Scratch coat (cement render)
Scratch coat is the first 5-8 mm cement render coat, mechanically scratched horizontally before set so the float coat keys to it. Cures 24-48 hours.
Ask Chalkline about this →A scratch coat is the first of three coats in a traditional cement-render system on masonry walls. It is applied 5-8 mm thick directly to the prepared substrate (brick, block, or concrete) and is mechanically scratched horizontally with a comb or scratching tool before it has fully set. The scratching forms a mechanical key that the next coat (float coat) bonds into. Together with the end coat on top, the scratch coat is the foundation of the three-coat render system used historically and still common on heritage or premium work in Australia.
Mix specification (typical):
| Component | Ratio (volume) | Function |
|---|---|---|
| Portland cement | 1 | Binder, sets the matrix |
| Hydrated lime | 1 | Workability, breathability, reduces shrinkage cracking |
| Sharp sand (washed plastering sand) | 6 | Bulk, aggregate skeleton |
| Water | To workable consistency | Hydration |
The 1:1:6 mix is the most common scratch coat specification. Some renderers omit the lime for cheaper or faster work (1:5 or 1:6 cement:sand), but the lime-free mix is more prone to shrinkage cracking and is less forgiving over the float coat.
Application sequence:
- Substrate prep: brush dust off the wall, dampen with a fine spray (suction control, not soaked).
- Apply scratch coat at 5-8 mm thickness with a hawk and trowel. Maintain consistent thickness; ridges and hollows become problems for the float coat above.
- Strike off with a long straight edge to remove high spots.
- Allow initial set (typically 30-60 minutes depending on weather): coat firms enough to take a horizontal scratch without smearing.
- Scratch horizontally with a wire comb or special scratching tool at 25-50 mm spacing. Lines should be 2-3 mm deep into the coat.
- Cure under damp cover for 24-48 hours minimum before the float coat is applied. In hot weather, cure 48 hours and mist-dampen periodically.
Why scratch horizontally? The float coat is applied as a flat plane. Vertical scratches would create vertical drainage paths for any moisture migration; horizontal scratches break that vertical path and discourage water tracking down the joint between coats.
Common defects:
- Scratched too late: if the coat has set hard, scratching smears the surface or pulls away material. The float coat then has no key and risks debonding.
- Scratched too early: the wet coat smears when scratched, the lines don’t hold, no effective key forms.
- Inconsistent thickness: too thin in places and the substrate suction overcomes the coat; too thick and shrinkage cracks open up.
- Skipped cure period: applying float coat the same day risks shrinkage cracks transferring through both coats as the scratch coat continues to shrink underneath.
- Substrate too dry or too wet: dry substrate sucks moisture out and starves the scratch coat; soaked substrate prevents bond.
Two-coat vs three-coat render. Most modern Australian residential render is two-coat (a thicker base + finish) rather than three-coat. The scratch coat is largely reserved for:
- Heritage repair where matching historical detailing matters.
- Very irregular substrates (rubble masonry, old patchwork brickwork) where the scratch coat fairs out the surface before any plane-true work.
- Premium full-render specifications.
Also known as: SC; render base coat; key coat; first coat render.
Category: Materials.
Related
See also
Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.