Lath and plaster walling techniques (see right) were commonly used from the early eighteenth until the mid twentieth century for internal, non-load-bearing walls.
The laths are horizontal strips of wood (normally about 25mm by 6mm (1 by 1/4 inch)) fixed by nails to vertical upright timbers forming the framework of the wall. The laths are spaced to give a gap of about 6mm (1/4 inch) between them – this gap provided the ‘key’ for the plaster coating.
Lath and plaster walls were traditionally covered with three coatings of different lime putty mixtures:
The first coating applied (referred to as the render layer) was applied so that it partly went through the gaps between the laths so achieving a strong bond – this coating was about 8mm (3/8 inch) thick and was left with a unsmoothed surface.
The second coating (referred to as the floating layer) was applied to provide a much smoother surface for the final coating – this second coating was about 6mm (1/4 inch) thick.
The third and final coating (referred to as the setting layer) was about 3mm (1/8 inch) thick and was smoothed off to give a suitable finish for decorating.
The first and second coatings were typically a 1:3 mixture of lime putty to clean sharp sand; often animal hair was mixed in to the mixture to help it bind together. The third layer was typically either just lime putty or a 3:1 lime putty to fine sand mixture.
One advantage of using lime putty for the coating, was that it was fairly flexible so was able to cope with movement of the timber framework and laths. However, each layer could take up to 3 weeks to dry before the next coating could be applied; so the practice developed towards the end of the nineteenth century to add cement or gypsum to the mixtures to decrease the delay between the application of each layer. These new mixtures were typically 1:1:6 (gypsum or cement:lime putty:sharp sand) for the first two coats and equal parts lime putty and gypsum for the final layer.
The technique of using lath and plaster to cover internal walls was generally replaced by the introduction of pre-manufactured plasterboard in the middle of the twentieth century,