The Preservation Pop Quiz from Friday March 16 asked if you could identify this architectural material:
Quiz subject.
This is located in the bathroom of my 1920s bungalow, and at first glance it looks like subway tile (most people think it is, and with a fresh coat of semi gloss paint, it continues to look like subway tile). There is a chair rail about 4′ up on the bathroom walls; what you see in the picture is below the chair rail, and above is the regular plaster surface. However, the peeling paint gave it away; this was not tile.
The truth is, until yesterday, I was not entirely certain as to this material. It looks and sounds like plaster, but without any holes in the wall, it is hard to accurately compare it to the plaster walls. I was not going to do any exploratory destruction. However, after some searching, I’ve learned that plaster scored to look like subway tile was fairly common for a 1920s bathroom. And it’s the subject of some online discussions (see This Old House).
After reading more on plaster, I came to the conclusion that my plaster walls have rock lath/plasterboard/gypsum board of sorts, meaning that there are only two coats of plaster necessary (brown and finish), as opposed to the typical three of earlier plaster (scratch, brown, finish). From NPS Preservation Brief 21: Repairing Historic Flat Plaster by Mary Lee MacDonald:
Rock Lath. A third lath system commonly used was rock lath (also called plaster board or gypsum-board lath). In use as early as 1900, rock lath was made up of compressed gypsum covered by a paper facing. Some rock lath was textured or perforated to provide a key for wet plaster. A special paper with gypsum crystals in it provides the key for rock lath used today; when wet plaster is applied to the surface, a crystalline bond is achieved.
Rock lath was the most economical of the three lathing systems. Lathers or carpenters could prepare a room more quickly. By the late 1930s, rock lath was used almost exclusively in residential plastering.
So, the answer? The picture shows plaster walls in my bathroom scored to imitate subway tile. The brown and finish coats are scored; beneath them is the rock lath.
Does your house have anything like this? I had never seen it before (or it was done so well that it fooled me into thinking it was tile).
Preservation in Pink 
I have seen this in 1920’s houses in northeastern Massachusetts, but thought it not fair to answer your quiz since I, an architect who works with old houses, am supposed to know about stuff like this.
Unfortunately I have not needed to dislodge it, so I cannot tell you more about its composition than you already know.
Fun to see it and read what you found out.
Hi Jane, You could have answered as an architect! Please always feel welcome to contribute. Sometimes I ask questions because I don’t know the answers, despite the fact that I “should.” The important thing is knowing where to find the answers and who to ask.
I’m hoping to not have to do demolition, and I shouldn’t have to in the bathroom. I figured out the rock lath based on an unfortunate plaster failure in my stair hallway. I’ll get to repairing it eventually.
I recently found the same thing in my 1918 Bungalow. My question is this….one of the previous owners attached drywall to part of the original plaster. Is there a way to remove the drywall without damaging the plaster beneath? Thanks.
I think the first question would be: how is the drywall attached?