How to Fix a Window That Lets in Rain Around the Frame Corners

Rain leaking around window frame corners is one of the most common weathertightness complaints in New Zealand homes. The good news is that many cases come down to failed sealant or minor flashing issues that a confident DIYer can tackle. However, if the leak is linked to deeper building envelope problems, you'll want a professional on the job quickly — water damage can escalate fast in our wet climate.

Why Do Window Corners Leak?

Corners are the most vulnerable point of any window frame because that's where two separate sealant runs meet, and where movement in the building is concentrated. Common causes include:

• Failed or cracked sealant — silicone and polyurethane sealants typically last 10–15 years before they shrink, crack, or lose adhesion
• Poor original installation — sealant applied over a dirty or damp surface won't bond properly
• Missing or damaged flashing — the head flashing above the window diverts water away; if it's bent, corroded, or absent, water tracks down into the corners
• Timber shrinkage or building movement — especially common in older NZ homes with timber framing (NZS 3604 construction), where seasonal movement opens gaps at joints
• Blocked weep holes — many aluminium window frames have small weep holes at the sill to drain any water that gets past the outer seal; if these are blocked, water backs up and finds its way through corners

Step 1 — Diagnose Before You Fix

Don't just slap sealant over the corners without understanding where the water is actually entering. On a dry day:

1. Inspect the outside of the frame corners closely — look for cracked, missing, or peeling sealant, gaps between the frame and the cladding, and any rust staining (a sign of long-term moisture)
2. Check the head flashing above the window — it should overlap the top of the frame and direct water outward
3. Look at the inside of the reveal for staining, mould, or soft plasterboard, which indicates how long the leak has been occurring
4. Clear any debris from weep holes along the bottom of the frame with a thin wire or toothpick

If you're unsure where the water is entering, a glazier or builder can do a hose test to pinpoint the source.

Step 2 — Remove Old Sealant Thoroughly

This is the step most DIYers skip, and it's why their repair fails. New sealant will not bond properly over old sealant.

• Use a sealant removal tool or sharp utility knife to cut away all the old sealant from the joint
• Clean the surface with a rag dampened with methylated spirits to remove any residue, dust, or grease
• Allow the surface to dry completely — never apply sealant to a damp surface

Step 3 — Apply New Sealant Correctly

For exterior window-to-cladding joints, use a paintable polyurethane sealant or a neutral-cure silicone rated for exterior use. Check the product is suitable for your cladding type (e.g., fibre cement, timber, or brick). Products from brands stocked at Bunnings or Mitre 10 will carry clear guidance on their labels.

• Press backer rod (foam rope) into any gap wider than about 6 mm before sealing — this gives the sealant the right depth-to-width ratio and allows it to flex
• Apply sealant in a continuous bead, working from one end of the joint to the other without stopping
• Tool the sealant smooth with a wet finger or sealant tool within a few minutes of application
• Do not paint over silicone — if the area needs painting, use a polyurethane sealant instead, or a silicone specifically labelled as paintable

Step 4 — Check and Repair Flashing

If the head flashing above the window is the culprit, this is where the job gets more involved. Under the NZ Building Code clause E2 (External Moisture), windows must be properly flashed to prevent water ingress. Flashing repairs that involve cutting into the cladding or wall underlay may require a building consent depending on the scope — always check with your local council first. MBIE Building Performance has useful guidance on what work requires consent and what falls under Schedule 1 exemptions.

For minor flashing repairs (e.g., resealing a lifted flashing edge), a competent DIYer can use a compatible flashing tape or sealant. For anything more involved — especially on homes with a history of weathertightness issues — engage a Licensed Building Practitioner (LBP). You can find one at Licensed Building Practitioners or search for a local glazier or builder on tradedirectory.co.nz.

Preventing Future Leaks

• Inspect window sealant every 2–3 years as part of routine home maintenance
• Keep gutters and downpipes clear so water doesn't overflow onto window heads
• Ensure any repainting of the exterior includes checking and refreshing sealant joints
• BRANZ publishes excellent guidance on weathertightness maintenance for NZ homes

When to Call a Professional

Call a qualified glazier or builder if:

• The leak has been ongoing for some time and you can see mould, rot, or soft wall linings inside
• The window frame itself is warped, cracked, or poorly fitted — this may mean the window needs to be removed and reinstalled
• The head flashing is missing or significantly damaged and the repair involves opening up the wall
• Your home is a monolithic clad (plaster/EIFS) property — these require specialist weathertightness assessment
• You're a landlord — under the Healthy Homes Standards, rental properties must be free from moisture ingress, so prompt professional repair is important

A skilled glazier can re-bed and re-seal a window frame properly, and a builder or LBP can assess and repair flashing. Find trusted local tradespeople on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance — Building Code clause E2 (External Moisture)
• BRANZ — Weathertightness guidance for NZ homes
• Licensed Building Practitioners — Find an LBP
• Tenancy Services — Healthy Homes Standards
• Bunnings — Sealants and weatherproofing products
• Mitre 10 — Sealants and flashing tape