Tile over Concrete Questions

[Ncerniglia]

I want to install porcelain tile in my basement bathroom. I have attached a rough sketch of the bathroom. Facts:


-- The house is 16 years old and the basement has never been finished before.

-- The basement is concrete slab with concrete foundation walls as the basement is underground (there is no place in the basement, except the garage, where the floor is less than 4ft below ground-level).

-- There are only 3 cracks anywhere in the entire basement (one is no closer than 8 ft to the bathroom; the other two are more than 20 ft away). None of the cracks resulted in elevation changes and none are gapped. They all appear to be minor.

-- The concrete in the bathroom is not completely flat. It is level overall, but there are two low spots. The slope of the spots is 1/16th of an inch over 1 foot maxing out at 3/16th of an inch in the middle.


Even barefoot, the lowspots in the floor are not noticable.

Do I absolutely need to install an underlayment?

Do I absolutely need some sort of backerboard to deal with the low spots in the floor?

Can I just use mortar directly on the concrete floor? If so, would I make the mortar thicker in the low spots?

 

[Floorist]

Just put thinset directly on the concrete. Do not use CBU. Yes, you can use a little extra thinset in the low places. I wouldn't worry about the cracks.

 

[Ncerniglia]

Thank you very much for your speedy reply.

A friend of ours highly recommended we use Ditra for the underlayment. Is this something that we need to install given our situation?

Thanks again.

 

[DarisMulkin]

I wouldn't worry about Ditra if there are no cracks in the area to be done.

Daris

 

[Incognito]

Make sure you have a POROUS slab. That means you drop a bead of water on top and see if it's absorbed or if it just sets on top.

Thin set needs to get into the concrete. Bigger tiles need to be "back buttered".

If there's no absorption you'll need to buzz that surface with diamond or carborundum wheels to rough it up.

I'd also fill and flatten out any depressions a day before the install rather than trying to mud it level with wet stuff.

Really GRINDING down any humps is the more critical procedure though. I find it feasible to work out some dips in wet mud but much more awkward to tile over a hump in the floor by slopping more mud on the back of the tile and the floor.

It's not always about right and wrong so much as what's easier for YOU.

 

[Ncerniglia]

The slab is definitely porous. The largest tiles we are using are 12inx12in.

There are no bumps in the floor that need to be ground. The slab is very smooth; there are just a couple of low spots. I've also had a couple of people tell me that the self-leveling stuff is a giant pain in the ass.