Pebbledash and roughcast are forms of render in which the top coat is roughly textured by pebbles or stone fragments. As the terms are used today, they each have different meanings. For pebbledash, clean material is thrown at the freshly plastered surface then pressed in, so the colour of the material is visible. For roughcast, on the other hand, this material is mixed with mortar and then thrown at the surface, so all the material is coated with the mortar. This produces a slightly softer texture and the surface is usually limewashed.
A contemporary account of roughcast (or pebbledash). The wall would first have been given a coat of ‘strong-haired coarse stuff’, that is to say a mortar of lime or hydraulic lime and aggregate with a high proportion of animal hair. This would then have been scratched to provide a good key. Next, when this coat had dried, a second coat of the same material, ‘well knocked up and of even consistency’ would have been applied, laid to an even surface ready for the shingle or other material to be dashed on. The material, he advised, should be well washed, passed through a quarter- to half-inch sieve, mixed with ‘hot lime (hydraulic for preference)’ and water in a tub. This suggests that quicklime was slaked with the pebbles, shale or gravel in it. When the second coat of render was ready, the material would then have been thrown quickly and evenly onto the soft surface using ‘a “scoop” or hollow trowel’, starting from the top and working downwards. The principal component of the finished surface is thus the pebbles or stone fragments with a thin coating of binder.
For exposed environments Millar recommended the use of Portland cement rather than lime. Examples from this period which have survived would suggest that this approach was commonly followed, although it seems likely that most examples from the Edwardian period onwards contain Portland cement either on its own or as a gauged lime mortar.
K Rend approved