Plastering By Hand With Lath
| Field | Construction |
| Went Obsolete | Late 20th Century |
| Made Obsolete By | Invention of plaster-board |
| Knowledge Assumed | Carpentry, plastering skills |
| When useful | Historically-correct restoration of a building |
Plaster walls in older wood-framed houses were made by hand. First you would nail horizontal slats (lath) to the frame, then build the plaster wall using a coarser mix first, and then a finishing layer of finer plaster. While this skill may not be completely obsolete, it is not in common usage.
This skill is not completely obsolete. I made a good living for a short while in Southern California in the early 1990s doing this for the restoration market.
Original split lath was hand split from boards, giving widths of about an inch and thickness of about 1/4 inch. In about the mid 1920s sawn lath was used - same dimensions but cut by a saw instead of split with a knife or hatchet.
We nailed the lath horizontally to the studs, with about 1/4 or 3/8 of an inch between the lath. This wasn't measured, we just nailed it by eyeball. This space was important because the "brown coat" - the first coat - had to push though the gaps to "key in" the plaster. It would ooze through like the head of a mushroom and when set, hold the plaster to the lath in much the same way the head of a rivet holds a steel plate to a beam.
The brown coat is composed of sand, Portland cement and some kind of fiber for structure and to prevent cracking. Chopped horsehair was used back in the day and we occasionally used this for authentic restorations. The modern equivalent is either chopped polyester fiber or chopped fiberglass. The brown coat goes on maybe 1/2 inch thick.
Once the brown coat is on, keyed in and more or less level but before it's completely set, it's roughed up to accept the plaster coat. This is done with special trowels which score the surface with shallow but rough grooves perhaps 1/4 inch apart.
After the brown coat sets, the plaster coat goes on. This is the "finish" coat. It's comprised of plaster of paris and sand, mainly. Sometimes vermiculite is substituted for sand for applications that need to be light, e.g. ceilings. Additives are sometimes used, latex sap, dyes or pigments, etc. It's spread on over the brown coat to a thickness of perhaps 1/4 to 3/8 of an inch; it takes some skill to get a perfectly smooth surface as it's all done by hand and it's important to bring the plaster "cream" to the surface to push the sand down and get a smooth, almost glossy finish on the final coat.