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Unread 01-12-2020, 04:40 AM   #1
SpyGuy
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How would you level this uneven wall?

We are doing TI construction in a commercial space for a small cafe inside a mall. The building was constructed with a vertical elevation change in the concrete floor, creating a natural division between a main room (which will comprise the kitchen and service area) and a back room (which will comprise the office and storage area). The back room is 16" higher than the main room. This 16 inch concrete "step" was formed as part of the building's concrete slab construction.

At this division exists a 2x4 wood stud partition wall sheathed in 1/2" standard gypsum board. This wall is anchored to the elevated floor of the back room at the edge of the 16" concrete step. However, the gypsum and the concrete are not in the same vertical plane: the gypsum is approximately 1/4" proud of the concrete. In other words, the gypsum wall extends ~1/4" into the main room beyond the vertical face of the concrete step. Additionally, the concrete face is not perfectly flat across the width of the room due to forming irregularities when the concrete was poured.

I need to make this dividing wall flush and smooth from floor to ceiling. The plan is to cover the bottom 48" of the wall with FRP (measured up from the floor of the main room) so that the FRP will hide the 16" concrete step and any transition from the concrete to the gypsum wall. But I need to fill out the ~1/4" gap where the concrete step is to bring it out flush with the wall before the FRP is applied.

My thought was to apply thinset onto the vertical face of the concrete step and trowel it flush and smooth with the surface of the gypsum. That will allow me to glue the FRP onto the thinset and the gypsum.

Two concerns I have with the thinset:

1. Will water from the thinset wick into the exposed cut bottom edge of the gypsum and cause it to swell (or otherwise damage it)? Should I prep the edge of the gypsum with masking tape or something like Red Guard to create a waterproof barrier?

2. Sometime in the past, this space was carpeted and the carpet was installed up the face of this 16" concrete step. (That may be why the wall was built to extend out from the face of the concrete: so that the wall was flush with the surface of the carpet.) The carpet has long since been removed but there is still hardened mastic on the face of the concrete. Most, but not all, of the mastic has been painted over with latex paint. This building was constructed in the early/mid 1980's. While I think it's unlikely, there's a possibility that the mastic could contain asbestos so I really don't want to try to remove it. Can thinset be successfully applied over this bare and painted mastic?

What do you think of this thinset idea? Do you have a better solution? Any advice or recommendations would be greatly appreciated.
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Unread 01-12-2020, 05:10 AM   #2
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Hey Michael,

Sounds like there is a lot going on with this project.

1. I can’t say if thinset will be effective in the application but it will not harm the drywall.

You may want to consider mechanically fastening 1/4 material to the concrete to fill that void. CBU with tap cons comes to mind but there are other options I’m sure.

2. The thinset will not be effective over mastic. Asbestos was on the list of known carcinogens in the 80s so little risk of it being in the mastic. You will need to use a scraper or some type of grinder to clean off the concrete.

Good luck and welcome to the forum!
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Unread 01-12-2020, 06:34 AM   #3
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I considered attaching 1/4" CBU panels to the concrete, but the gap is not even nor consistent across the width of the room. I also considered attaching lathe to the faces of the 2x4 studs to enlarge the gap to 1/2" so I could run the 1/2" sheetrock all the way down to the floor. The problem with that being I would still need something in the remaining small gap between the sheetrock and the concrete to give backing strength to the sheetrock. Otherwise, if something bumps against it, the sheetrock will crack.

I thought about putting thinset on the concrete behind the sheetrock for structural support, and it probably wouldn't matter if the thinset adhered well to the mastic because the thinset would be "captured" by the sheetrock. But then I thought, why not just sandwich the thinset between the FRP and the concrete. That would save me from having to put lathe on all the studs.

I realize that asbestos was already a known carcinogen by the time this mall was built. But I've read that products still existed at that time that contained asbestos and that they were still being used in new construction even well into the 1980's. As I said earlier, it's unlikely, but it is possible.
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Unread 01-12-2020, 06:47 AM   #4
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Btw, the reason I don't want to simply furr out the walls with 1x2's is because the space is very small and losing even 1-1/4" from the length of the room will negatively affect the floor plan.
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Unread 01-12-2020, 09:54 AM   #5
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Welcome, Michael.

Were it my project I'd treat the potential for asbestos as a don't ask, don't tell, issue. I'd straighten the bottom of that wall using a long straight edge and either solid blocks of some suitable material or perhaps your thinset mortar (not the best choice), installing such blocking along the bottom of the concrete wall in such a manner as to create a straight, but not necessarily continuous, edge with which to support the bottom of my FRP. Very similar to your capturing dollops of thinset mortar, but I'd want to create the straight edge and let that set up before installing the FRP panels. Attaching the bottom of the panels could then be done with mechanical fasteners or, more likely, just some construction adhesive.

The only potential for disturbing any possible asbestos might be when cleaning a bit of that glue off small areas sufficiently to adhere your blocking. I would worry about that for perhaps five or ten seconds before spraying some water on it and scraping it off.

My opinion; worth price charged.
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Unread 01-12-2020, 12:06 PM   #6
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I appreciate the peace-of-mind given by both of you that there's likely nothing to worry about regarding the potential for asbestos. But I don't think it will be possible to apply water and scrape off the mastic because most of it has been painted over.

I could try a heat gun and scraper.
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Unread 01-12-2020, 01:23 PM   #7
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The easiest way of handling it would be to install a 1/4 inch, or even 1/8, inch sheet of foam board across the bottom. Foam board can be installed over concrete without fasteners. Wedi is what I would use but there are other manufacturers that make thin boards.

Another option would be to coat the concrete with a bonding primer like Mapei's Eco Prim Grip or Custom's MBP primer. Then go over it with a trowelable patching compound like Mapei's Planipatch or Ardex's Feather Finish. There might even be more suitable products that are easier to trowel and don't set up as fast.
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Unread 01-12-2020, 03:41 PM   #8
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The problem I have with mounting a board on the concrete is that the concrete face is neither flat nor parallel with the wall above it. So any angles would be transmitted by the board and I won't get a flush, co-planar surface to mount the FRP. That's why I had the idea of applying a trowelable product to the concrete face.

The Eco Prim Grip sounded good. Mapei says it can be used "over substrates that have a residue of well-bonded old cutback adhesive, floor-covering adhesive or polyurethane adhesive." But later they say, "Thoroughly clean all surfaces of any substance that could interfere with the bond of the installation material, including paint, asphalt, wax, oil, sealers, curing compounds, and poorly bonded or incompatible adhesive."

I don't know if carpet mastic is a compatible adhesive. I suspect it would be ok because it's >30 years old and hard as a rock. But then there's the issue of the latex paint on top of the mastic.
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Unread 01-12-2020, 03:54 PM   #9
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Quote:
Originally Posted by Michael
But I don't think it will be possible to apply water and scrape off the mastic because most of it has been painted over.
The paint will scrape off, too, Michael. The water, just a spray bottle, is simply to keep any potential asbestos from becoming airborne. It's to make you feel better about the scraping. But just scrape/chisel down to bare concrete so you can use some sort of cementitious patching compound to do your flattening/straightening.

You could even drill the concrete and drive a row of flat-head Tapcons to act as guides to support the bottom of your FRP panels as you fasten the FRP. A bit tedius, maybe, but could be made to work.

Lots of ways to skin that cat.

My opinion; worth price charged.
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Unread 01-12-2020, 05:03 PM   #10
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Ah, Ok. I thought you meant to spray water on the mastic to soften it for scraping off.

There were linoleum tiles on the floor in the back room and when I scraped them off, large areas of the tile mastic were still very wet. I had to use water to wipe off the mastic because it was so messy.
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