionals.
Why Drainage Matters When Selling
Buyers and their building inspectors will scrutinise drainage carefully. Poor drainage can indicate deeper problems: foundation damage, leaky pipes, non-compliant stormwater systems, or health hazards. Under the NZ Building Code, drainage systems must comply with clauses E1 (surface water), G13 (foul water), and G12 (water supplies) — so any obvious failures are red flags for informed buyers.
Doing your own checks first gives you time to fix problems before they appear on an inspector's report.
Safety First
Important: Never enter a confined space such as a manhole or underground chamber. This is a serious safety risk and is governed by WorkSafe NZ confined-space regulations. All visual checks should be done from ground level.
1. Check Stormwater and Surface Drainage
Start outside after a decent rain, or run a garden hose for 10–15 minutes near the house:
- Walk around the perimeter of the house and watch where water flows. It should drain away from the foundation, not pool against it.
- Check downpipes are connected and discharge into a stormwater drain, soakpit, or approved outlet — not onto a neighbour's property or into the sewer.
- Look for ponding water in the garden, on paths, or near the garage. Standing water that persists for more than a few hours after rain suggests poor grading or a blocked drain.
- Inspect subfloor vents (if you have a suspended timber floor) — blocked vents combined with poor drainage can cause moisture and rot.
2. Test Indoor Drains
For each sink, basin, bath, and shower:
- Fill the fixture to the brim, then release the plug and watch the speed of drainage. Slow draining usually means a partial blockage in the trap or branch drain.
- Listen for gurgling sounds from other fixtures when one drains — this can indicate a venting problem or a partial blockage further down the line.
- Check under vanities and kitchen sinks for moisture, staining, or soft cabinetry around waste pipes — signs of slow leaks.
3. Flush and Observe Toilets
- Flush each toilet and confirm it clears completely and refills without running continuously.
- Check around the base of the toilet for any soft flooring, staining, or odour — these can indicate a failed wax seal or a cracked pan.
- If you have more than one toilet, flush both simultaneously and see if either backs up or drains slowly.
4. Locate and Visually Inspect Manholes
Most NZ homes have one or more inspection chambers (manholes) in the yard. You can lift the lid (use a flat bar and wear gloves) and look from ground level with a torch:
- The chamber should be clear of debris and water (unless it has just rained heavily).
- You should be able to see the channel at the bottom running cleanly between pipes.
- Any sign of sewage backing up, roots, or collapsed walls is a serious issue requiring a licensed drainlayer.
5. Check for Odours
- A persistent sewage smell inside or outside the house — especially near manholes, garden beds, or the subfloor — can indicate a cracked or blocked drain.
- Dry traps (from infrequently used fixtures) can also cause odours; simply run water into unused drains to reseal the trap.
What a Simple Hose Test Can Reveal
| Test | What to look for | Likely cause if problem found |
|---|---|---|
| Run hose at downpipe | Water backs up or overflows | Blocked stormwater drain |
| Run hose at boundary | Pooling near foundation | Poor grading or blocked soak channel |
| Fill and drain sink | Slow or gurgling | Partial blockage or vent issue |
| Flush toilet | Incomplete clear or backup | Blockage in branch or main drain |
Limitations of DIY Testing
Your own checks are useful but have real limits. You cannot see inside buried pipes, assess pipe condition, or confirm compliance with the Building Code without specialist equipment. A licensed building inspector or drainlayer can carry out a CCTV drain camera inspection, which is the gold standard before selling. This will identify root intrusion, pipe collapse, offset joints, and illegal connections that no surface check can reveal.
If your home is older (pre-1980s), there is a reasonable chance it has clay or concrete pipes that may have cracked or been infiltrated by tree roots — a camera inspection is strongly recommended.
When to Call a Professional
Call a licensed drainlayer (registered with the Plumbers Gasfitters & Drainlayers Board) if you find:
- Sewage backing up into fixtures or manholes
- Persistent odours you cannot trace
- Wet or sunken patches in the lawn above drain lines
- Any work needed on buried pipes — this is restricted work under the Plumbers, Gasfitters, and Drainlayers Act and must be done by a licensed professional
A licensed building inspector can give you a comprehensive pre-sale report covering drainage alongside structure, weathertightness, and compliance. You can find qualified local inspectors and drainlayers on tradedirectory.co.nz.
For any questions about whether drainage work requires a building consent, check with your local council — some repairs are exempt under Schedule 1 of the Building Act, but confirm this before starting work. MBIE Building Performance has useful guidance on consent exemptions.
Licensed Building Practitioner (LBP)
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