Before plasterboard existed, sawn or riven laths would be attached to timber studs, with narrow gaps in between, and lime plaster would be applied. This created something not dissimilar to plasterboard; a moderate weight partition, reasonably strong, and a smooth surface. When applying the plaster, it squidges between the gaps in the laths, and a certain amount of it drops into the cavity behind. I’d experienced this when on a course; we were each plastering onto a test section of laths, but since we could walk round behind and pick the excess plaster up and use it, I had never really pondered how much material is wasted during this technique when it’s done on actual walls. As part of current renovation works (an ongoing series of blog posts starting here), a section of plaster was removed from laths in the bathroom, and revealed a solid lump of lime at the base of the wall.
Having done some rough calculations, assuming a 3mm finish, it seems that approximately one third of the material has actually fallen to the base of the wall and is not contributing to the actual lath and plaster.
What I don’t know is whether laths had been fixed to both sides of the partition before starting; I am assuming so (and therefore this is the amount wasted when plastering both sides of the wall). If the laths were only on one side, allowing the excess material to be picked up and reused, then this volume of waste is created by plastering just the second side of the partition.
I have no idea if this wastage rate was typical, comments below most welcome if others have experience of this issue.
In the spirit of always showing your workings… Total plastered area of lath (both sides of the partition): 3.4m2. Assumed thickness 3mm, so volume of material forming the wall is 0.02m³. Volume of solid material in base of wall: 0.01m³. So the plasterer would have mixed a total of 0.03m³ of plaster, wasting around one third.