Water tracking under a sliding door during heavy rain is one of the most common complaints from NZ homeowners — and the good news is that most causes are straightforward to diagnose and fix yourself. The usual culprits are worn or missing weatherstripping, a blocked drainage channel in the door track, or a poorly sealed threshold. Left untreated, persistent water ingress can lead to timber rot, mould, and damage to interior flooring — all issues that matter under the NZ Building Code's E3 clause on internal moisture.
Understand How Sliding Doors Are Designed to Handle Rain
Modern sliding doors aren't designed to be completely watertight — they're designed to manage water. The bottom track has small drainage holes or channels that direct water back outside. When everything is working correctly, a small amount of water entering the track simply drains away. Problems start when this system breaks down.
Step 1 — Inspect and Clear the Drainage Channel
The first thing to check is the bottom track drainage holes. These small slots or holes sit at intervals along the outer edge of the track and allow water to escape.
- Slide the door fully open and look along the bottom track.
- Use a thin screwdriver, a piece of wire, or a compressed air can to clear any debris, dirt, or dead insects blocking the holes.
- Flush the track with a small amount of water to confirm it drains freely.
- Clean the track thoroughly with a stiff brush and a damp cloth.
This simple step alone solves the problem for many homeowners.
Step 2 — Check and Replace Weatherstripping
Weatherstripping (also called pile seal or brush seal) runs along the edges and bottom of the sliding door panel. Over time — especially in NZ's UV-intense summers and wet winters — this seal degrades, flattens, or falls out entirely.
- Inspect the pile seal along the bottom and sides of the door. It should look full and upright, not flattened or missing in sections.
- Check the fixed panel seals as well, particularly where the sliding panel meets the fixed panel in the middle.
- Replacement pile seal is available in rolls from Bunnings or Mitre 10. Match the width and pile height to your existing seal — take a sample with you if possible.
- To replace, simply pull out the old seal from its channel and press the new one in. No special tools are needed.
Step 3 — Inspect the Door Threshold and Sill
The threshold (the raised section at the base of the door frame) acts as a physical barrier to water. If it's cracked, has gaps at the ends, or wasn't installed with adequate height, water will track straight under.
- Check for gaps where the threshold meets the door frame jambs at each side. These are common failure points.
- Apply a neutral-cure silicone sealant (not acetoxy/acetic cure, which can corrode aluminium) to any gaps. Sika and other brands offer suitable products for aluminium and timber interfaces.
- Check that the threshold itself is sitting flat and hasn't shifted or lifted.
Step 4 — Check the Door Alignment and Roller Height
If the door has dropped or is misaligned, the bottom seal won't make proper contact with the track.
- Most sliding doors have adjustable rollers accessed via a small screw hole at the bottom of the door panel (check your door manufacturer's instructions).
- Raising the door slightly using the roller adjustment screws can restore proper contact between the door and the track seal.
- The door should glide smoothly and sit evenly in the frame when correctly adjusted.
Step 5 — Assess the External Drainage and Paving
Sometimes the issue isn't the door itself — it's water pooling against the threshold because the external paving or deck slopes toward the house rather than away from it.
- Stand outside during rain and watch where water flows. Ideally, paving should fall away from the building at a minimum gradient (check BRANZ guidance for recommended falls).
- If water is ponding against the door, improving the external drainage or adding a channel drain just outside the threshold can make a significant difference.
- A small door canopy or awning above the door can also dramatically reduce the volume of rain hitting the threshold in the first place.
Step 6 — Consider a Waterproof Membrane at the Threshold
If the door opens onto a deck or a wet area, the junction between the door frame and the deck or concrete slab should have a proper waterproof membrane and flashing detail. This is particularly important under NZ Building Code clause E2 (External Moisture). If this detail was never done correctly — or has failed — water will track under the door regardless of how good the seals are.
Applying a flexible waterproofing product (such as those from Ardex or Mapei) to the threshold area can help, but if the underlying flashing is missing or failed, a more thorough repair is needed.
When to Call a Professional
If you've worked through all the steps above and water is still getting in, it's time to call a qualified waterproofer or joiner. Persistent leaks can indicate:
- Failed or missing flashing behind the door frame (a building defect that may require consent to repair if structural work is involved — check with your local council).
- Rot in the surrounding timber framing, which needs assessment by a Licensed Building Practitioner (LBP).
- A door that is fundamentally unsuitable for the exposure level of the site — NZ's coastal and high-wind zones can require joinery performance ratings appropriate for the exposure zone.
A qualified waterproofer or building inspector can identify the root cause quickly and advise on the right fix. You can find experienced local tradespeople on tradedirectory.co.nz.
Safety note: When working around door frames, be aware of any electrical wiring near the threshold area (e.g., for alarm sensors or underfloor heating). If in doubt, consult a registered electrician — see the Electrical Workers Registration Board for guidance on restricted electrical work.
Sources & Further Reading
- MBIE Building Performance — NZ Building Code clause E2 (External Moisture)
- BRANZ — Weathertightness guidance for homeowners
- Sika NZ — Sealants and waterproofing products
- Ardex NZ — Waterproofing membranes
- Licensed Building Practitioners — Find a licensed professional
- Bunnings NZ — Weatherstripping and door seals
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