How To Prep A Wavy Cement Slab For Tile

Nick S.

I know i cannot use only adhesive to bind backer board to the slab. Should i level out the existing slab and then use masonry nails and adhesive to bind the backerboard? Any help would be greatly appreciated.

Mike Finley

If it is just a few places or small I would use thinset. I wouldn't be installing back board on a slab.

mdshunk

I was on a commercial job a while back where they had a use a terazzo grinder to shave off a few high spots before tiling. I'm not sure how bad your deal is or if that's even an option for you.

R&D Tile

Once again like Mike said, No CBUs over slabs, they won't level anything either, for small depressions, maybe some modified thinset, otherwise use SLC, then a crack isolation membrane if there are cracks in the slab, also, make sure the slab isn't painted or sealed.

Nick S.

Thank you all. After what u have told me I intend to use a chisel to bust down a high spot, and then use thinset on the low spots. Should i scar the slab up a little bit for a better bond?

R&D Tile

As long as water doesn't bead up on it, you'll be fine, no cracks right?

Nick S.

no cracks . Is it necessary to scar the slab or not?

Mike Finley

No.

Another issue is control joints.

Nick S.

The slab is the entire bathroom floor and it is about 7x5. How many control joints should i cut? And where?

R&D Tile

NONE, leave a gap around the room 1/8" for expansion and cover with tile or molding.

Mike Finley

None.

I leave a 1/8 inch gap because I suck, and it's going to be covered by molding. Now I can use a real excuse, thanks R&D!

747

If your going to tile over that cement. You should do a old fashion mud job before you lay the tile. This way you can level it perfectly.

R&D Tile

That would be fine if they knew how, it also adds 1 1/4" height to the floor, they would also have to know how to bond it to the slab or use slip sheet and lath, for a such a small area, SLC would work better and be less labor for a first timer attempting this, yes, mud is cheaper to do, but alot more involved, just my .02 worth.

Eric K

Another technique you can use is use the modified thinset and use a skreet board as long as the room and skreet the low spots out. Kind of like smoothing out cement at first.

747

I was thinking more like seeing how he kind of messed the concrete up it is salvageable if he had a oldtimer come in and do a mud job. Key being oldtimer tile man. I had a tile floor put down in my bathroom got a oldtimer to do it. He did a full mud job first made it look easy

Nick S.

when i leave the 1/8 gap do i grout that or leave it open? also what is SLC. Thanks

Teetorbilt

Self Leveling Concrete. It can save a lot of grinding dust.

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Mike Finley

If you are leaving it as an expansion gap then you wouldn't want to grout it right?

We silicone it as another preventative measure for water infiltration into the sub floor.

ApgarNJ

i have prepped my last few shower pans with that redguard sealer that helps tile bond to concrete. homerdepot is the only place i saw it at though. not that i ever go in there or anything.......shhhhhhh.
i used two coats of it and it turned out great.
makes for a great clean surface.

R&D Tile

Redgard is a waterproofing membrane, doesn't help in adhereing the tile, a good quality thinset does, where did you use it, on the floor of the pan?, yes, it's rated for this, but a sheet membrane or Kerdi is the way to go.