What is the difference between roughcast and pebbledash?

Roughcast (also called harling in Scotland and wet dash in Ireland) is a render where the aggregate is pre-mixed into the mortar and thrown as a wet mix onto the wall. Pebbledash involves throwing dry aggregate onto a wet receiving coat. The result looks different — roughcast has a more uniform, embedded texture, while pebbledash has individual exposed stones sitting proud of the surface.

What mix ratio should I use for roughcast?

The traditional roughcast mix is 1:1:4 to 1:1:5 (cement:lime:aggregate) by volume. The lime improves workability and allows the mix to be thrown more easily. For a lime-free mix, use 1:4 cement:aggregate with a plasticiser. The aggregate should be a well-graded mix (6-20mm) with some sand fines to fill voids. The mix should be sloppy enough to throw but stiff enough to stay on the wall.

How thick should roughcast be applied?

Roughcast is typically thrown at 15-20mm thickness in a single coat. The minimum recommended thickness is 15mm to ensure full stone coverage. Standard applications use 18mm. For exposed or high-traffic areas, 20mm provides better durability and weather resistance. Unlike smooth render, roughcast does not require multiple coats — it is a single thrown application over a scratch coat.

Do I need a scratch coat under roughcast?

Yes — always apply a scratch coat (1:3 cement:sand at 8-10mm) first and allow it to cure for at least 24 hours. The scratch coat must be combed or scratched to provide a key for the roughcast. On blockwork or brick, dampen the wall before applying the scratch coat. Without a proper scratch coat, the roughcast is likely to debond, especially on smooth substrates.

Can I repair patches of roughcast?

Patch repairs to roughcast are possible but achieving an invisible match is difficult. Hack out the damaged area back to sound substrate, apply a scratch coat, and throw new roughcast to match the existing thickness and texture. The main challenge is matching the aggregate colour and size — try to source the same stone type. New patches will look lighter until they weather over 6-12 months.

TradeCalculator

Roughcast Calculator — Thrown Coat Mix Ratios & Coverage

Calculate roughcast (harling) materials: thrown coat mix ratios, aggregate grading, and coverage by throw thickness from 15-20mm.

Wall Area

Net area after subtracting openings

Aggregate Grading

10-14mm is standard UK roughcast

Throw Thickness

18mm is standard; 20mm for exposed walls

Aggregate Price Per Tonne (optional)

Enter your supplier price per tonne

How We Calculate This

This calculator works out the quantities of cement, lime, and aggregate needed for roughcast (harling) rendering. Unlike pebbledash, roughcast is a wet-thrown mix where the aggregate is pre-mixed into the mortar before throwing.

The formula

Mix volume = Wall area × Throw thickness × Wastage factor

Total weight is calculated from the volume using a wet mortar density of 2,000 kg/m³. The weight is then split by the mix ratio — for example, a 1:1:5 mix (by weight approximately 15% cement, 5% lime, 80% aggregate) gives: 100 m² at 18mm = 1.8 m³ × 2,000 = 3,600 kg × 1.20 wastage = 4,320 kg total mix.

The calculator also estimates coverage per mixer batch (based on one 25kg bag of cement per batch) to help plan your work rate.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.