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itcanadian 03-19-2011 01:13 PM

Bathroom Remo: Is this Plaster over Concrete Block? Mounting Backerboard over top?
I'm in the middle of a bathroom remodel. We had a leak behind the tile, and after removing the tile I found a lot of mold all around the tub, I'm glad we're moving forward with this!

At this point, I'm about 60% through the demolition, I'm tearing down the plaster to the studs and intend to install a proper vapor barrier, and backer board to make sure this lasts. Two of the three walls are plaster over studs. The southern wall is a concrete block wall which has what I believe to be plaster over top.

I have a few questions (which I haven't been able to pinpoint after a thorough search):

  • I'd like to put 1/4" cement board over the concrete wall, what's the best way to do this? Should I put a vapor barrier under this cement board? Do I need to remove the paint/old mastic prior to putting up the cement board? Should I forgo the cement board over concrete idea all together?
  • If I do need a vapor barrier under the cement board between it and the plaster over concrete, what's the best way to adhere it?
  • What's the best way to repair the corner where the plaster over concrete meets the studded wall? I'd like to patch it to reverse the damage of the corner to a flush surface before laying the concrete board over top.
  • Along the corners of each wall, there is chicken wire overlapping from one wall face to the next (see pictures). This is true for the concrete wall as well. I'm thinking I'll just knock out the plaster/plasterboard embedded in the chicken wire, and fasten it behind the new cement board. Any thoughts on this? Can I cut it out all together for the walls I'm demo'ing?
Thanks in advance for your help, here are some pictures to help with my questions:

Corner where plaster/concrete wall meets a studded wall:
Plaster over Concrete by itcanadian2, on Flickr

Same corner, different angle:
Plaster over Concrete by itcanadian2, on Flickr

Close up to show texture of what I'm calling plaster:
Plaster over Concrete by itcanadian2, on Flickr

Ron6519 03-20-2011 08:41 AM

Don't you think you need insulation on those walls? You reference Canada in your name. I'm thinking it gets cold where you are.
If you can manage the space, I'd at least furr out the wall. You will need a mechanical connection to the wall with the cement board and it gives you an opportunity to add insulation.

itcanadian 03-20-2011 10:50 AM


Thanks for the reply. I am planning on insulation for the other concrete wall, which is already furred out. However, I wasn't considering it here because I was thinking the south wall (the wall in question here) is mostly an interior wall. After your reply though, I realized that the top half of the wall is an exterior facing wall too. As you can see in the picture below, there isn't a lot of space to work with to furr out the wall a little bit. I suppose I could do what they did on the other exterior walls, furr it out with some 1x2, and attach some 1/2" cement board to that. One thing I like about that approach is it does give me space to install reflectix (insulation) and it also provides a platform for me to mount a vapor barrier to. The vapor barrier is really what was throwing me through a loop, how would attach it to the plaster under the cement board etc...

So, with it somewhat decided that I'll furr out the wall, what mechanism would you recommend to most effectively fasten the studs to the concrete/plaster wall? Drill and tapcon? Is there an adhesive that would work (although I suspect I'd still have to drill/tapcon the upper portion due to the mastic left behind from the tiles on the lower half). Thanks for any feedback you can provide! And by the way, this remodel has just shone a light on a very unfortunate fact: if the bathroom is representative of the rest of the house, none of our exterior walls are insulated, yippy!

Bathtub meets the concrete wall:
Bathroom Remodel by itcanadian2, on Flickr

Just for grins, here's another pic of some oddities we found in one of the walls during some of the demo yesterday. I wondered if this remodel would reveal anything weird...
Treasures found by itcanadian2, on Flickr

That's about an eighth of all the razors we found in the north wall during demolition... Thanks in advance for your help!

Ron6519 03-20-2011 11:17 AM

Old medicine cabinets used to have a slot where you could dispose of old razor blades. I can't tell you how many of thos I've found over the years in customers houses.
By space, I meant moving the tub so the insulation process could be done. A more arduous task, if the drain is in the basement, encased in concrete.

itcanadian 03-20-2011 01:28 PM


When they installed the tub, they plastered around it, the space doesn't exist to remove the tub, furr out the wall, insulate etc. and then installed the tub back in place. The 1-2" taken up by the new furred out wall would prevent the bathtub from fitting back in place. I could cut out portions of the opposite wall's studs, but frankly I'd rather just furr out, around the bathtub, and seal the seam where the cement board meets the tub. Thoughts?

If you were thinking that I build out a standard studded frame over the concrete, that's not going to jive. The bathroom as it currently exists is literally as wide as the bathtub is long. The only reason the tub fits in there right now is because they installed it flush to the concrete block wall, and studded wall, and then applied the plaster to the concrete block, laying it around the tub body.

Thanks for your help!

Ron6519 03-20-2011 02:27 PM

Building out the wall over the tub generally doesn't work unless the tub has a really wide, flat top. As you deepen the wall it gets to where the tub starts to curve down. Solutions are smaller tub, shower only, bath redesign or leave it as it is.

itcanadian 03-20-2011 02:54 PM


Thanks for the feedback. Changing the tub really isn't an option at this point (I'm chewing on it though). Do you think a 1 1/2" addition to the concrete would be significant enough to run into trouble against the tub?

Back to some of my initial questions:

1. I'm now thinking I'll anchor the cement board using tapcon screws, do I need thinset behind it? Can I anchor the cement board directly over top of a vapor barrier? I'm thinking I'll double up with liquid sealant over the cement board to be extra sure. Also, would I have to remove the wallpaper and old mastic before adhering the cement board?

2. If I do mount directly to the plaster over concrete wall, should I repair the corners where the plaster on concrete was ruined, or just let the cement board act as a repair?

3. What are my options for removal or concealing the chicken wire used along the corner seams? Can I cut it out, or simply mount cement board over top of it?

Thanks again for all of your help!

Ron6519 03-20-2011 03:34 PM

Rest a 2x4 on the tub ledge and see what it looks like.

itcanadian 03-20-2011 10:07 PM

I placed a level across the end that meets the concrete wall. It's pretty flat across the width of the bathtub... I think I'll give a shallow frame a shot.


itcanadian 03-21-2011 09:21 AM

So, I'm back to not framing, and just laying cement board over the plaster/concrete wall. The reason being: this wall is about 1/3 exterior, the rest is all interior. While it would be a bonus to add insulation, the added effort to do so would take up space in an area of already limited space, and it seems the gains would be limited given the fact that the rest of the house lacks insulation anyways. So, my thought now is install insulation on the fully exterior wall, but just lay 1/4" cement board directly over the plaster/concrete wall. So, now my questions are refined:

1. How can I sandwich a vapor barrier between the plaster and the cement board? I'd like to have the vapor barrier as an added layer of protection, I intend to water proof the cement board too.
2. Do I need thinset behind the cement board, or are concrete screws sufficient? I'd like to avoid thinset because it would require surface preparation by way of removing wallpaper and either covering or removing old mastic.
3. The rest of this wall doesn't need a vapor barrier, what's the easiest way to mount drywall directly to this type of wall?

Thanks guys!

itcanadian 03-21-2011 12:06 PM

Problem solved, I just got off the phone with Wedi... They said mount their product directly to the plaster wall after removing any brittle plaster sections and flattening it out with thinset. Then, thinset wedi panels directly to the plaster, and support with masonry anchors into the block wall. Voila, waterproofed, slightly insulated, and prepped for tile all in one move without needing to remove wallpaper or mastic. Thoughts?

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