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oakland76 10-08-2010 08:43 PM

Self leveling concrete...Help needed badly
I converted a bathroom to a laundry room which involved everything from shoring up the joists to all plumbing supply and waste lines. My carpenter sistered the floor joists with laminated veneer lumber because the house was built in 1929 and there has been some settling over the years. He put down tongue and groove 3/4 in plywood with 1/4 inch concrete board on top of the ply. He recommended using self leveling concrete to fill in the sways of the floor and to bring up the back corner which was about one inch below the high side of the room. Ultimately I will be putting down ceramic tile on the concrete but I would like a level surface for the washer and dryer and a flat surface to make tiling easier.

Here is the problem. I added another layer of the concrete board on the low side as recommended by my carpenter and fastened it. I primed the backerboard with the suggested adhesive primer then I poured a total of about 150 pounds (3 bags) onto the floor starting in the low areas and tapering off as I got to the level side. My wife assures me this is wrong and I had to pour the entire floor for the product to work. I did not listen to her and let the product cure. As it turns out I am still about 3/8 low on the low side of the room and I have a couple more dips in the floor to deal with.

1. Can I prime the concrete (which has set for 7 days) and simply add more to the needed areas without covering the entire floor with the product?
2. Do I have to hammer out all of the poured concrete and repour the entire floor? I am worried about adding unnecessary weight to the floor.
3. Once the concrete is in place can I thinset and tile over it or do I need further surface preparation?

Davy 10-08-2010 08:56 PM

Jason, you would have been much better off to nail cleats onto the sides of the joists, dropped the plywood down between the joists and went with a thick mud bed. I'm not a SLC expert, I'll let someone else answer who is.

cx 10-08-2010 09:00 PM

Welcome back, Jason. :)

I'm kinda curious why you could have a floor with "sways" in it and a corner a full inch low after a carpenter sistered the joists and installed new subflooring. Just doesn't compute for moi. :scratch:

If by "concrete" you actually mean a SLC product, you should be able to prime and add another layer after a week with no problem. But it might be helpful if you'd identify the product you're using so we can be sure.

When your carpenter installed the CBU over the new subflooring, did he put a layer of thinset mortar under the CBU panels? How 'bout when you installed the second layer of CBU over the first?

My opinion; worth price charged.

dhagin 10-08-2010 09:02 PM

Well Jason, if you installed the plywood, CBU & SLC properly and in accordance with all manufacturers installation instructions, all over a proper structure, then you may be able to prime over everything and pour a little more... maybe.

Lets back up a little. Has the floor structure been checked for deflection? Plug your numbers (before LVL's) into the Deflecto and tell us what you get, and what joist spacing you have.

Then, tell us how both layers of CBU were installed. Were both layers bed into mortar? What CBU did you use?

What SLC did you use? How thick is it at the deepest area?

oakland76 10-08-2010 11:01 PM

First of all thanks for the quick replies. The carpenter sistered the joists where they were without paying any attention to fixing the problem from the get go. There was no mortar between the plywood and the backer nor did I put any between the first layer and the area I built up. There is however a screw every 8 inches.

The room is 13 x 6 and the floor slopes as you walk through the door toward the back left corner. The left side of the room was on average about 1/2 inch lower than the high side (right side) with the worst area being the rear left corner (about 3/4 inch lower than the high side).

The Deflecto is good. The joists are 2x10 old Douglas fir and laminated veneer lumber with the longest unsupported span being 10 feet. The joists are 16 oc. The plywood was secured with adhesive and framing nails and the backer was placed perpendicular to the ply. The CBU was a solid material (not wonderboard) similar to Hardibacker but I dont remember the brand name (and it is currently covered with SLC). I used Quickcrete SLC but ran out and my wife picked up the brand sold at Home Depot.

I dont know if "sways" was the best word choice. There are a couple of areas between joists that are lower than the surrounding area (similar to an empty puddle) but those area are 1/4 in lower at the most.

Thanks again.

Levi the Tile Guy 10-09-2010 12:07 AM


There was no mortar between the plywood and the backer nor did I put any between the first layer and the area I built up. There is however a screw every 8 inches.
Sorry to be the bearer of bad news but this is a major problem, and going ahead with tile is possibly asking for failure of your entire tile job.


There are a couple of areas between joists that are lower than the surrounding area (similar to an empty puddle) but those area are 1/4 in lower at the most.
I would have all of these fixed and flat before you tile. Industry standard is no more than 1/8" in 4'. If they aren't corrected then you will either have a floor that is far from flat or have lippage.

As for the SLC I have never used quickcrete's SLC before so you may want to call their tech support and ask. I have however put slc over other slc before with no issues. I always drill little holes (approx. 1/2" x 1/2") in the existing stuff to give the new legs, but that I'm told is overkill it just makes me feel better.

At this point though you wouldn't be out that much money to rip it all out and start over the correct way, and IMHO that is what you really should do.

oakland76 10-09-2010 11:18 AM

no mortar between ply and cbu
$#it. Is there any alternative other than smashing out the SLC i have already poured, removing the cbu and thinsetting it?

cx 10-09-2010 11:44 AM

That would be the only way to repair that part of your floor deficiency, Jason. Whether you choose to do it is, of course, up to you. It's not a guarantee of failure, but it's considered a serious problem by the manufacturers of CBUs.

Not sure what flatness standard Levi is quoting there, but tile industry standards for flatness call for no deviation of more than 1/4" in ten feet and no more than 1/16th" in any one foot. For large format tiles that's still not really flat enough for comfortable setting.

My opinion; worth price charged.

torquedude 10-09-2010 01:35 PM

I'm a carpenter looking for flooring advice but I think I can give Jason some advice here. He's not going to like it but will save a lot of weight on the floor, and maybe expense and time.

The carpenter who beefed up the structure was correct in sistering those engineered laminated joists. However, he should have done you good by leveling the floor. You don't need to jack the house up which would cause a nightmare with cracked plaster and roof problems to name a couple. Did the carpenter put the glue lams in from beam to foundation rather than just the span of the room?

If you decide to rip all that other stuff out, may as well rip the plywood up also. You can level the room and then no need for more slc, or at least very little. It's easy and not as time consuming as leveling the floor with slc.

All you need to do is sister stud grade 2x6 to the existing joists inside the room so that the 2x6's rise above the floorjoists to make level. If you intend to do this this, please state and I'll tell you how to do it in more detail if you want.

You will need to use stud grade 2x6 because they will be used for structural purposes. 2x4 might work but I always use 2x6

d :redwings:

jadnashua 10-09-2010 03:35 PM

When all else fails, read the manufacturer's instructions. They ALL say to set the cbu in thinset, then nail or screw it down. This isn't an adhesion thing, it is to ensure there are no gaps and 100% full support. It's the nails or screws that hold it down, the thinset holds it UP.

oakland76 10-09-2010 03:45 PM

flooring fun
Thanks Dave. The new joists as well as the sistered joist spanned from the sill plate to the center beam of the house not just the room. My wife just changed the design plan and bought terracotta tile rather than ceramic. I really dont want to smash the 150 pounds of slc which is already cured only to find the slc has filled the screwheads of the cbu and I still wont be able to get the cbu up. It just occured to me that I did use a generous zig zag bead of construction adhesive between the ply and the cbu. Will that be enough or is the thinset still required?

cx 10-09-2010 03:52 PM


Originally Posted by torquedude
You will need to use stud grade 2x6 because they will be used for structural purposes.

Stud grade would be about my last choice for that application, Dave. Even though it won't be structural in this application, I'd want a better grade of lumber than that.

Originally Posted by Jason
It just occured to me that I did use a generous zig zag bead of construction adhesive between the ply and the cbu.

That can actually make it a good deal worse, Jason. Depending upon the adhesive, how it was applied, how quickly you got the boards down and fastened, you can cause more gaps under the panel than might have been there with nothing at all between them and the subflooring. Not a good situation, I'm afraid.

My opinion; worth price charged.

jadnashua 10-09-2010 04:35 PM

As I said, the goal of anything underneath the cbu is NOT to hold it down, it is to ensure that the board is 100% supported. You almost certainly ended up with minor (but anything in ceramic that can cause deflection - a gap in support can do that) gaps, and may have increased the chance of failure. Sometimes these things take awhile, and you may be lucky. It's your call how to proceed. Terra cotta is weaker than most 'normal' tile, and even more subject to problems with minor deflection issues.

CSS 10-09-2010 04:57 PM

Its only a matter of grout cracking out possible broken or cracked tile its your house do what you want. if it were my job and someone working for me installed it like that no question. it would come up and get thinset under it.

oakland76 11-15-2010 08:40 AM

terracotta grout tips
I fixed the floor and installed the tile in both my laundry room and bathroom. I installed 8in hex terracotta in the laundry room and sealed them with the recommended sealer. The tiles are hand made and somewhat irregular. I have 1/2 in grout lines so there is a good amount of grout that has to get in there. I planned on using a grout bag to fill the joints (as recommended by the manufacturer) but I am not clear how much grout to put in. Do I go to the top of the tile lip, just below that or just above that?

Secondly, I installed about 90% of my bathroom tile (second project) and I am not sure if it is better to seal the travertine tile before grouting or after. Finally, I installed 18x18 travertine in an alternating brick pattern with 9x18 travertine in the shower over the Schluter. I installed the Kerdi to the ceiling because I intended to tile to the ceiling but after seeing the pattern I have decided to stop 5 inches from the ceiling to prevent having an awkward size tile at the top. Should I cut the Kerdi off and scrape the thinset or can I have my drywall guy texture over it?

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