Pebbledash Calculator — Aggregate, Cement & Sand Quantities

For standard 6-10mm stone, allow about 12 kg/m² — this is the figure given in the Irwin Dry Dash specification (a 25kg bag of dry-dash covers roughly 1.5-2 m²). Fine 3-6mm stone needs a little less (~9 kg/m²) and coarse 10-14mm stone a little more (~15 kg/m²); these two are rule-of-thumb adjustments rather than published rates, as the manufacturer notes coverage can only be estimated. These rates assume a single even coverage thrown by hand. Always add 15% wastage as some stones fall and bounce off during application.

Pebbledash uses two render coats, and the cardinal rule of rendering is that each coat must be WEAKER (less cement) than the coat beneath it — NHBC 6.11.6 states render coats should not be stronger than the background or any previous coat. The scratch coat (first coat) is a 1:3 cement:plastering sand mix at around 10mm, scratched with a comb to provide a key. The receiving (butter) coat is a weaker 1:4 cement:sand mix at 8-12mm (Irwin Dry Dash specification). The dash sticks because the stones are thrown while this coat is still wet and tacky and then tamped in — NOT because the mix is rich in cement. A butter coat stronger than the scratch coat traps shrinkage stress and leads to cracking and the dash blowing off. For a plain rendered finish without the dash, our sand and cement render mix calculator covers standard 1:5 and 1:4 render coats instead.

Yes, provided the existing render is sound, well-bonded, and free of loose material. Clean the surface with a pressure washer and apply a bonding agent (SBR or PVA diluted 1:4) before the scratch coat. If the existing render is hollow-sounding or cracked, it should be hacked off and replaced before dashing. Never dash over painted surfaces without mechanical keying.

The dash must be thrown while the butter coat is still wet and sticky — typically within 10-20 minutes of application. Work in manageable sections (2-3 m² at a time) to ensure the render doesn't start to set before you throw the stones. In hot weather, dampen the scratch coat before applying the butter coat to slow the set.

Traditional UK pebbledash uses rounded flint, quartz, or granite chippings in natural colours (white, cream, grey, pink). Spar aggregate (crushed and washed stone) is the most common type available from builder's merchants. Avoid limestone or soft stones in exposed locations as they can weather and discolour. Recycled glass dash is also available for a modern look.