Crumbling brick windowsills are a common sight on older New Zealand homes, particularly those built before the 1970s. The good news is that minor deterioration — such as spalling mortar joints or surface erosion — is often a DIY-friendly repair. More serious structural damage, however, is best left to a qualified bricklayer.
Why Do Brick Windowsills Crumble?
NZ's variable climate plays a big role. Freeze-thaw cycles in colder regions (think Central Otago, the Manawatū, or elevated parts of Canterbury) cause moisture trapped in bricks and mortar to expand and contract, breaking down the surface over time. Salt attack, UV exposure, and simply the age of the original lime mortar all contribute. Older homes often used soft lime mortar, which is more permeable than modern cement-based mixes — and ironically, using a modern hard cement mortar as a replacement can actually accelerate brick decay by trapping moisture. Always match your repair mortar to the original as closely as possible.
Assessing the Damage
Before you reach for the trowel, take a good look at what you're dealing with:
- Minor spalling or crumbling mortar joints — suitable for DIY repointing
- Surface erosion of the brick face — can sometimes be stabilised, but may need professional assessment
- Cracked, loose, or hollow-sounding bricks — individual bricks may need replacing
- Structural movement or cracking extending into the wall — stop here and call a professional; this could indicate foundation or lintel issues
Tap bricks gently with a small hammer. A hollow sound suggests the brick has delaminated internally and will need replacement.
Tools and Materials You'll Need
- Cold chisel and club hammer (or an angle grinder with a mortar-raking disc — use with care)
- Wire brush and stiff-bristled brush
- Pointing trowel and margin trowel
- Bucket and mixing paddle
- Lime-based mortar mix (for pre-1940s homes) or a suitable bagged mortar mix
- Safety glasses, dust mask (P2 rated), and gloves
- Plastic sheeting or drop cloth
For lime mortar, specialist suppliers or PlaceMakers and Mitre 10 stock bagged hydraulic lime and heritage mortar products. Always check the product is appropriate for your situation — if in doubt, ask a bricklayer or check with BRANZ for guidance on heritage materials.
Step-by-Step: Repointing a Brick Windowsill
1. Rake Out the Old Mortar
Using a cold chisel and club hammer, carefully remove deteriorated mortar to a depth of at least 15–20 mm. Avoid damaging the brick edges. An angle grinder speeds this up but requires a steady hand — always wear eye protection and a P2 dust mask, as old mortar dust can contain silica. WorkSafe NZ has guidance on working safely with silica dust.
2. Clean the Joints
Brush out all loose debris with a wire brush. Dampen the joints with clean water — this prevents the dry brick from drawing moisture out of the fresh mortar too quickly, which weakens the bond.
3. Mix Your Mortar
Follow the manufacturer's instructions carefully. For heritage homes, a lime-based mortar (typically NHL 3.5 natural hydraulic lime with sand) is usually recommended. Avoid standard Portland cement mixes on soft, older bricks.
4. Apply the Mortar
Press mortar firmly into the joints using a pointing trowel, working in small sections. Overfill slightly, then finish flush or with a slightly recessed (bucket handle) profile to match the existing joints.
5. Cure and Protect
Keep the repair damp for a day or two, especially in hot or windy weather. Avoid working in direct sun or when frost is forecast — mortar needs stable conditions to cure properly.
Replacing a Damaged Brick
If individual bricks are cracked or hollow, they can sometimes be carefully removed and replaced. This is more involved:
- Chisel out the surrounding mortar and extract the damaged brick without disturbing neighbours
- Source a matching replacement brick — this can be tricky with older NZ bricks; try demolition yards or specialist suppliers
- Bed the new brick in appropriate mortar and repoint to match
Colour-matching mortar and sourcing period-correct bricks takes experience. If the sill has multiple damaged bricks or the coursing is complex, a professional bricklayer will get a much better result.
Waterproofing and Finishing
Once repairs have fully cured (allow at least four weeks for lime mortar), consider applying a penetrating masonry sealer to help repel water. Products are available from Bunnings and specialist paint stores. Avoid film-forming sealers on older bricks, as these can trap moisture. If you plan to paint the sill, Resene and Dulux both offer masonry paints suited to NZ conditions — check their technical data sheets for compatibility with lime-based substrates.
Building Consent Considerations
Repair and maintenance work on existing brick windowsills is generally exempt from building consent under Schedule 1 of the Building Act 2004, provided it is like-for-like repair and does not affect the structure. However, if your home is heritage-listed or in a special character zone, your local council may have additional requirements. Always confirm with your council before starting work. See MBIE Building Performance for more on consent exemptions.
When to Call a Professional
Call a qualified bricklayer if you notice:
- Cracks extending beyond the sill into the surrounding wall
- Sagging or displaced bricks suggesting lintel failure
- Multiple bricks needing replacement
- You're unsure about mortar type for a heritage property
- The damage is high up and requires scaffolding
A licensed bricklayer will assess whether there are underlying structural issues and ensure repairs comply with the NZ Building Code. You can find experienced local bricklayers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
