Roughcast
Print this adviceThis article outlines how to repair this long-used protective wall covering, which is not to be confused with pebbledash.
What is roughcast, why use it?
Roughcast (also called ‘wet-dash’ in Cumbria and ‘harling’ in Scotland) is a render with a coarse finishing coat containing gravel thrown in a premixed state onto a wall. It is distinct from pebble dash (or ‘dry dash’) in which the aggregate is applied separately onto wet plaster. Historically, roughcast was popular for rural buildings across much of the UK, except the south-eastern half of England. It was also sometimes employed for high status Arts and Crafts buildings.
Compared to smooth render, roughcast is particularly advantageous in exposed districts because its heavy texture creates a larger surface area that aids moisture absorption and evaporation. Additionally, the application method allows use of a mortar mix less prone to shrinkage while curing.
What mixes are suitable for roughcast?
When patch-repairing, materials should be selected to ensure the existing roughcast is matched in strength and appearance, unless an inappropriate mix was used previously. In most cases, existing roughcast can be adequately assessed from visual inspection coupled with local knowledge. Lime (not Portland cement), gravel and sand are typical ingredients suitable for use on a pre-1800 building but other constituents might include, for example, earth, stone dust or brick dust. Hair is sometimes added to the underlying coat(s). The more water used in the finishing coat, the less heavily textured it tends to be.
When not replicating an existing mix, new roughcast must take into account site exposure and the strength and condition of the wall below.
How is roughcast applied?
Before applying new roughcast, remove detached render beyond saving. Loose edges repeatedly dampened become sufficiently pliable to resecure with lime slurry grout (and, if needed, screws and washers); the new finishing coat is ‘feathered’ over the reattached edges. It is unnecessary, in most cases, to rake out mortar joints, and always inadvisable to hack masonry faces, to provide a key for new roughcast.
Several thin coats are better than one thick one. The underlying coat(s), as well as the finishing coat, may be roughcast, rather than laid on smooth, if preferred. Roughcast can be machine-applied but hand-casting is as quick for an average-sized house. One method is to throw the mix ‘backhand’ from a casting (‘dashing’) float or cut-down coal shovel. The texture can be modified by wet or dry brushing as the surface firms up. Allow for weather protection.
What is roughcast, why use it?
Roughcast (also called ‘wet-dash’ in Cumbria and ‘harling’ in Scotland) is a render with a coarse finishing coat containing gravel thrown in a premixed state onto a wall. It is distinct from pebble dash (or ‘dry dash’) in which the aggregate is applied separately onto wet plaster. Historically, roughcast was popular for rural buildings across much of the UK, except the south-eastern half of England. It was also sometimes employed for high status Arts and Crafts buildings.
Compared to smooth render, roughcast is particularly advantageous in exposed districts because its heavy texture creates a larger surface area that aids moisture absorption and evaporation. Additionally, the application method allows use of a mortar mix less prone to shrinkage while curing.
Presumably, I should retain old roughcast?
Early lime-based roughcast is becoming rarer and should be retained on walls wherever possible. If it has deteriorated, repair is often possible but investigate the reason for any defects first to prevent, or reduce the likelihood of, their recurrence.
Deterioration, which is often localised, may result from, among other things:
- Defective weatherings (such as copings, tablings and string courses)
- Dampness, notably at the foot of walls.
- Exposure to the prevailing weather.
Unlike with cementitious renders, areas of old lime-based roughcast that are hollow-sounding and not fully keyed to their backing do not automatically require replacement. Especially ancient and fragile roughcast that is slightly loose can frequently be consolidated and grouted by a conservator.
How is roughcast best detailed, what should I use to paint it?
If there is no plinth on the ground storey against which to finish the roughcast, take it down to ground-level. Historic precedent should normally be followed with repairs when deciding how to finish roughcast at openings, corners and mouldings etc. For instance, much early roughcast on masonry dies out onto dressed quoins whereas later examples wrap around corners. Sometimes imitation quoins were pressed into roughcast at the margins. Where no roughcast already exists, detailing is affected by the building construction (for instance, presence of raised quoins).
It is often appropriate to limewash lime-based roughcast. When repairing roughcast, decide early whether any finish is to be applied because the need for good colour matching if left uncovered will influence material selection.
English Heritage (2011) Mortars, Plasters and Renders, Practical Building Conservation, Farnham: Ashgate Publishing Ltd
