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In a Philadelphia-area house built in the early 1950s: I cut out a piece of wall to fit an old work box. The piece I removed is shown below. The original walls were finished in plaster, but I can't understand what I'm seeing here: There's half an inch of plaster on top of what appears to be a modern-style half-inch paper-faced gypsum panel! Is this consistent with known plaster wall construction from that era? If so, what is it called, what does it consist of, and how was it done?

(Exposed, painted face of wall on left:)

Plaster over gypsum?

(The house has gone through various renovations over the years, so there are places where walls are just modern sheetrock. But I can't imagine somebody putting up gypsum and then plastering over it!)

  • FYI, "sheetrock" isn't a generic term, though it's used that way by laypeople and a few trades folk. "Gypsum wallboard" or "drywall" are generic terms. SheetrockTM is a brand name. – isherwood Aug 27 '18 at 18:53
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The term for that drywall like product to my knowledge has been called "rock lath".

All the demo I have done in remodeling, the sheets are/were 3/8" thick and 16" tall by 4 ft. wide. Before rock lath come along, wood lath was installed on the walls as 4 ft. long by approx 1 1/2" strips. It was nailed up in sections that were 16" wide. This was done over the whole wall or ceiling, so that each 16" X 4' section was staggered over the preceding section below. The picture below illustrates this even though it is a ceiling. It gets the idea across.

enter image description here

The rock lath was installed in much the same way. Some of the makers of it added holes through the rock lath so the plaster would "key in" the same way the spaces the wood lath allowed. I have also seen rock lath without these holes.

The plaster is installed the same way it is done with the wood lath with the exception of the "scratch coat" needed for wood lath. The layers you see are the "brown coat" (even though this is gray) and the white coat on top. White coat is the actual plaster surface that is painted and very durable. The brown coat, to my knowledge is not plaster, but a portland cement based product used to flatten the walls. Older versions on the east coast would have been different, that is how the term brown coat came to be. Nevertheless, the older scratch coat, brown coat and white coat all make up a plaster wall. The rock lath is installed much the same way the larger drywall sheets are done these days with nails.

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What you see there is a hybrid gypsum/plaster from the 1950s. It represents a transitional stage between traditional wood lath and plaster to modern drywall techniques. You'll probably find metal lath at inside corners and metal corner bead at outside corners and door openings. The gypsum panels are 18 or 24" high.

I owned a home with exactly that in it for 17 years and consider it to be the best wall treatment ever devised due to its strength and durability, along with the fact that it can be easily cut to, say, add an electrical outlet.

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Believe it or not, drywall goes back to 1916. At the time, plaster was considered the superior product. I found this article which describes your scenario

With all its uses and benefits, why were builders hesitant to use something as simple as drywall? At the time, drywall was thought of as a cheap fix, with none of the fine art associated with making plaster. People didn't want to live in homes that were shoddily constructed, so they stuck with the tradition and expense of plaster.

While drywall became the preferred material after World War II, it's not hard to see someone with money (and old-school thinking) in the 50's wanting plaster so you get this strange inversion, where they put the cheaper drywall under the plaster.

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