A scratch coat is the first structural layer of a two-coat (or three-coat) render system, applied directly to the substrate to create a mechanical key for the finish coat. Getting it right is critical — a poorly applied scratch coat leads to delamination, cracking, and costly repairs down the track. Follow these steps and you'll give your finish render the best possible foundation.
What Is a Scratch Coat and Why Does It Matter?
A scratch coat (sometimes called a base coat or undercoat) is a cement-rich mortar layer typically 6–10 mm thick. Once it has partially set, the surface is deliberately scored with a notched scratcher or devil float to create a rough, keyed texture. This scratched surface gives the finish coat something to grip mechanically, dramatically reducing the risk of the finish coat sliding off or cracking away from the wall.
In New Zealand, external render systems must comply with NZ Building Code Clause E2 (External Moisture), which sets performance requirements for weather-tightness. Using a proper scratch coat is a core part of meeting those requirements on masonry and fibre-cement substrates. Always check with your local council if you are unsure whether your project requires a building consent.
Tools and Materials You'll Need
- Portland cement (GP cement) or a bagged render undercoat mix
- Plastering sand (washed, fine-to-medium grade)
- Fibreglass reinforcing mesh (for fibre-cement or lightweight substrates)
- Bonding agent (for smooth or low-absorption surfaces)
- Scratch coat mix ratio: typically 1 part cement : 3–4 parts sand by volume (check your product datasheet)
- Hawk and steel trowel
- Devil float or notched scratcher
- Straight edge / darby
- Mixing drill with paddle attachment or a mortar mixer
- Clean water
- Spray bottle or hose for curing
For bagged undercoat products, suppliers such as Sika and Ardex offer pre-blended render base coats with polymer additives that improve adhesion and flexibility — particularly useful in New Zealand's variable climate.
Step 1 — Prepare the Substrate
Preparation is everything. The substrate must be:
- Clean and free of dust, oil, efflorescence, and loose material
- Structurally sound — fix any cracks or damaged masonry before rendering
- Dampened (not saturated) to reduce suction and prevent the scratch coat drying too fast
On fibre-cement sheet (such as James Hardie products), embed a layer of alkaline-resistant fibreglass mesh into the scratch coat to control cracking. On very smooth or dense surfaces, apply a bonding agent and allow it to become tacky before applying the mortar.
Step 2 — Mix the Scratch Coat
Mix your mortar to a firm, workable consistency — it should hold its shape on the hawk without slumping. A common site-mixed ratio is 1 part GP cement : 3 parts plastering sand, with just enough water to achieve workability. Avoid adding too much water; a wet mix shrinks more as it dries and is more prone to cracking.
If using a bagged product, follow the manufacturer's water ratio exactly. Always mix in a clean container and use within the pot life specified on the packaging.
Step 3 — Apply the Scratch Coat
- Load your hawk with a generous amount of mortar.
- Using a steel trowel, apply the mortar to the wall with firm, upward strokes, working from the bottom up.
- Build the coat to an even thickness of 6–10 mm. Use a straight edge or darby to rule off high spots and fill low areas.
- Where mesh is required, press it into the wet mortar with your trowel so it is fully embedded — it should sit in the middle third of the coat, not on the surface.
- Work in manageable sections to avoid the mortar skinning over before you can scratch it.
Timing the Scratch
This is the most critical step. Wait until the mortar has stiffened but not fully hardened — it should resist a firm thumb press but not feel rubbery or wet. In warm, dry NZ conditions this may be as little as 30–60 minutes; on a cool, overcast day it could take 2–3 hours.
Using a devil float or notched scratcher, drag horizontal lines across the entire surface to a depth of roughly 3–4 mm. Keep the lines consistent and avoid going so deep that you break through to the substrate.
Step 4 — Cure the Scratch Coat
Curing is non-negotiable. Cement-based render needs moisture to hydrate properly. Mist the scratch coat lightly with water 2–3 times a day for at least 48 hours, and keep it shaded from direct sun and wind if possible. In hot, dry conditions (common in many parts of New Zealand over summer), hang hessian or shade cloth over the work.
Allow the scratch coat to cure for a minimum of 7 days before applying the finish coat. Some manufacturers recommend up to 28 days for full strength — check your product datasheet.
Common Mistakes to Avoid
| Mistake | Why It's a Problem | Fix |
|---|---|---|
| Scratching too early | Mortar tears rather than scores cleanly | Wait until it's firm to the touch |
| Scratching too late | Surface is too hard to key properly | Time your sections carefully |
| Mix too wet | Excess shrinkage and cracking | Use minimum water for workability |
| Skipping mesh on sheet substrates | Cracking at sheet joints | Always embed mesh over joints |
| Applying finish coat too soon | Scratch coat still shrinking | Wait the full cure period |
When to Call a Professional
While applying a scratch coat is within the reach of a capable DIYer on a small project, larger or more complex jobs are best left to a qualified renderer. This includes:
- Multi-storey work (working at height carries serious risks — see WorkSafe NZ for scaffolding requirements)
- Weather-tight remediation on leaky homes, where incorrect render systems can cause significant structural damage
- Any work requiring a building consent — check with your local council first
- Rendering over EIFS (polystyrene) systems, which require specialist knowledge and specific product systems
A Licensed Building Practitioner (LBP) with a plastering or bricklaying designation is the right person for restricted building work. You can find qualified local renderers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
