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Unread 08-23-2007, 09:01 PM   #1
FCollins
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Tile over new plaster walls

Hi,

Great forum, been lurking for several months gathering knowledge to install new bathroom in our house. This will be my first tile project and am feeling more confident after absorbing the wealth of information that is available on this site and the willingness of professional tradespeople to share their expertise and knowledge with DIYers.

Our house was built in 1870 and due to a fire was recently gutted to studs and new 3 coat plaster was installed over rock lathe.

We are planning on putting in the new bathroom (5' x 8') in a landing on the 2nd floor. The walls in this area are all new plaster that is fully cured but has not been primed or painted. The plaster has a hard smooth almost mirror-like finish. I was planning to install a Kerdi shower.

I am looking for advice regarding attaching the Kerdi to the new plaster walls. What kind of surface preparation and adhesive is necessary apply the Kerdi to the plaster? If possible would like to avoid attaching CBU to walls.

Any advice would be greatly appreciated.
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Unread 08-24-2007, 09:51 PM   #2
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Hi Francis, welcome. I would carefully rough up the plaster using a course sandpaper to do away with the mirror like surface. A sander or grinder may be needed. I would then use a lightly modified thinset like Versabond to stick the Kerdi to the walls. You might want to do a small test piece first and check the results.
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Unread 08-24-2007, 10:57 PM   #3
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Welcome, Francis.

Is that a Portland cement-based plaster you have there?
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Unread 08-28-2007, 03:14 PM   #4
FCollins
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Davy/CX,

Thanks for the replies. Sorry it took me so long to respond. I thought if I subscribed to the thread the replies would show up in my email.

Davy, will take your advice about test area. Question: How do I determine that all is good. I guess my worry is that everything will appear fine initially and then a couple years down the road I will start experiencing failures.

CX: I believe the plaster application is all gypsum based. As I'm not able to post URLs yet, if you do a google search for '3 coat plaster rock lath' the first result fairly accurately describes the application that was done except we used rock lath instead of metal lath.
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Unread 08-28-2007, 04:41 PM   #5
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Make one more dummy post, Francis (the post is a dummy, not you ), then you'll be able to post a link.
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Unread 08-29-2007, 02:44 PM   #6
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One more post, ok here it is....
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Unread 08-29-2007, 02:46 PM   #7
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Here is the link that fairly describes the plaster installation I have, except we used rock lath instead of metal lath.

http://www.bobvila.com/HowTo_Library...lls-A1921.html
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Unread 08-29-2007, 03:26 PM   #8
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Well, according to that article, Francis, that's a gypsum-based plaster. Not at all sure just how a fella wants to approach that with a thinset mortar bond coat for his Kerdi. Maybe what Davy has suggested is all you need, but I know with poured gypsum floors there are always primers recommended before tiling or installing membranes.

I'd suggest you call the good folks at Schluter (800-472-4588) and get their take on it.

And I'm not familiar with "rock lath" as opposed to metal lath. Can you gimme more hints? Not that it matters at all, just for my own edification.

My opinion; worth price charged.
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Unread 08-29-2007, 04:24 PM   #9
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CX,

Rock lath is a gypsum board (drywall) designed for plaster installation. The sheets we used were 2'x8'.

Here is link to USG product page for what we used: http://www.usg.com/navigate.do?resou...aster_Base.htm

I'll take your suggestion and talk to Schluter (thanks for phone #) and see what they say, also will call USG.

Other than tiles falling off the wall in the future, I'm not sure what problem I am trying to anticipate.

Assuming a roughed-up gypsum/lime (plaster) surface, is the issue the plaster sucking the moisture out of the thinset, or is it more a concern of the thinset bonding properly to the plaster?
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Unread 08-29-2007, 04:50 PM   #10
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There will certainly be a problem with the gypsum sucking moisture outa the thinset. The problem I see is the gypsum coming off the wall. May not be a problem at all, I'm just not familiar with that kinda wall treatment.

Do I understand correctly that with this rock lath you would only have applied a single finish coat of plaster over the wall boards? Kinda like mudding a sheetrock wall for a slick finish?
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Unread 08-29-2007, 05:36 PM   #11
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No, there are 3 coats over the gypsum (rock lath) board. Scratch coat, brown coat and finish coat. The scratch and brown coat are gypsum based and the finish coat is lime plaster. There is another gypsum board product (blue board?) that is used for just a single veneer plaster finish coat.

The thickness of the walls (including the 3/8" rock lath is between 3/4"-1 1/4". I'm thinking after I rough up the finish coat, I would prime it and then apply the thinset/Kerdi.

If I'm not able to get a definitive answer that this will create a sound installation surface fallback is to install CBU and then Kerdi. Unless absolutely necessary, I want to avoid the CBU.
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