Firstly, a base or “scratch coat of sand and cement is applied this may contain additives such as an integral waterproofer, PVA or latex. Once cured this base coat is normally saturated with water to reduce suction, this reduces the rate of water absorption from the additional render coat. On top of this base, a second coat or “butter coat” is applied. Into this coat (whilst it’s still wet) is thrown the dry dash or aggregate. Some trades then tamper and press the aggregate into the render with a steel float to reduce shedding (common in Spain). A roughcast or harling finish is achieved by mixing the aggregate into the actual render mix and throwing it or “casting” the mixture onto the wall.

Pebbledash and roughcast was a common finish on pre-war houses. Nowadays advances in render technology have allowed for renders that are made specifically for pebble dashing. They are known as dash receivers. Usually, polymer-based they can be coloured to match the aggregate, i.e. white coloured dash render for a white dolomite pebble. Dash receivers are less prone to shed off the aggregate but some loss of pebble will still happen to a degree.

Rendering over existing pebble dash can present problems. If the backing render coats are not solid and they fail, then whatever is applied to it will also come away.

Also if the pebbledash has been painted a good key may not be possible. Cracks can form in the old pebble dash layer and appear through the new finish. Definition and neat angles can be difficult or impossible to form, detail such as corners, window frames, stone or brick features etc can get swallowed up and buried in the new render finish. Old metal rusting corner beads that may have been used in the original dash render can quickly spoil any new render or paint you apply.

If the pebbledash is of a concrete material (which it mostly will be)  it will not breathe, on solid walls, this may cause problems, going over the top of the pebbledash or render finish even in a vapour permeable material such as lime render or a breathable coating will not allow the wall to breathe as the existing pebble dash or render is unable to do so.