What Is Roughcast, Why Use It?
Roughcast 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.