Filling a small gap between brickwork and a window frame is a common maintenance task most NZ homeowners can tackle themselves. The key is using the right product — a flexible sealant, not rigid mortar — and preparing the surface properly so the seal lasts. Done correctly, it keeps out wind, rain, and insects, and helps your home meet the weathertightness intent of the NZ Building Code.
Why the Right Product Matters
Brickwork and window frames move independently. Bricks expand and contract with temperature changes, and timber or aluminium frames do the same — often at different rates. If you fill the gap with rigid mortar or cement, it will crack within a season or two as movement occurs. The correct solution is a flexible polyurethane or silicone-based sealant that can accommodate this movement without breaking down.
For most NZ conditions, a polyurethane sealant (such as those available from Sika) is a popular trade choice because it bonds well to masonry and frames, can be painted over, and handles our variable climate. Silicone sealants are also effective and very durable, though most cannot be painted.
What You'll Need
- Flexible polyurethane or silicone sealant (exterior grade)
- Backer rod (closed-cell foam rope) — for gaps wider than about 6 mm
- Caulking gun
- Utility knife or scissors
- Wire brush or stiff brush
- Scraper or old chisel
- Methylated spirits or appropriate cleaner (check sealant manufacturer's instructions)
- Masking tape
- Disposable gloves
Step-by-Step: How to Seal the Gap
1. Remove Old Sealant or Loose Mortar
Use a scraper, old chisel, or utility knife to remove any existing cracked sealant, loose mortar, or debris from the gap. Be thorough — new sealant won't bond properly over old, failed material.
2. Clean the Surfaces
Brush out dust and loose particles with a stiff brush. Wipe both the brickwork and the window frame with a clean cloth dampened with methylated spirits (or the cleaner recommended on your sealant's data sheet). Allow to dry completely.
3. Insert Backer Rod if Needed
If the gap is wider than about 6 mm or deeper than 10 mm, press a backer rod into the gap first. This foam rod gives the sealant something to sit against, controls the depth of the sealant bead, and improves flexibility. The sealant should be roughly twice as wide as it is deep for best performance.
4. Apply Masking Tape
Run masking tape along both edges of the gap — on the brickwork and on the frame — to keep your sealant line neat and protect surrounding surfaces.
5. Apply the Sealant
Cut the nozzle of the sealant tube at a 45-degree angle to match the width of the gap. Load the tube into your caulking gun and apply a smooth, continuous bead into the gap, moving at a steady pace. Try to fill the gap in one pass.
6. Tool the Sealant
Within a few minutes of application (before a skin forms), use a wet finger or a caulking tool to press the sealant firmly into the gap and create a slightly concave, smooth finish. This improves adhesion and sheds water away from the joint.
7. Remove Tape and Allow to Cure
Peel the masking tape away carefully while the sealant is still wet. Allow the sealant to cure fully before painting — check the manufacturer's data sheet for cure times, as these vary by product and temperature.
Choosing the Right Sealant: A Quick Guide
| Sealant Type | Paintable | Best For | Notes |
|---|---|---|---|
| Polyurethane (e.g. Sika) | Yes | Brick-to-frame joints | Excellent adhesion, flexible |
| Silicone (exterior grade) | Usually no | Aluminium frames, wet areas | Very durable, long-lasting |
| Acrylic/Latex | Yes | Interior or low-movement gaps | Less flexible, not ideal here |
Always read the product data sheet and confirm the sealant is rated for exterior use and compatible with both masonry and your frame material (timber or aluminium).
NZ Climate Considerations
New Zealand's weather — particularly in wetter regions like the West Coast, Northland, and Wellington — puts real pressure on window-to-wall junctions. Keeping this joint properly sealed is an important part of maintaining your home's weathertightness, as outlined under Clause E2 (External Moisture) of the NZ Building Code. BRANZ provides good guidance on weathertight detailing for NZ homes.
If your home has a Healthy Homes obligation (rental properties), maintaining weathertight seals also supports draught-stopping requirements. See Tenancy Services for more information.
When to Call a Professional
This is generally a safe DIY job, but consider calling a qualified bricklayer or plasterer if:
- The gap is large or irregular (more than 15–20 mm wide), suggesting structural movement or settlement
- You notice cracking in the surrounding brickwork or mortar joints — this may indicate a deeper issue
- There is evidence of water damage, rot in the frame, or mould inside the wall cavity
- The window frame itself is loose, damaged, or needs replacing — window installation can involve restricted building work depending on the scope
For significant repairs, it's worth checking whether a building consent is required with your local council. Minor maintenance like resealing generally falls under Schedule 1 exemptions, but always confirm with your council if you're unsure. You can find experienced local bricklayers and builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
