Plaster on cement board

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  #1  
Old 02-01-06, 09:04 PM
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Plaster on cement board

I am renovating a bathroom in my 85-year old home, where all interior walls and ceilings are 3-coat plaster over wooden lath. With respect to moisture, is it ok to put an new plaster ceiling over the wooden lath?

I am using cement board on the walls (both around the tub and elsewhere). Is it ok to used a finish coat of plaster (gauging plaster and lime) on the part of the wall that will not be covered with tile?

I plan to paint the plaster ceiling and walls with latex paint. Is that enough protection from moisture.

If the answer is 'no' to any of the above, what would you recommend instead?

Thanks.
 
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  #2  
Old 02-01-06, 10:57 PM
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I would hang drywall instead of trying to learn how to plaster (on a ceiling no less).

For the walls I would stop the CB just before where the tile stops and hang drywall where there is no tile.

Unless you prefer to do things the most difficult way possible.
 
  #3  
Old 02-02-06, 06:42 PM
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bigmtk,
Thanks for your suggestions. I would like, however, to preserve some of the character of my old house. Plastered walls and ceilings are part of that. I understand also that plaster holds up well to moisture, when compared to sheetrock.
 
  #4  
Old 02-02-06, 07:27 PM
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Good advice in earlier posts. If you decide to go with the cement board put on a plaster base coat first. You do not want to try to whitecoat (lime putty and guaging plaster) directly to the cemnt board.

If you want to plaster the lid you have a couple options. You can lath them with metal lath and put on two coats of plaster, a brown coat and finish. You might need to use three coats if there is not enough suction.
Or you can hang veneer plaster base also called blue board and use either a one coat or two coat plaster finish system over that.

Plastering is easy when you watch someone who is good at it do it. It isn't so easy when you are learning. I don't know any plasterers who learned the trade in their thirties or beyond.
 
  #5  
Old 02-03-06, 02:36 PM
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Thank you for your advice, Tightcoat.

If I applied only a white coat on the cement board without first putting a brown coat, would the white coat crack or peel later? A bonding agent would be used on the cement board.
 
  #6  
Old 02-03-06, 04:07 PM
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Maybe, just maybe you could do this if you use a prepared product like UniCal or Diamond finish. I think if you visit the web sites of either USG or National Gypsum that they will tell you they should not be used over Portlanc cement base coats. There might be a clause in there about unless it is over a suitable bonding agent. I haven't looked. Those two materials are made for either two coat, in which case they are used over a plaster brown coat or a veneer plaster base coat, or a one coat system in which they are used directly over a veneer plaster base board, often called blue board and not to be mistaken for green board. I have never tried it over cement board though I have a piece I might try it on. What I think you will find, and if you put it over a bonding agent it will likely be even worse, is that you can't lay it down well because it has no suction. You might try talking to a tile man who has set tile over cement board and ask him how much suction it has compared to a plaster brown coat. If it has comparable suction then you can probably lay it down and trowel it down pretty well. Be sure to tape the joints and let the mud over that set before you do the whole field.
Now there are cement based products that would bond well to cement board and some of them can be trowled smooth.
I, of course prefer plaster. I just think if you have your heart set on plaster you should do it with a tried and trusted system rather than experiment with some misused concoction. Plaster does inded stand moisture a bit better than drywall. It can't stand soaking or staying wet. If you think you need something to withstand a lot of moisture use cement board and tile it up a bit higher or all the way to the top. Other wise you should not have to deal with more than an occasional drop and humidity (Install an exhaust fan). Gypsum drywall will stand humidity as well as you need it to in a bathroom.
About what kind of paint to use: I don't know.

Let us know what you do and how it works especially if you plaster over the cement board. That is an intriguing idea.
 
  #7  
Old 02-03-06, 07:12 PM
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Thanks Tightcoat

Thanks again!

I am tiling directly on the cement board. The plaster finish coat is for the upper part of the wall, which will be painted. Around the tub, I am using tile all the way to the ceiling, which will have 3-coat plaster. I will be priming (using an alkaline-resistant primer -- I don't know which one yet) and painting the ceiling. Any recommendations on a primer?

Based on earlier posts, I suppose I should be using an adhesive for the wall tile, to help serve as a vapor barrier, as opposed to thinset. Any advice on that?
 
  #8  
Old 02-03-06, 10:29 PM
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I'm not a tile man. Seems to me like adhesive would weaken over time and lose its bond. If you use thinset it should not lose its bond tile to a cement based mortar to cement board sounds pretty permanent to me. Why do you need a vapor barrier between the tile and cement board? A vopor barrier behind the cemnt board might be a good idea. Check with some builders about that.
bigmtk's advice about using drywall above the tile is good. If you are dead set on plaster then use blue board and plaster to that or use galvanized metal lath and three coat plaster. It sounds like you don't have a very large area to do so why not do it right. If you are really concerned about damage from water then use Portland cement plaster and do three coat work and a mission finish material to put on the smooth finish. It takes a lot of troweling but can be polished off as well as the whitecoat.
At least if you use cement board put the rougher side out so you get some mechanical bond.

I think you are wanting to do something less than the best if you try to plaster especially without a base coat using whitecoat finish over cement board. But like I said, I haven't tried it. I will when I get a few minutes spare time.
I think you must be bound and determined to try plaster. My best advice to you is if you use plaster then use it conventionally. I have used Diamond finish over a Portland cement plaster brown coat. it isn't reccomended but I did it and got away with it. You might get away with it over your cement board too but unless you have worked with the material a lot you will have to overcome some difficulties.
If you already know how to plaster then why not do it the way you are used to doing it?
 
  #9  
Old 02-04-06, 12:35 PM
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Thanks again.

Tightcoat,
Thanks once more.
 
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