Lath and plaster walls are common in New Zealand homes built before the 1960s, and while they're durable, they can crack, sag, or crumble over time. The good news is that small to medium repairs are well within reach of a capable DIYer — you just need patience, the right materials, and a good understanding of how the original system works.
Understanding Lath and Plaster Construction
Traditional lath and plaster walls consist of narrow timber strips (laths) nailed horizontally across the wall studs, with a gap of roughly 6–8 mm between each strip. Wet plaster was then applied in multiple coats, squeezing through the gaps to form keys — the hardened plaster that locks the wall together from behind. This three-coat system typically includes a scratch coat, a brown (float) coat, and a finish coat.
When plaster fails, it's often because the keys have broken away from the back of the laths, causing sections to become hollow or fall away entirely. Tapping the wall with your knuckles will reveal hollow areas with a dull thud rather than a solid knock.
Assessing the Damage
Before you start, work out the extent of the problem:
- Small cracks (hairline to 3 mm wide) — can be filled with a flexible filler or setting compound.
- Larger cracks or loose plaster — the surrounding area needs to be checked for further delamination.
- Missing sections — you'll need to re-key or patch with a suitable backing material.
- Widespread failure — if large areas are hollow or sagging, it may be more practical to remove the lath and plaster entirely and line with GIB plasterboard.
Always check for the cause of damage first. Water leaks, subfloor moisture, or structural movement can all cause plaster failure, and patching over an unresolved problem won't last.
Tools and Materials You'll Need
- Cold chisel and hammer
- Stiff-bristled brush
- Spray bottle with water
- Bonding agent (e.g. a PVA-based primer)
- Base coat plaster or patching plaster
- Finish plaster or setting compound
- Plasterer's trowel and hawk
- Fibreglass mesh tape (for cracks)
- Fine sandpaper (120–180 grit)
- Safety glasses and dust mask
Products from Ardex and Mapei include suitable patching and bonding compounds available at most NZ trade and hardware stores.
Step-by-Step Repair Process
1. Remove Loose and Damaged Plaster
Using a cold chisel and hammer, carefully chip away all loose, crumbling, or hollow plaster back to a solid edge. Don't be tempted to leave questionable sections — they'll fail again. Brush out all dust and debris.
2. Check and Repair the Laths
Inspect the exposed laths. Replace any that are broken, rotten, or missing. If the gap in the lath is too large to support a new plaster key, cut a piece of fibreglass mesh or metal lath and staple or nail it across the opening to provide a backing.
3. Apply a Bonding Agent
Dampen the surrounding plaster edges and the exposed laths with water — this slows suction and helps adhesion. Apply a bonding agent (diluted PVA or a proprietary bonding primer) to all surfaces and allow it to become tacky before plastering.
4. Apply the Base (Scratch) Coat
Mix your base coat plaster to a firm, workable consistency. Apply it firmly into the repair area, pressing it through the lath gaps to re-form the keys. Build up to roughly 6–8 mm below the surrounding wall surface. While still wet, scratch the surface with a comb or the edge of your trowel to create a key for the next coat. Allow to firm up — this can take several hours depending on conditions.
5. Apply the Float Coat
Once the scratch coat has set but is not fully dry, apply a second coat to bring the surface to within 2–3 mm of the finished wall level. Feather the edges carefully into the surrounding plaster. Allow this coat to cure.
6. Apply the Finish Coat
Mix your finish plaster or setting compound to a smooth, creamy consistency. Apply a thin finish coat, bringing it flush with the surrounding wall. Use a damp trowel to smooth and polish the surface. Feather the edges carefully so there's no visible step.
7. Sand and Prime
Once fully cured (allow at least 24–48 hours, longer in cooler or humid conditions), lightly sand the repair with fine sandpaper. Spot-prime the repair before painting — new plaster is very porous and will show as a dull patch if painted directly. Resene and Dulux both offer suitable primers for plaster surfaces.
Tips for a Seamless Finish
- Work in thin coats — thick coats shrink and crack as they dry.
- Keep the repair damp between coats in hot or dry weather to prevent too-rapid drying.
- Match the texture of the surrounding wall before painting — older plaster often has a slightly textured finish that can be replicated with a sponge or brush technique.
- In older NZ homes, be aware that pre-1980s plaster may contain asbestos fibres, particularly in textured finishes or if the home was built or renovated between the 1940s and 1980s. If you're unsure, do not sand or disturb the material — have it tested first. WorkSafe NZ has guidance on identifying and managing asbestos safely.
When to Call a Professional
Consider engaging a qualified plasterer if:
- The damaged area is larger than roughly half a square metre.
- You suspect widespread key failure across multiple walls.
- The damage is caused by ongoing moisture, structural movement, or subfloor issues that need a builder's assessment.
- You find or suspect asbestos-containing materials — this work must be handled by a licensed asbestos removalist.
- You want a seamless, professional finish that matches the original wall perfectly.
A skilled plasterer will also be able to advise whether full re-lining with GIB plasterboard is a more cost-effective solution for your situation. You can find qualified local plasterers on tradedirectory.co.nz.
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