Pebbledash / Roughcast Calculator
Render coat quantities, aggregate, and primer for dry dash, wet roughcast, and Tyrolean systems
Pre-mixed render typically sold in 25 kg bags
Pea shingle / chippings typically 20 or 25 kg bags
1
Scratch Coat
Typically 10-15 mm. Keyed surface for float coat adhesion.
At 12 mm: ~1.2 m² per 25 kg bag of sand/cement render
2
Float Coat (3-coat systems only)
Typically 6-10 mm. Floated smooth to receive dash coat.
At 8 mm: ~1.8 m² per 25 kg bag
3
Dash / Throwing Coat
Typically 5-8 mm wet. Aggregate thrown in while coat is fresh.
At 6 mm: ~2.0 m² per 25 kg bag
Pebbledash: typically 5-8 kg/m². Roughcast: aggregate mixed in - leave at 0 if pre-mixed.
Bonding primer applied to substrate before scratch coat. Typically 5-8 m²/L.
High-suction substrates (aircrete, lightweight block) may need 2 coats.
15% standard for pebbledash - aggregate throw wastes more than flat renders. 10% for scraped / Tyrolean.
Wall Sections
Job Costing (optional)
Per 25 kg bag
UK Market PricesFills in the material cost fields with current UK market prices. You can review and edit each price before applying.
Results
Net Wall Area
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m² (less openings)
Scratch Coat
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bags
Float Coat
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bags
Dash Coat
-
bags
Aggregate
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bags
Primer
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litres
Total Render Bags
-
all coats combined
Wall Breakdown
| Wall | Gross | Openings | Net | Scratch Bags | Float Bags | Dash Bags | Agg Bags |
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Frequently Asked Questions
How much pebbledash aggregate and render do I need per square metre?
Scratch coat (1:3 mix, 15 mm depth): approximately 25 kg/m² dry materials. Aggregate: 8-12 kg/m² for 6-10 mm pebbles; 12-18 kg/m² for 10-14 mm chippings. Add 10% waste - aggregate inevitably falls to the ground when thrown and some is always lost.
What is the difference between pebbledash and roughcast?
Pebbledash: pea gravel or chippings thrown onto a wet buttercoat and pressed in - stones proud of the surface. Roughcast (wet dash): aggregate mixed directly into the render before application - denser, more even texture. Both are traditional finishes common in Scotland, Northern Ireland, and northern England, highly durable and weather-resistant.
What mix should I use for the pebbledash base coat?
Scratch coat: 1 cement : 3 sharp sand at 10-15 mm. Buttercoat (wet layer for pebble adhesion): use a weaker 1:4 or 1:5 mix - too strong a mix cracks and won't embed pebbles well. For improved workability and crack resistance, add lime: 1:0.5:4.5 cement:lime:sand for the undercoat is a traditional specification.
How do I repair or patch pebbledash?
Chip out loose or hollow areas back to sound render. Apply bonding agent or PVA primer. Re-render in two coats to match existing depth. Throw matching aggregate into wet buttercoat and press in. The patch will be lighter than the surroundings for several years until weathered. Where colour matching is critical, re-rendering the full elevation may be preferable.
Can I paint over pebbledash?
Yes - use exterior masonry paint applied by brush worked into the surface. Coverage is only 3-5 m²/litre on rough pebbledash vs 8-12 m²/litre on smooth render. Two coats always required. Note: once painted, pebbledash is difficult to return to unpainted finish and will require regular repainting thereafter.
What waste allowance should I use for pebbledash materials?
Plain rectangular wall: 10%. Many openings and reveals: 15%. Mix only as much buttercoat as can be used in 30-45 minutes - it stiffens quickly and pebbles won't adhere to a set surface. Order aggregate from the same batch for consistent colour across the full elevation.