Applying a scratch coat to a timber-framed wall is something a capable DIYer can tackle, but it's not as simple as slapping on some cement mix. Timber-framed walls require careful preparation, the right lath or mesh system, and a correctly proportioned mix — get any of these wrong and you risk cracking, delamination, or worse, moisture getting trapped behind the render and causing serious damage to your framing.
What Is a Scratch Coat and Why Does It Matter?
A scratch coat is the first layer of render applied to a wall. Its job is to bond firmly to the substrate and provide a keyed surface (scratched with a comb or scarifier tool) for the next coat to grip. On timber-framed walls, the scratch coat is especially important because timber moves with temperature and humidity changes — your render system must accommodate that movement.
NZ Building Code Considerations
Before you start, it's worth understanding the regulatory context. External render on timber-framed walls is covered under NZ Building Code Clause E2 (External Moisture). The goal is to keep water out of the wall cavity. MBIE Building Performance has detailed guidance on acceptable solutions for external cladding systems, and render over timber framing must typically include a drained and vented cavity behind the cladding — this is a critical detail that many DIYers overlook.
For most external render work on a habitable building, a building consent may be required. Schedule 1 of the Building Act lists some exemptions, but these are narrow. Always check with your local council before starting — don't assume your project is exempt.
Choosing the Right Substrate and Mesh
Timber framing alone is not a suitable direct substrate for render. You'll need one of the following:
- Fibre cement sheet (such as James Hardie products) as a base board, fixed with the correct cavity battens behind it
- Metal lath (expanded galvanised steel mesh) fixed over a building wrap
- Fibreglass reinforcing mesh embedded in the scratch coat
Fibreglass alkali-resistant (AR) mesh is strongly recommended for the scratch coat on timber-framed walls. It helps control cracking caused by timber movement. Make sure the mesh is fully embedded — not sitting on top of the coat.
What You'll Need
| Item | Notes |
|---|---|
| Cement, sand, lime | Standard 1:1:6 or 1:0.5:4.5 mix (cement:lime:sand) |
| AR fibreglass mesh | Alkali-resistant, min 145 g/m² |
| Scratch/scarifier tool | To key the surface for the next coat |
| Steel float & hawk | For applying and spreading render |
| Mixing drill or paddle mixer | Consistent mix is essential |
| Building wrap / cavity battens | If not already in place |
| Corner beads and stop beads | Galvanised or PVC |
Step-by-Step: Applying the Scratch Coat
1. Check Your Wall Build-Up
Confirm that a building wrap (such as a Type B wrap) and cavity battens are correctly installed. The render system must drain — water that gets behind render needs somewhere to go.
2. Fix Your Base and Beads
Fix your base board or metal lath securely to the framing. Install galvanised stop beads and corner beads at all edges and external corners. These control thickness and give clean edges.
3. Mix Your Scratch Coat
A common mix for a scratch coat is 1 part cement : 1 part lime : 6 parts clean washed sand by volume. Keep your mix consistent — too wet and it'll slump and crack; too dry and it won't bond. Products like Sika and Ardex offer pre-blended render systems with technical datasheets — these can simplify the process and give more predictable results.
4. Apply the Scratch Coat
Apply the scratch coat to a thickness of around 8–10 mm, working from the bottom up. Press the AR mesh into the wet render, overlapping joins by at least 100 mm. Flatten the mesh fully into the coat — no bubbles or dry spots.
5. Scratch the Surface
While the coat is still green (firm but not fully set — usually 1–3 hours depending on conditions), use a scarifier or notched scratcher to create horizontal grooves across the entire surface. These grooves give the next coat mechanical grip.
6. Cure Slowly
Keep the scratch coat damp for at least 24–48 hours by misting with water, especially in hot or windy NZ conditions. Rapid drying causes shrinkage cracks. Avoid applying render in direct summer sun or when frost is forecast.
Common Mistakes to Avoid
- Skipping the cavity — render directly on framing is not compliant and traps moisture
- Using too much water in the mix
- Not overlapping mesh joins adequately
- Applying the next coat before the scratch coat has cured (wait at least 7 days)
- Forgetting movement joints on large wall areas (typically every 4–5 m)
When to Call a Professional
While a scratch coat is within reach of a competent DIYer, there are situations where you should engage a qualified renderer from the outset:
- If the wall build-up (wrap, cavity, substrate) is not already correctly installed — this may involve Licensed Building Practitioner (LBP) work; check at LBP
- If a building consent is required — the work may need to be done or supervised by an LBP
- If you're rendering a large area or a multi-storey building
- If you're unsure about the existing wall construction or moisture performance
A professional renderer will also know how to detail around windows, penetrations, and junctions — areas where moisture entry is most likely. You can find qualified local renderers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
