[diy888]

On page 3 here:
http://www.nationalgypsum.com/ng/FILE/plasterresman.pdf

Is the plasterer applying a thick scratch coat to a diamond mesh metal lath that isn't quite visible?
Is one kind of metal lath better suited than another for what the document in the section on Performance refers to as "hollow partitions"?

Would it be appropriate to use felt paper behind the mesh when applying plaster over a hollow partition?

Thanks

 

[AGWhitehouse]

There is a wire mesh there, though it's difficult to see it. Expanded wire mesh is pretty standard and I've personally never seen an alternative used or specified. The hollow partition is essentially standard stud wall system. They are simply saying that using only wire mesh on a stud wall will likely result in cracking. If you apply it to a solid backing, then you stand a better chance at a lasting finish.

 

[diy888]

Thanks for the reply.

Can you use standard 1/2" drywall as the subtrate for a 3/8 inch "brown coat" when you need to patch a large section of "hollow partition"? Or is the "gypsum lath" the document refers to something different than standard drywall?

 

[AGWhitehouse]

it's my understanding that the sheetrock is just a backer and you will still need the expanded mesh with scratch coat. The brown coat cover the scratch coat and gives you the secondary finish. The top coat finishes off the brown coat.

 

[Jackofall1]

I admire your enthusiasm toward restoring a section of plaster wall, but if you don't mind my suggestion, why not just shim some drywall out to the same elevation as the existing plaster and fix or path it that way.

Mark

 

[diy888]

it's my understanding that the sheetrock is just a backer and you will still need the expanded mesh with scratch coat. The brown coat cover the scratch coat and gives you the secondary finish. The top coat finishes off the brown coat.

I've read about what you describe. In our house the walls were built like this in the 1940s:

some kind of gypsum board
brown coat
1/16+ skim coat

There are strips of diamond metal mesh only at changes of plane.

What I'm hoping to find out is whether standard drywall can be used beneath the brown coat, or whether there's a special kind of gypsum board for this purpose. The document I linked to above distinguishes between the metal lath and "gypsum lath". Is "gypsum lath" the same as "drywall"?

 

[AGWhitehouse]

What I'm hoping to find out is whether standard drywall can be used beneath the brown coat, or whether there's a special kind of gypsum board for this purpose. The document I linked to above distinguishes between the metal lath and "gypsum lath". Is "gypsum lath" the same as "drywall"?

That is a question above my current knowledge of plaster systems. Hopefully someone else will chime in an help you on that one.

 

[TrapperL]

No you can't use a regular drywall under the scratch coat. It can't stand up to the moisture that comes with plaster. You can use it if you apply a water proofing over it first like RedGard. You can also use a felt over the drywall for the moisture barrier and then apply the metal lath over the felt but there must be a moisture barrier of some kind in place. There are special sheathings for this application and your supplier should be able to help you choose the right one for your application.

 

[diy888]

Thanks for the helpful information.

 

[diy888]

I admire your enthusiasm toward restoring a section of plaster wall, but if you don't mind my suggestion, why not just shim some drywall out to the same elevation as the existing plaster and fix or path it that way.

Mark

I've done what you suggest above on a couple of sections of wall and it worked well. I furred out the studs so that 5/8" drywall was flush with the adjacent plaster wall. But there are two kinds of sections remaining where that it isn't going to be as easy and straightforward.

1. In places the plaster got torn out in a large curve rather than a rectangle. When I tried to clean up this edge to make a rectangular area, two things happened: immense amounts of dust everywhere if I use a diamond blade in an angle grinder. My wife has just about had it with the dust, and I cannot blame her. If I forego the diamond blade and just try to score the plaster deeply with a knife, it's very hard to keep from ruining adjacent sections of good plaster. Pretty soon I'd have taken off the entire wall. I seem to remember a Three Stooges episode where this happened.

2. Where the underlying framing members had rotten because of a slow leak, and were reinforced with "sistered" studs and joists and Simpson Strong-Tie metal plates with dome-head screws, the framing members in these places are not all flush and are in a variety of planes. The nose end of a joist is proud relative to a sill, or the metal tying plates add thickness here and there. It just seems easier to mask these bumps with plaster over mesh.

 

[Maintenance 6]

The "gypsum lath" you speak of is actually nothing more than drywall. It is available in 16"x48"x3/8" sheets and the scratch (brown) coat was applied directly to it. It was/is commonly known as "rock lath". The paper surface was never meant to be finished like drywall, but to act as a place for the brown coat to adhere. Also, it is common to apply brown coat directly to wire lath (expanded metal lath) without any backer. It is probably better to not have a backer, as the brown coat "keys" through the openings in the lath. On page 3 of the booklet you linked, you see a plasterer applying scratch coat directly to wire lath without a backer. The idea is to set your wire lath deep enough to allow room for a good solid foundation of brown coat. That is what gives the wall strength. The white finish coat then levels out and flattens the wall surface.

 

[diy888]

Also, it is common to apply brown coat directly to wire lath (expanded metal lath) without any backer. It is probably better to not have a backer, as the brown coat "keys" through the openings in the lath. On page 3 of the booklet you linked, you see a plasterer applying scratch coat directly to wire lath without a backer. The idea is to set your wire lath deep enough to allow room for a good solid foundation of brown coat. That is what gives the wall strength. The white finish coat then levels out and flattens the wall surface.

Thanks for the info.

The plaster-on-gypsum-lath has held up well for 60 years in my house. The brown coat that was applied to the gypsum lath board is about 3/8" thick. How thick should a brown coat be if it's not backed by gypsum board and metal lath alone is being used?

By "setting the wire lath deep enough" do you mean instead of stretching it taut across the studs, you sort of tuck it down somehwat into the void, so it's recessed?

 

[Maintenance 6]

A brown coat should be at least 3/4 inch thick. By deep enough, I mean far enough below the finished surface of the wall that you can get a good coat of brown coat on it and still have room for the white coat. No need to create bellies in it. It should be pulled taut.

 

[ebaretta]

all the plaster at my house was applied after making a concrete wall so every plastered wall is studs, wire, concrete, plaster. pretty solid 60 years later.... the only thing that is giving some fits in the past year or so is the plaster is starting to chip in the bathrooms only...the only down side i can find is its not water sealed in the shower. The wood behind all this in the shower was rotted through and through from about chest high down.

the cinderblock walls are cinderblock, concrete, plaster...

if you ask me good places to go for tornado...lol

 

[maxsmoothwall]

The photo shows inidications of wire lath, notice the grey circle overhead of the man. That is overlap of the higher and lower sections of lath. Also to the right of the hawk you will see a grey triangle running along the rafter. This too is overlap of wire lath.

I've used this type of lath, the same as tilemen use (used to use)for mud jobs on walls, to plaster round walls and I have always backed up the lath with sheetrock, masonite, 1/4" luan, felt paper and even cardboard. I have been a drywaller for decades but I am not a plaster expert (though I can smooth plaster like a plasterer). Back it up.