Water ingress around window reveals is one of the most common weathertightness problems in New Zealand homes, particularly in older houses or those with direct-fixed cladding. The good news is that with the right preparation, materials, and technique, many reveals can be effectively waterproofed as a DIY project — though some situations will require a licensed professional.
Understanding Why Window Reveals Leak
A window reveal is the recessed area between the outer face of the wall cladding and the window joinery frame. Water gets in when:
- Sealant fails — UV exposure and thermal movement cause sealant to crack or pull away over time.
- Flashing is missing or incorrectly installed — particularly at the sill (bottom) of the reveal.
- Cladding or render is cracked — allowing water to track behind the reveal lining.
- No back-dam — the sill has no upstand to redirect water away from the interior.
Under the NZ Building Code, clause E2 (External Moisture) requires that buildings are constructed to prevent water from penetrating to cause undue dampness or damage. Getting your window reveals right is a key part of meeting this requirement.
Tools and Materials You'll Need
- Utility knife and scraper
- Wire brush and cleaning cloths
- Primer suitable for your substrate (timber, fibre cement, or plaster)
- Flexible polyurethane or silicone sealant (paintable polyurethane is generally preferred for reveals)
- Self-adhesive flexible flashing tape (e.g., butyl or modified bitumen type)
- Fibreglass mesh tape (for plaster reveals)
- Waterproof membrane or coating (liquid-applied, for rendered reveals)
- Paintbrush and roller
- Safety glasses and gloves
For fibre cement reveals, check the James Hardie technical documentation for compatible sealants and primers specific to their products.
Step-by-Step: Waterproofing a Window Reveal
1. Inspect and Prepare the Surface
Remove all old, cracked, or loose sealant using a utility knife and scraper. Check for any rot in timber reveals — soft or spongy timber must be repaired or replaced before waterproofing. Clean the entire reveal with a damp cloth and allow it to dry completely. Moisture trapped under a new membrane will cause it to fail.
2. Apply Flashing Tape to the Sill
The sill (bottom) of the reveal is the most critical area. Apply a self-adhesive flexible flashing tape across the full width of the sill, lapping it up the sides (jambs) by at least 50 mm and onto the face of the joinery frame by at least 10–15 mm. Press it firmly into all corners. This creates a continuous waterproof layer that directs any water outward rather than inward.
BRANZ recommends that sill flashing be installed with a slight outward fall (slope) to shed water — aim for a minimum 5° fall where possible.
3. Reinforce Corners with Mesh (Rendered Reveals)
If your reveal is plastered or rendered, embed fibreglass mesh tape into the internal corners (where the reveal meets the joinery and where the sill meets the jambs) using a compatible base coat or flexible adhesive. This bridges any movement cracks that commonly form at these junctions.
4. Apply a Liquid Waterproof Membrane
For rendered or plaster reveals, apply a liquid-applied waterproof membrane over the entire reveal surface. Products from Ardex, Mapei, or Sika are widely used in NZ and are suitable for this application — always follow the manufacturer's data sheet for coverage rates and number of coats. Typically two coats are required, applied in perpendicular directions, with the first coat fully cured before the second.
For timber reveals, a quality exterior primer followed by a good exterior paint system (such as those from Resene or Dulux) may be sufficient if the flashing tape and sealant joints are sound.
5. Seal All Joints and Junctions
Once the membrane is cured, apply a flexible polyurethane sealant to all junctions:
- Between the reveal and the joinery frame (all four sides)
- Between the reveal and the outer cladding face
- Any penetrations or fixings
Tool the sealant smooth with a wet finger or sealant tool to ensure full adhesion on both sides. A properly tooled sealant joint should be slightly concave (not convex) to shed water. Allow full cure time before exposing to rain — check the product data sheet.
6. Check Drainage and Finish
Confirm that water can drain freely from the sill. If your joinery has weep holes, make sure they are clear and unblocked. Paint the reveal with a suitable exterior topcoat to protect the membrane and improve appearance.
Common Mistakes to Avoid
| Mistake | Why It Matters |
|---|---|
| Sealing over damp or dirty surfaces | Causes adhesion failure and early sealant breakdown |
| Bridging the sill joint with rigid filler | Rigid materials crack with thermal movement |
| Skipping the sill flashing tape | The sill is the highest-risk area for water entry |
| Using interior-grade sealant outdoors | Breaks down quickly in UV and weather |
| Painting over uncured membrane | Traps solvents and causes peeling |
When to Call a Professional
Some window reveal situations are beyond a straightforward DIY fix:
- Extensive rot or structural damage to the framing around the window — this may require a Licensed Building Practitioner (LBP) and could need a building consent depending on the scope of work. Always check with your local council.
- Direct-fixed cladding systems (no cavity) where water has tracked behind the cladding — this is a complex weathertightness issue requiring professional assessment.
- Recladding or window replacement — replacing windows may constitute restricted building work under the Building Act depending on the scope — check with your local council and an LBP to confirm whether the work requires LBP involvement or a building consent.
- Ongoing or unexplained leaks despite previous repairs — a qualified waterproofer or building inspector can identify the true source.
You can find experienced local waterproofers and building tradespeople on tradedirectory.co.nz.
Site Safe
SiteWise
Licensed Building Practitioner (LBP)
