What is this very thin drywall-like wallboard?

Question

I'd like to repair some damage to an interior wall around a light switch box. My home is oldish (early 1900s) and has been updated numerous times, so there's a wide variety of materials and techniques in play in any given wall. As far as I can tell, this wall was originally lath and horsehair plaster, then covered by wallpaper, which was subsequently removed and the wall was painted. But in between the old plaster and the new paint, there's a very thin (~1/8") wallboard that resembles drywall (white and crumbly if scraped at), but is far too thin and has backing paper only on one side. See the photos below (indicated by green arrows). I don't mind just cutting it and using some joint compound to smooth it, but if it's easy to buy another sheet of it, that seems like less effort. Any ideas what it is?

Answer 1

That certainly fits the general profile of transite® or some other asbestos-cement board, with the age of the house, so verifying that it is or is not an asbestos containing product would be advisable before proceeding. There are fiber-cement boards where asbestos is not the fiber, but asbestos was the default fiber until it was removed from most things.

We can't tell that from a picture - you need a lab test, (unless you can read a name off the board itself, which is sometimes possible) and you should shop around as there are labs that vastly overcharge (which probably use some of the vast profits to come up first in search results...)

As for a similar product, there do seem to be 1/8" fiber cement (new products, so definitely not asbestos) tile backer boards available, though they are not as common as the 1/4" and 1/2" versions.

Answer 2

It could very well be thin drywall.

If the underlying wall was rough, it's easier to cover it with new drywall than than to repair the existing wall.