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Not lath and plaster, not modern drywall

My house was built in 1951 in Madison, WI. When we originally bought the house we thought the walls were lath and plaster. The walls are bumpy, cool to the touch, and very thick.
When we did our basement remodeling we discovered that there was no lath and that they appeared to just be giant, thick, dense boards. Not quite a full inch thick, but definitely thicker than modern drywall and not smooth and papery on both sides. It's mounted on 16" on center studs in the walls.
One exterior wall has a rather giant crack that needs to be repaired, and I'm planning on widening a closet door opening in a couple weeks.

So 3 questions:
1. What is this 1950s era plastery-board stuff?
2. Do I patch it like it was plaster? We installed curtain hangers and it took out huge chunks of "plaster" and nails tend to do in with more difficulty than in drywall.
3. Once I've re-framed a new door opening, clearly drywall will be much thinner than what currently exists, what material(s) should I use for having consistency in transition along the wall?

Thanks in advance for any advice!

Re: Not lath and plaster, not modern drywall

IT may be Rock Lath which is 3/8 inches thick with 3/8 inch plaster applied.
The bumpy part may be thicker. If it is rock lath check the back side it would be 16 inches X 48 inchec.
Also it may be what was known as Button Board which would have holes for keying of the base coat plaster these holes would be about the size of a half dollar also rock lath could have a foil backing.Rock lath was very popular in the 50's.

Re: Not lath and plaster, not modern drywall

Next question:

I need to surgically cut through this stuff to gently remove bits of the wall and not risk cracking the walls.

Dremel tool or rotozip?

Re: Not lath and plaster, not modern drywall

Your best tool would be to use Carbide burs they make very nice cuts in plaster walls and ceilings.
Select the bit that best fits your needs. You can get bits that will plunge cut or cut very straight edges.
Some will under cut the plaster edge.
Cost is $ 5 to $ 25 per bit

Re: Not lath and plaster, not modern drywall

I have rock lathe and plaster in my 1940 house and I'm gradually getting used to working with it. I have been installing new outlet boxes and I have not had any problem with causing significant cracks due to my work. I use serrated cutting blade tool I bought at Home Depot and its designed so you can hammer on the end of the handle to drive it into the wallboard.

I just mark where I want to cut, and then drive the blade in about 1/4" to 12/" inside the line. Any cracking is usually not more than 1/4", so its within the area I plan to cut away. From there I just use the blade to saw out the hole and then smooth it out after I am done.

I have also found the plaster layer to be different thicknesses for inside and outside walls. The outside walls and some inside walls are a full inch thick, while other inside walls are only about 3/4" thick. I found the thinner plaster layer and 2x3 framing on closet walls, so maybe they just thought that is all that was needed.

You can easily patch the plaster with the dry plaster mix from Home Depot. However, its not fun to work with. Your working time is only about 8-12 minutes and you can't extend that time by adding water. Just about the time the consistency feels right it starts to harden too much and it may ball up when you try to trowel on a layer. However, each coat dries quickly, so its easy to put on multiple thin layers.

If the cracks are big you can dig out the area around them and use fiberglass tape to reinforce the new plaster.


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