Could you please describe the components of a typical plaster wall?–or a good reference book!
- I removed molding from the base of a wall and got a peek into the structure. I expected to find plaster on wood lath, but instead saw a thick layer of concrete covered with a thin layer of plaster. The opening is only 4″ high, so I can’t tell if the entire wall is the same plaster-on-concrete.
- A small section of the concrete had been removed to make room for chunks of wood. The molding had been nailed to the wood.
- There is metal lath behind the concrete, with a sheet of something shiny and black behind it.
- A piece of plaster chipped off the surface of the wall, revealing another layer of plaster that is smooth (smoother than the surface layer, in fact) and has a pencil mark for aligning the molding. Does this indicate re-plastering that took place some time after the house was finished?
How thick is a typical plaster wall? Is more than one layer applied? Are they usually all plaster, or concrete with a skim coat of plaster? When did they start using metal lath instead of wood?
Janet
Replies
Also, how is molding attached to a plaster wall?
I do a lot of remodeling on old houses and usually what I find is plaster on wood lathe. It sounds to me what you have is a newer system that has a base coat a scratch coat and a skim coat. Usually molding is attached to wood strips like you have seen otherwise I use powergrab adhesive and predrill through the plaster and use trim screws. I have never seen concrete used as a wall covering so you are probably looking at the base coat, do you have wood studs underneath?
There are many styles of (more or less) "old fashioned" plaster. The oldest style of lath is wood strips, of course, but "expanded metal" mesh or "rocklath" (gypsum board) might be used instead (with several variations of each). (Actually, the oldest plaster used twigs for lath, but that would only be seen in very old buildings.) And the plaster may be just a thin coat over one of the gypsum board products, or may be two or three layers over the wood or mesh. Where there are at least two coats (which is fairly normal), the first is usually a cement-based coat (though in the oldest plaster portland cement was unavailable and it's apt to be horsehair plaster instead).
> A small section of the concrete had been removed to make room for chunks of wood. The molding had been nailed to the wood.
The cement-based first coat of plaster doesn't take nails very well, so various tricks such as you describe might be used to facilitate nailing.
> with a sheet of something shiny and black behind it.
Likely some sort of tarpaper. Not sure why it would be used behind interior plaster (with no sheathing behind), but it's commonly used behind stucco.
> revealing another layer of plaster that is smooth
Could have been replastered (or simply patched) at some time, or you could just be seeing a bunch of coats of paint peeling off. And if the original plaster was painted with "milk paint", someone may have later skim coated it with plaster, since regular paint won't stick to "milk paint". (But "milk paint" seems unlikely if the old plaster is "new" enough to have been on expanded metal mesh.)
If you are looking at the very bottom of the wall, it's possible that the wad of concrete you are seeing is actually just a bunch of base coat plaster that fell to the bottom of the wall when the plaster was squeezed through the lath on the first coat.Sounds like a wire-lath plaster job. What's the approximate date of construction?STeve
I've lived in two '30s houses in my area, and they all had a form of cement board under the plaster layers. I wouldn't assume there to be wooden lath underneath the plaster much after the 1920's.
I'll betcha that if you were to go around the country you'd find all sorts of variations as to which technique was used when. Workers would tend to copy the techniques of the folks they learned from, plus there'd be variations based on local conditions and materials availability. Likely even the qualities of the local sand figured in at times.
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How thick is a typical plaster wall?
- Depends on the age of the house and lath type - but normally you'd expect around 7/8"
Is more than one layer applied?
- Yes - normally brown coat, scratch coat and finish coat
Are they usually all plaster, or concrete with a skim coat of plaster?
- You may be mistaking a cementitious plaster brown coat for concrete - some plaster was cement-based (more like stucco).
When did they start using metal lath instead of wood?
-1920's-1930's
Also, how is molding attached to a plaster wall?
- typically nailed to studs right through the plaster
Jeff
Edited 11/8/2009 10:22 pm ET by Jeff_Clarke
Jeff, you were right about it being brown coat instead of concrete.
A search on "brown coat plaster" turned up a lot of useful information from the National Park Service, of all places (when I hear "national parks," I think of Yogi Bear, not Technical Preservation Services). They offer about fifty free publications on maintaining and restoring historic buildings.
Okay, our house is hardly historic, but Repairing Historic Flat Plaster Walls and Ceilings contains a lot of information that I can use.
Janet
Thanks - the NPS Technical briefs are very good.
Jeff
It varies from region to region and builder to builder, and the transitions happened over many years. I've seen wood lath used as late as the early 1950's. About that time, they even had radio commercials for "genuine lath and plaster". There was also "rocklath" -- gypsum board sorta like drywall, only with holes thru it to key the plaster. It went up in strips about 12 - 16" wide, with gaps again to key the plaster. My 1926 place had this in the stairwell and around the water heater, with wood lath for the rest of the flat work, and expanded metal where they needed fancy curves.
-- J.S.
I've heard what I have called "rock lath," so that's probably it. Down side is that plaster washers don't work as well on rock lath ceilings to secure plaster as they would on wood lath.
Lots of variations on rock lath. They did my parents' house with it ca 1970, using panels that were something like 18x32, with no holes or other features for "keying". Then 3-coat plaster over that.It was interesting watching the guys hang it. The only tool they used was something like a shingle hatchet. A pair of guys could do an entire room in maybe twenty minutes. (Of course, then someone else had to spend twice that much time carrying out all the bits and pieces on the floor.)In the building where I used to work (built ca 1960) they had a different system. Panels about 32x120, attached vertically to steel studs made from wire welded together like a baker's rack. The attachment was via some metal clips. Then about half an inch of plaster over that.
This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in. --Theodore Roosevelt
IIRC, they still make something similar. It's basically a wet version of drywall. Gypsum between two layers of heavy paper, only one surface is coated with a bonding agent like the PVA used for concrete, intended to bond to traditional wet wall plaster. It's sort of lath, scratch, and brown coats in one piece, over which you make a real plaster surface.
-- J.S.
Yeah, now when you get "real" plaster it's generally a skim coat over "blueboard" or some such -- 4xwhatever sheets hung like drywall.
This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in. --Theodore Roosevelt
Yeah, now when you get "real" plaster it's generally a skim coat over "blueboard" or some such -- 4xwhatever sheets hung like drywall.
This is veneer plaster - a really superior finish to drywall and about 6 times stronger. In some areas, like Boston, it is competitive in cost too.
Jeff
Here we're not allowed to do "real" lath and plaster. You can patch a wood lath wall, but if the wood lath is taken down, you have to put up that blue board stuff instead of replacing it. That, or ordinary drywall.
-- J.S.
I can see why it wouldn't be done (too expensive an no one knows how), but it seems odd that it would be verboten.
This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in. --Theodore Roosevelt
I removed some paint down to the plaster and am surprised at how smooth and shiny the plaster is. I had expected to see a matte finish. Did they do something to give it the burnished look, and why?
Janet
I've seen that "polished" look also. I assume that it's a result of the technique used by the plasterers. I rather like it and would love to see an entire unpainted wall with this finish, but I've never seen it on a wall that doesn't have numerous cracks.
I think the "polished look" may simply be the result of the plaster somehow interacting with the paint for decades.
This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in. --Theodore Roosevelt
The polished look of plaster is done through the final troweling process. This is done with a large very fine spatter brush and clean trowel. Using two hands, the trowel follows the wet brush. There's a point where the plasterer should stop though as not to make the surfacr too fine where paint will not adhere as well.
The lime-putty mix can give this type of surface easily if you know how. The new veneer plasters don't give way to it. You can even buff fine white plaster after it's cured and just leave it. I have also seen it waxed. I think it's a great look.Venetian plaster is a cool shiny surface that has a lot of depth to it with varying hues. It's a different technique altogether than regular plastering.
I've seen this polished surface look done with exterior stucco. It looks fine for several months, then you see cracks everywhere.
-- J.S.
I think maybe because the wood lath shrinks and the plaster therefore comes loose and cracks. Having demoed both wood lath and rocklath walls in the same 1926 building, I can say for sure that the rocklath holds up better over the long haul. In a fire, once the plaster falls away, the wood lath is a large amount of excellent fuel with lots of air around it.
-- J.S.
I can see the fire issue. Whether it falls apart or not has to do with the artistry of the plasterer, and I don't think you can codify that.
This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in. --Theodore Roosevelt
Back in the late 70's when I learned how to plaster, they still sold rock lath. I still have my plastering hatchet somewhere.Best plaster wall was black ribbed mesh lath with the three coat. I haven't seen that for sale in years, Or maybe they don't sell it around these parts.
Plaster's the best wall to have, very strong, hard, doesn't ding so easily, and if the mechanic was good, the corners would be really square, unlike the buildup you get from spackle.
The shiny black stuff was a vapor barrier to help the plaster cure better and also it provides a backing for the scratch coat "keys" (wet plaster that pushes through the other side of the mesh), to push up against and stay on the lath and not drop off inside the wall.
Typically they nailed wood strips to the studs and ran them along the top & bottom of the walls. These were called 'grounds' that were the thickness of the plaster from the stud face to the finished surface. when doing the scratch coat, they would run a screed rod along top of these grounds and the plaster in between was "rodded off" giving you a flat consistent brown coat on which to run your white coat. It's the same thing when you throw a concrete floor. The white was anywhere from 1/16 to 1/8" thick.
Later, the baseboard was nailed to the grounds that were left on and if there was to be a crown molding of any type, the upper grounds were left on as well, otherwise it would be removed and filled after the brown coat had cured. You can mix a little dry lime to a brown mix to set it off quickly (45 min).
Brown coat plaster was the same material basically, made from baked gypsum. It was grey and had a rougher,grittier consistency than the white guage plaster you usually see on the surface. Brown coat plaster was mixed with sand or perlite as an aggregate. The scratch coat of this plaster is often mixed with fibers of some sort for strength. In the 18th & 19th century, they used straw or horse hair for this.
The white guage was mixed with a lime putty and water usually in a 3:1 to produce the fine finish coat. Nowadays, the lime-guage is not used so much as is a white "veneer plaster".
There are a million different plaster finishes and almost as many types of plaster.
Good descriptions from Dan and Jercarp of the varid plaster wall systems found in old, and not so old buildings.Strengthing fibers mentioned by Jercarp are sometimes asbestos. I have found asbestos in both the scratch or brown coat, and the finishing coat. Sometimes in either coat but usually in the scratch coat for hard plaster walls and ceilings. Acoustical (soft) plasters typically have asbestos in the finish coat. Asbestos was used in plaster mostly from the 1930s to the 1970s. Older buildings usually have scratch coats with horse hair or wood fibers.The vapor barriers may also be ACM.It is imperative to have plaster tested to avoid the significant exposure risk when working with asbestos plasters.Bart
I demo'd a house waaay up north in Michigan. It was plaster on lath and having done this many times before, I hit the wall with the sledge anticipating the normal result of such action.........nothing, barely a crack.
I found out why once I finally got through the plaster, the whole house was lathed in rough sawn 1x4's.
I guess Pine was really plentiful up there 90 years ago.
Family.....They're always there when they need you.