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mrosspa 09-19-2020 12:07 PM

Raising A Sunken Floor
The house I live in is slab on grade, and the 180 square foot living room is sunken. It is about 5 1/2 inches lower than the surrounding floor. We will have someone we trust pour the concrete.

I'll lay a porcelain tile on the surface and want it be level with the surrounding tile floor.

I need to think backwards- The porcelain tile is 3/8 inch thick. (10mm). Considering thinset and an anti-fracture membrane, the concrete needs to be low enough so the tile installation will be flush with the surrounding tile.

I've not decided on an antifracture membrane for this project- I've used Red Guard in the past, but I'm open to suggestions.

The porcelain tiles are 18 x 18 inch (remember when that was considered large?)

How thick do you think the tile, thinset, and antifracture membrane will be?

I realize I'm adding another question on the same post, but here goes. I'd rather have the depth of concrete lower, and raise the tile with more thinset. How thick can I make the thinset without affecting the bond between the tile and the substrate? :scratch:

Lazarus 09-19-2020 12:24 PM

Depending on the tile, the morter should raise it between 1/16 and 1/8....give or take.

I don't know if a concrete pour is better than having floor mud put it. I would price them both....:scratch:

cx 09-19-2020 01:16 PM

Welcome back, Mike. :)

You really, really, don't want to try to raise the finished floor level by adding to the thickness of your bonding mortar.

If you want to pour concrete to partially fill the depth of your depressed floor, you can do that, but I'd strongly recommend you leave approximately 1 1/2 inches or more to be filled with deck mud to exactly the height you need to make your tile installation finish out where you want it.

You will not be able to finish a concrete pour sufficiently flat to accommodate your large format tiles if you intend to follow the industry guidelines for substrate flatness. Won't happen. The requirement for tiles with any side longer than 15 inches is no deviation from intended plane of more than 1/8th-inch in ten feet nor 1/16th-inch in two feet. A mortar bed (deck mud) and SLC are about the only ways to reasonably achieve that flatness and will also allow you to more precisely set the finished level.

It is possible to fill the entire void with deck mud, but it's a bit trickier when dealing with that much mud.

As for the thickness of your installed membrane and tile, the only real way to determine that with any accuracy is to do a mock-up using your membrane, your thinset mortar, and your tile and measure the result.

mrosspa 09-19-2020 02:45 PM

CX and Lazarus:

Thanks for your suggestions.

As you suggested, I had figured I would need to make a mock up for my tile thickness also. I didn't mention it, because I figured you guys would fall asleep from my droning on...:blah:

The concrete pour being 1 1/2 inches low might do the trick, once I factor in uncoupling membrane, thinset, and tile.

MUD BED- 5:1 sand:portland cement? Same stuff in John's instruction page? ( same stuff in my shower floor...)

How thin can a mud bed be before it is too thin?

mrosspa 09-20-2020 02:24 PM

Bump! :wave:

cx 09-20-2020 03:35 PM

1. Yes.

2. 3/4" if bonded, 1 1/4" if unbonded and reinforced.

My opinion; worth price charged.

Davy 09-20-2020 06:17 PM

1 Attachment(s)
If I picture this correctly, the surrounded area that is raised already has tile installed and you want the lowered area to be mudded up so the new tile will be flush with the tile you now have. If you have cut tiles along the edge that will need to be removed, take them out before you mud up the sunken area. Then the mud can be made flush with the concrete that's under those cut tiles.

I'd do like was mentioned, pour the concrete 1 1/2 to 2 inches low and mud up the rest with deck mud. I'd probably put a broom finish on the concrete and then bond it too.

Best I remember, this deck mud is about 3 inches on the left and 1 1/4 on the right, bonded with gray Versabond.

mrosspa 09-20-2020 07:02 PM

2 Attachment(s)

Your correct as you've described. It's time to add a picture of some side tiles removed. We'll see if the file size is too big. Sorry, the progam placed the picures in a horizontal format.I'll remember for the future.

As I was doing the math, I was wondering if 1 1/2 inches will be too high. We will see after I put together a mock up. I've got over 2 weeks before the concrete is set.

1. The person doing the concrete asked that the side tiles be removed, so he can insert metal dowels.

2 He also said that the porcelain tile didn't need to be removed from the floor. I put those floor tiles down and I'm certain they have over 90% coverage. I'd rather not remove then if I don't have to, but I respect your opinion.

3. CX raised an important point, but I'll ask that question tomorrow. You guys have done enough for me already. :clap1:

Davy 09-20-2020 07:26 PM

Heck, we're just getting warmed up. :D Yep, those tiles can stay down and pour concrete over them. I'd remove the diagonal cut tiles up on top along with the 3 inch tiles along the edge.

The concrete height isn't a big deal as long as you leave close to 1 1/2 inches for deck mud. 2 inches would tickle me to death.

Davy 09-20-2020 07:34 PM

One of the first things that pops in my head on a job like this is if the original layout is square and accurate enough for more tiles to fit in once the concrete is raised. Also, do you have any of the original tile left to fill in the area?

mrosspa 09-20-2020 08:23 PM

Great question about running the same tile into the raised living room floor. Sadly, no. I never thought I would raise that floor...

I've resigned myself to using a tile that will transition nicely from one tile to the other. Also I want the tile to be different enough that it defines the living room as a separate space.

Treyaflash 09-22-2020 01:24 PM

One alternative to consider: it was quite labor intensive, but I raised my living room floor 5.5” with 2x6 framing and plywood over top, which was then carpeted. Attached are some pics. We used sand to fill in spaces between the 12” boxes we created to give it some weight.

mrosspa 09-27-2020 02:13 PM

2 Questions-

1- What is in Bonded deck mud?

2- Uncoupling membrane. 17 years ago, Ditra was barely mentioned. Any other recommendations beside Ditra now?

(I've a reprieve on time- concrete is poured in 1 month.)

jadnashua 09-27-2020 03:30 PM

Deck mud either needs to be reinforced and a minimum thickness (nominally 2" and with metal latch embedded in it), or if over a solid surface like a slab, bonded to it with either a cement slurry or thinset so that it gets attached to the slab. An unbonded mudbed can go over a suitable wooden subfloor, a bonded one cannot.

If you want to understand what Schluter says about Ditra, it's best to review their website www.schluter.com . It is an easier to install tile interface than say cement board and, can help decouple differential horizontal (not vertical) movement between the layer beneath it and the tile. They claim it emulates the methods used in tiled buildings throughout the ages with modern materials.

cx 09-27-2020 03:34 PM

1. Same thing that's in un-bonded, reinforced deck mud, Mike; a mixture of about 5 parts sand and 1 part Portland cement with just enough water to hold it together.

2. And after all those years there is still no industry standard for such membranes. The ceramic tile industry standards committees are "working on a standard" and have been for most of those years you mention.

Despite the lack of an industry standard, there are a number of uncoupling membranes on the market these days. The only information you have to rely upon for their performance is the manufacturers' advertisements.

My opinion; worth price charged.

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