Roughcast Calculator — Thrown Coat Mix Ratios & Coverage

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.

A common roughcast top-coat mix is 1:1:4 to 1:1:5 (cement:lime:aggregate) by volume; lime improves workability so the mix throws more easily. Weber publishes an alternative of 1:2 cement:sand plus 4-5 parts stone aggregate. For a lime-free mix, use 1:4 cement:aggregate with a plasticiser. Whichever you use, the throw coat must not be stronger than the base coat beneath it (NHBC 6.11.6 / BS EN 13914-1). Aggregate is typically 6-10mm rounded or angular stone (Weber recommends no larger than 10mm); coarser traditional roughcast may use larger graded stone with sand fines to fill voids. The mix should be sloppy enough to throw but stiff enough to stay on the wall. These are rule-of-thumb proportions — always follow your render or aggregate supplier’s data sheet.

The roughcast (top) coat is typically thrown at 15-20mm in a single application — 18mm is standard, with 15mm the practical minimum for full stone coverage and 20mm used on exposed walls. Note this is the thrown-coat thickness over a separate base/scratch coat, not the total render build-up. BS EN 13914-1:2016 sets the minimum nominal overall render depth (base coat plus throw, excluding texture) at 16mm for sheltered/moderate exposure zones and 20mm for severe/very severe zones — a 15-18mm throw over a 8-12mm base coat comfortably meets both. Check your local exposure zone (BRE / BS 8104 driving-rain index) and increase thickness in severe coastal or upland locations.

Yes — always apply a base/scratch coat first and allow it to cure (typically 2-3 days before the next coat). Use a gauged cement:lime:sand mix, not neat cement:sand — Weber recommends 2:1:4 (cement:lime:sand) for roughcast/wet-dash base coats, and NHBC 6.11.6 lists designation ii-iv mixes (e.g. 1:1:6 or 1:½:4½ cement:lime:sand) for render backing. The key rule (NHBC 6.11.6 / BS EN 13914-1) is that no coat may be stronger than the background or the coat beneath it — a strong neat 1:3 cement:sand backing shrinks more and is prone to cracking and debonding. Scratch (comb) the base coat to form a key, apply it about 8-12mm thick, and dampen blockwork or brick beforehand to control suction. Throw the roughcast coat no stronger than the base coat below it.

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.