3/8" leveling needed under Ditra - ideas?

[remodelboy]

Read John's book and took the Schluter workshop, but didn't notice this covered.

All ready to lay down some Ditra over a 5'x5' bathroom floor when I noticed that there were some problem low spots behind where the toilet and vanity will be. The dip is mostly confined to one corner and goes from a low of 3/8" to zero (3/8" low in the corner tapering to zero towards the middle of the room in both directions "width and length and diagonal").

I know that we are not supposed to put a latex modified product under the Ditra as it will not be able to dry "predictably" whereas a non-modified product like Ditraset will set and develop full strength even with the moisture trapped under the Ditra.

Would it be okay (advisable) to use Ditraset to level this concrete floor. I was going to rip some strips from zero to 3/8" and use them for forms and then pull a screed over the thinset to knock it down to the forms. The other 3 quarters of the floor are flat and level and would be what the other end of the screed is laying on.

I was considering Ardex K15, but see that the P51 primer is recommended and then some have mentioned that SLC's can be a little fussy about how they are mixed and spread and how they set up (sometimes lumpy). Also, it seems like most of the SLC's are modified (I'm not sure about the K15).

The tile for the bathroom is 18x18 porcelain and I would be willing to use a deeper notch trowel and some of that no sag Ardex x7Gplus that I used for the walls.

[jadnashua]

You didn't mention if the floor was a slab or wood...that can make a difference. Ardex recommends their laytex addative when using over a wood floor. Many others don't. All SLC's require lath over a wood floor, so tapering to zero is tough to impossible, so flooding the whole floor is best. If it is over a slab, SLC would work. All SLC's require the floor to be primed first with their primer.

A medium bed mortar can handle that depth in one layer...a thinset doesn't like more than 1/4". If you let that mortar cure well before covering with DItra, you should be okay...and, a plywood floor would allow some to migrate out...just don't cover it up right away.

[remodelboy]

Jim,

Thanks for the quick response! I need to be doing something with this tomorrow morning.

It is a concrete slab. It had some vinyl that I was going to leave on as a slip sheet, but it was poorly attached around the toilet. Good thin that I removed it as I found some slight cracks and the low areas became more obvious.

I'm willing to go with the SLC. The Ardex K15 is supposed to be a premium Portland based leveler. I thought that this would be good as it is not so dependent on drying out as the more heavily modified levelers might be (it will be under the Ditra). Would this stuff be fairly easy to work with for a first timer (leveling) on a small area (about 5 square feet)?

What about a rapid set medium mortar? I know that they are modified, but when they set up, don't they pretty much cook off much of the moisture? Or how about the brand Rapid Set Cement - it seems like it is portland and sand and some kind of catalyst. The Rapid Set boasts of fibers mixed into it. The bag/box mentions that it is good for leveling.

The thinset that I was going to use is Ditraset (the premium unmodified thinset. I also have some bags of Versabond laying around, but then there is that concern about having a modified thinset under the Ditra.

[muskymike]

Hi Roland, I would use the SLC. Be sure to prime the floor before you pour it with the proper primer for the SLC.

[jadnashua]

SLC's are a little runnier than pancake batter, but will leave a rolled edge unless you spread it to a feather edge. You do not have a lot of time - mix it well, pour it out and do what you need to with it. At the depth you are talking about, I'd want to spread it around, feather the edge, then leave it alone. Because of the thickness, if you aren't careful, you'll end up with it thicker over the high areas if you don't move it a little. Think of honey - it will dome a little because of surface tension. But, if you spread it over the thin areas, you can get it to level quite well. The thicker the stuff is and the quicker you put it down, the less problem. It gets easier the thicker you apply it to flood the area. If you use cold water, you'll get a little more time, but you want to be done futzing with it in less than 10-minutes after you start to mix it up or you'll run into it starting to set - think of a puddle that is just starting to freeze over - you walk through, and you end up with ragged surface rather than a nice flat smooth one.

Just have a good idea what you need to do, have all the tools handy so there are no delays, then let it do its thing and you should be fine. A flat board or piece of plywood with a factory edge should be fine to help spread, and a trowel to feather the edges.

[remodelboy]

Thanks for the walkthrough with the SLC. I did a search on SLC and Ditra (titles only) and found a lot of addtional threads on the subject although most were pretty old, but still had a lot of useable info. Your encouragement has helped me shake off this paralysis - I'm heading out to get my Ardex K15 and P51 primer.

Thanks for the replies!