Tile question for a pro

[Steves32]

Helping a friend tile his house.
Tiles are 20x20 porcelain.
House is a slab.
About 1800 sq ft install.
House built in 70's
All floors were carpet before
No cracks in slab (amazing) except a few hairline shrinkage ones. Used Redguard on those.
All walls are square & floors level.................except 1 room.

Family room has an area, probably 10x8 ft that's 1" low in middle.
Using 1/2x1/2 notch trowel
Using Flexbond thin set.

Tried a couple of experiments to float floor in small test area.
Used mortar patch on floor- let dry & comes up easily w/ chisel. Not bonding.
Tried using same mortar but used a bonding agent. Same thing.
Tried a leveling concrete. Comes up easily w/ chisel.
Tried leveling concrete w/ bonding agent- better but still not good enough.

So- we tried bonding agent along w/ 1" of Flexbond thin set- the grey stuff.
48 hours later- you would need a jackhammer to get this up!
Doesn't shrink or crack but am willing to bury a fiberglass mesh in it if deemed necessary.
Plan is to use 8 ft 2x4 & level middle low spot w/ this stuff- feathering out edges. After completely dry over a weekend, install tile w/ 1/2 trowel & same thin set.

Another guy where we bought tile says we might try multiple layers of Schluter uncoupling mat w/ thin set between layers to build up low spot.

One other guy suggests using a thinner Spiderweb uncoupling mat glued to floor using the thin set & then build & level on top of that. We are leaning towards this approach.

Opinions?

 

[astroracer]

Does it have to "bond" to the existing floor? If it's solid and supports the tile what else do you need?
I'm asking because I don't really know but, once the tile is down, where is it gonna go?
Mark

 

[C96]

I’ve seen many tile contractors that will only install tile using a slip sheet between the concrete slab and tile. Basically, the entire floor is floating and wont crack with the slab. In this type of installation it’s the grout that bonds everything together. Using the Redguard is similar; it also isolates the tiles from direct contact with the concrete. Good product but pricey.

I personally like using Versabond for thin setting. Have had excellent results with zero issues. Never tried their Flexbond, sounds like a good product though.

I think once you make the repairs as you have stated, coat them with the Redguard and proceed with thin setting, you should be good to go.

I think most problems stem from the installer trying to save time by just adding more water and re-spinning the thin set rather than tossing it out and mixing new from scratch when it starts to stiffen up. This goes for the grout as well.

Good luck

 

[Steves32]

Both the mortar & the leveling stuff cracked while drying, the thin set didn't.
Just FIY.

We are only mixing as much thin set as needed & not adding any water.
This flex bond is very sticky & bonds to everything. Says you can even use it over old tile if you scuff it. At 30 bucks a bag, it's no cheap!


I would think it needs to bond to floor & tiles. We are trowelling thin set before using notched trowel and also back buttering all tile. The Spiderweb decoupling mat seems like a good idea though to me.