A strong first coat, keyed (scratched) to bond the next coat. Slightly richer than the undercoat above it so shrinkage stresses point inward.
Mix ratio
1 : 0.5 : 4.5
Cement : Lime : Sand
Proportions & strength
Render scratch (first) coat: proportions by volume, typical compressive strength and use.
Designation
Scratch coat
Ratio (by volume)
1 : 0.5 : 4.5
Cement
1 part
Lime
0.5 parts
Sand
4.5 parts
Typical use
First (keyed) coat of a two/three-coat render.
Planning reference. Strengths are typical values for a well-made mix at 28 days, not a guaranteed design strength. Structural work must follow your local building code and a qualified engineer's mix design.
Where Scratch coat is used
A strong first coat, keyed (scratched) to bond the next coat. Slightly richer than the undercoat above it so shrinkage stresses point inward.
Render is built up in coats, each one weaker than the coat beneath it. That way shrinkage stresses stay in the outer layer instead of pulling the render off the wall.
Need quantities? The volume calculators give the cubic yards, cubic metres and bag counts for the pour this mix goes into.