What is roughcast?
Roughcast is a coarse decorative render with a heavy, entirely random relief: the surface looks like rough rock or tree bark. It is thrown onto the wall rather than floated flat, so whatever sticks stays as it lands. In Dutch the same finish is called rotspleister; in Scotland the closely related wet-dash technique is known as harling.
What it is used for
- Decoration. The main reason. The technique belongs to nineteenth-century Romanticism and Victorian eclecticism, when natural, “grown” forms were fashionable: artificial grottoes, follies, garden walls, plinths and summer houses.
- Concealment. An unsightly, uneven or patched wall disappears under a coarse texture. A flat float finish reveals every blemish; a rock texture does not.
- Protection. A thick, coarse coat can add resistance to driving rain — provided the background can tolerate it.
How it is applied
Two working methods, often combined:
- Throwing. The coarse mix is flicked quickly at the scratch-coated wall from roughly 30 to 50 centimetres away, with a trowel or a mortar-loaded brush struck against a batten.
- Working. The fresh coat is stippled, dragged or picked before it sets, using a trowel, nail float or brush.
The mix is coarser than ordinary render. Gravel, chippings, coarse sand or occasionally crushed glass are added; the aggregate size determines how coarse the texture turns out.
Where it goes wrong — and what to do
Roughcast almost always sits on old masonry, and that is the problem. Historic roughcast is usually cement-rich, therefore hard and dense. A hard render on soft, porous brick acts like a plastic coat: moisture drawn into the wall can no longer escape outwards and finds the weakest point. The result is hollow, locally detached render and frost damage to the brick behind it.
A practical approach to repair:
- Tap the surface over. A hollow sound means the coat has lost its key; those areas have to come off, even where they still look sound.
- Repair with a softer, vapour-open mortar based on lime or natural hydraulic lime, always on masonry built before about 1900.
- Matching the texture is a craft skill. Closing a cut-out patch invisibly with the same aggregate and the same throw rarely succeeds; work bay by bay instead.
- Check the head detail. On garden walls and follies, water often enters from the top rather than through the face.
Related terms
- Decorative render
- Granol render
- Rough rendering
- Facade rendering
- Render coat
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