Expert: Dan Griffin - 11/7/2007

Question
QUESTION: Our house is in central texas. It's a single story built on-grade, over a concrete slab. The previous owner had built an additional bedroom/bath on the back of the house. The the entire house, including the add-on was floored with old 8" tile, which has been cracking and coming up since we've owned the place.
Anyway, we are in the process of having the tile removed to replace with new flooring.
When the outer room tile was removed, it apparently was attached to about and inch or more of some kind of morter, on top of the concrete slab. When it was removed down to the slab, consequently, the subfloor in there is now much lower than it used to be. This makes it floor sit quite a ways below the door, and also the comode area.
We are trying to figure out how to raise the level of the subfloor, back to about the height it was, before laying new flooring. If we leave it as is, there will be quite a step down from the entry into the room. Also, we will have to do something about the commode, which can't be reinstalled, as is, because it will no longer sit flush against the floor as it used to.
What is the best and most cost effective way to bring the level of this subfloor up?
The room area is about 200sf.


ANSWER: You indicated that it is quite a distance, but never said how much.  This makes an answer problematic.

I assume it was a patio slab and that it was probably pitched away from the house so it is not level as well as being lower.  If the addition had been done correctly, they should have added a footing to the patio before enclosing it and making it an interior space.  I fear this may not have been done.  Whether you remedy this situation is your call, but all codes would demand it.

If the patio had been down a full step ( about 7 inches) there would be space to build a wood sub floor and come up to meet grade or bring in a sand fill and pour a slab at grade.

It sounds as if it may have not been down that low.  There are floor products made to level and raise slabs.  The prime choices are between gypsum based or portland based levelers.  My personal choice would be portland based.  None of them are cheap.  If you are adding 1 1/2" or more, you can use concrete made with small stone usually called 3/4 minus.  It is immportant to get the original slab prepared to hold the new material.  This may involve sand blasting, acid etching, or, at least, proper use of a bonding agent on the old concrete and mixed in the new.  Work with a local ready mix company.

The huge problem will be how the outside wall has been done.  If it is sitting on the old patio, the fill will be quite thick at the wall.  It would be best to remove the wall, pour the slab to the outside edge and reset the wall.  I'm guessing this will not fit your plans.  There is the potential to create a potential mold sidtuation and any doors may be seriously affected.

It would be best to contact or work with a local contractor who can see the issues or an old carpenter who has been there/done that/ seen it all.

---------- FOLLOW-UP ----------

QUESTION: Actually, the additional room was not a part of a patio, and it is level. The previous owners had it professionally built, according to blue prints I've found. However, I think to cut costs, they probably laid the tile themselves.
It appears that the subfloor needs to be raised between 1 and 1.5 inches.
I have been looking into cement board, or possibly Sturd-I-Floor, which I found in 1 1/8 inch.
I'm just not sure how it is applied or anchored to the concrete subfloor.

Answer
I assume you are going back with ceramic again.

Cement board should be set in thin set.  I would plan on pinning at least the corners with some type of anchor, probably a Tapcon product, though the thin set only might be adequate if you have a good bondable surface.  The very best answers possible about all things to do with ceramic tile work is here:  http://www.johnbridge.com/

I respect the information about tile work at this site a bunch.  Notice the comment from "Trask" on this page:
http://www.johnbridge.com/vbulletin/showthread.php?t=39138

Remember these are real tile guys that do this for a living every day.  They say there is plenty of cement board bonded to slab floors with thinset only.  Most of them would mud the floor with lath, though it requires more skill, more labor, and more time.

It worries me that the original floor delaminated.  I would think your previous owner would have used commercial materials and followed directions, but then I have dealt with DIY work and repairs before.  Thinset will stick to good sound clean concrete and will stick to backer board.  Curing compounds and sealers can interfere with adhesion.