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Pebbledash roughcast calculator
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Pebbledash / Roughcast Calculator

Calculate render bags and aggregate for pebbledash, wet roughcast, and Tyrolean finishes - 2-coat or 3-coat systems with multiple walls and opening deductions.

Pebbledash / Roughcast Calculator

Render coat quantities, aggregate, and primer for dry dash, wet roughcast, and Tyrolean systems

Dry aggregate (pea shingle or granite chippings) thrown onto a wet dash coat - the most common exterior finish in the UK. Durable and low maintenance once set.
The most widely used pebbledash aggregate. 6-10 mm grades are standard. Rounded profile - key well into the dash coat.
Traditional method: scratch coat keyed, float coat levelled, dash coat receives the aggregate. Best quality and adhesion. Allows more time between coats.
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%
15% standard for pebbledash - aggregate throw wastes more than flat renders. 10% for scraped / Tyrolean.

Wall Sections

Job Costing (optional)

Per 25 kg bag

Results

Net Wall Area
-
m² (less openings)
Scratch Coat
-
bags
Float Coat
-
bags
Dash Coat
-
bags
Aggregate
-
bags
Primer
-
litres
Total Render Bags
-
all coats combined

Wall Breakdown

Wall Gross Openings Net Scratch Bags Float Bags Dash Bags Agg Bags

Finish Types

  • Pebbledash (dry dash) - Dry aggregate thrown onto a wet dash coat immediately after application. The most common UK exterior finish - durable, low maintenance, hides wall imperfections well.
  • Roughcast (wet dash) - Aggregate mixed into the final coat before application and applied together. Common in Scotland, Ireland, and northern England. Slightly different texture to pebbledash.
  • Tyrolean - Machine-applied render flicked onto the wall with a Tyrolean projector. Creates a spatter texture without aggregate. Quicker to apply on large areas.
  • Scraped / textured - Render applied and scraped or combed before hardening to create a textured pattern. Finer finish than pebbledash - can be coloured render.

3-Coat vs 2-Coat

  • 3-coat system - Scratch coat keyed (comb / devil float), float coat levelled, dash coat applied fresh and aggregate thrown. Best quality and longevity. Each coat allowed to firm before next is applied.
  • 2-coat system - Scratch coat goes on, dash coat is applied once the scratch is firm but not set. Faster and cheaper. Adequate for sheltered locations on good-quality backgrounds.
  • On poor, soft, or uneven backgrounds (e.g. crumbling lime render) a 3-coat system is strongly recommended
  • Total build-up: 3-coat ~25-35 mm · 2-coat ~16-22 mm

Application Tips

  • Never apply render in frost, strong sun, or high wind - protect with hessian for 24-48 hrs
  • Dampen the background before scratch coat on high-suction substrates to prevent dry-out
  • Key the scratch coat with a comb or devil float while still plastic
  • Throw aggregate at 45° from close range (~300 mm) with a firm flicking action - too far and it bounces off
  • Aggregate coverage is always wasteful - expect 15-20% landing on the floor; budget accordingly
  • Do not mix aggregate into the throwing coat too early - it will start to set before the aggregate keys in

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