How much pebbledash aggregate do I need per m²?

The amount of dash aggregate depends on the stone size. Fine 3-6mm stones require approximately 5 kg/m², standard 6-10mm stones need 7.5 kg/m², and coarse 10-14mm stones require about 10 kg/m². These rates assume a single even coverage thrown by hand. Always add 15% wastage as some stones will fall and bounce off during application.

What mix ratio should I use for pebbledash?

Pebbledash uses two render coats. The scratch coat (first coat) should be a 1:3 cement:sharp sand mix at 10mm thickness, scratched with a comb to provide a key. The butter coat (receiving coat) is a richer 1:2 cement:sand mix at 6-8mm thickness — this needs to be tacky enough for the stones to stick when thrown. Never use a weak mix for the butter coat or the dash will fall off.

Can I pebbledash over existing render?

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.

How long between applying the butter coat and throwing the dash?

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.

What types of stone can I use for pebbledash?

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.

TradeCalculator

Pebbledash Calculator — Aggregate, Cement & Sand Quantities

Calculate scratch coat, butter coat, and dash aggregate for pebbledash rendering. Correct cement:sand ratios by coat type.

Wall Area

Net area after subtracting openings

Stone Size

Standard UK pebbledash is 6-10mm

Aggregate Price Per 25kg Bag (optional)

Enter your supplier price for aggregate

How We Calculate This

This calculator works out all materials needed for a traditional pebbledash finish, including both render coats and the dash aggregate. It uses industry-standard cement:sand ratios and aggregate consumption rates based on stone size.

The formula

Render weight = Area × Thickness × Density × Wastage factor

Cement and sand quantities are then split according to the mix ratio. For the scratch coat (1:3 mix), cement is 25% of total weight and sand is 75%. For the butter coat (1:2 mix), cement is 33% and sand is 67%. Dash aggregate is simply area × coverage rate per m² for your chosen stone size.

Coat build-up

Scratch coat: 1:3 cement:sand, 10mm thick, scratched for key
Butter coat: 1:2 cement:sand, 6-8mm thick, applied wet
Dash: 3-14mm stones thrown by hand while butter coat is wet

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.