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Old 08-03-2015, 03:15 PM   #1
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Raising a sunken floor and travertine tile


I have a 16' x 20.5' sunken (sound) slab floor in a living room. I intend to raise this finished floor to the level of the surrounding slab floors and then tile with antique pattern cream colored travertine (1/2" thick). I intend to use white Mapei Ultraflex #2 thinset with a 14" x 1/4" notch and back buttering..

The wood framing under the raised floor is #2 spruce. The existing slab had been prepped with Ardex 8+9. The layout will be covered 16" on center squares over the entire slab and any low spots brought into contact to the framing using self leveling mix (max unsupported run is 16". The perimeter at all room junctions is bolted to the slabs with epoxied in bolts to ensure no movement at the junction of raised floor to slab. Given the stiffness of the support framing I envision using a single layer of 3/4" OSB T&G nailed down with 3" galvanized ring shanks. Next will be 1/2" Hardie board screwed into the subfloor and framing. The entire surface, especially the raised floor to slab joints, to be coated with Ardex 8+9 before tiling.

I think this ought to be strong and stiff enough... Or does this really need a double layer of 3/4" OSB w offset joints?

Fixing to get old and do NOT wish to revisit this issue in my last home.
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Old 08-03-2015, 05:42 PM   #2
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Hi,

It's difficult to follow what you're planning to do, but I know it's not an approved method. You're building a floor on "sleepers" which often fail when a solid base is required.

Aside from the framing issues, your plan for a single layer subfloor does not meet minimum specs for natural stone tiles. You're also wrong if you think .42" Hardie is gonna help stiffen the floor to meet specs. All natural stone floors require a double layer subfloor that meet L720 deflection which is much stiffer than the requirements for ceramic tiles.

How much lower is the slab and is it a consistent amount? Are you positive you want to do that and it won't mess up the structure?

Jaz
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Old 08-03-2015, 09:00 PM   #3
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The LR floor is 5.25 inches lower than the surrounding slabs. Yes, it is a system of fully supported joists (16" ctc) in full contact with the existing slab. Then a series of crossbraces the same height and thickness as the joists is added and spaced 16" ctc and also in full contact with the slab. This forms a network of supporting boxes, 16" ctc. The main question was whether a single or double layer of OSB subfloor was needed. Since a typical residential wood joist floor has little or no support along its length and this design provides full support every 16" along the length of the joist (what I meant by it forming a box), making it much more solidly supported than the conventional floor you describe. With a double layer of OSB (w non-overlapping joints, of course) why would this be less supportive (from a load carrying standpoint) than the spec you mentioned? No issue of the cement board providing structure, it is only there to provide a stable bonding surface.

Since I cannot seem to attach a pic of the framing layout, this description is the best I can manage.

Feel free to poke holes as long as a cogent rationale is also supplied. Thank you very much for your response.
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Old 08-03-2015, 09:26 PM   #4
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I think one of the biggest problems is joists in contact with the slab and the possibility they'll distort over time. Trapping moisture is another. Of course given the right circumstances this may not be much of a problem. If you wanna continue with the sleepers, why not use long joists with blocking every 16" instead?

As for the subfloor, you need a double layer for sure. The first must be min. " subfloor grade, the 2nd min. " where the face grade is no lower than CC. Make sure the plies of the underlayment are laminated with waterproof adhesive. The underlayment may be called 'exposure 1' for example. The underlayment is offset from the subfloor and fastened only to the subfloor and not the joists using ring shank nails or special screws that are just long enough to poke through the subfloor. Use no glue for the underlayment.

Then install your favorite " concrete backer or membrane such as Ditra.

BTW, you'll have to honor the new cold joints and install expansion joints above them into the tilework.

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Old 08-03-2015, 10:38 PM   #5
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Okay... The reason for the coat of Ardex 8+9 on the sunken slab prior to construction is to create a vapor seal to keep the framing and subfloor dry and stable from moisture wicking up (although I live in a relatively drier climate)

Is there a particular type or brand of subfloor adhesive that works better? I was thinking the older (smelly) type Tough as Nails construction adhesive. I would not think the newer water based adhesives a good choice.

I don't really follow your mention of an expansion joint (I do know what they are and their purpose), but not how one is constructed in this application. If you would not mind educating me or pointing me in the right direction it would be appreciated.

RE: expansion... The Framing will be constructed With relatively loose joints so that If the members should swell there is room for them to move without bowing or buckling. The same is true of the OSB, Though that is inherently a more dimensionally stable product than #2 dimensional lumber.

As far as what you termed as "honoring" the joints (in the cement backer board?), there is little that can be done since the antique layment pattern of various size tile in a mostly non-linear arrangement and is such that I anticipate little or no juxtaposition given that I plan to also lay the backer board in as much a non-linear pattern as can be had. The backer board also gets a double coat of the Ardex 8+9 to essentially casket the framing and subfloor prior to tiling.

Thank you very much for pointing out the need for a double layer of OSB... I will be using the 3/4" for both layers. Makes construction easier as now I need not rip the members to height.
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Old 08-03-2015, 11:20 PM   #6
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On the subfloor adhesive, PL 400 seems to be very popular among carpenter types. I would think any quality construction adhesive will do the job. Do "S" beads on the joists.

How you handle the expansion joints from dissimilar substrates depends on what you chose as your setting base. (s) Is the rest of the area, (the higher level), concrete or is the house wood framed with an adjoining lower slab LR?

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Old 08-04-2015, 08:20 AM   #7
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This house has a concrete perimeter foundation In the interior Floors are all concrete slab. Exterior Walls are all 2x 6 construction with a brick veneer and that is pretty standard approach around this area for 30 years ago. Newer homes are much the same but use a 2x4 exterior wall.

Generally the slabs look to be in pretty good shape with a couple of non-structural cracks found here and there, but those have been stabilized with a high pressure Epoxy injection. These have apparently been there for quite a long time judging by the cracking In the old tile and the amount of tile wear along the crack.

Roger on the glue-down to frame and in between the sandwiched OSB sheets. Been using that adhesive on and off in projects for decades with uniformly dependable results, but always looking for improvements in methods/materials. Better materials are most always a good investment.
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Old 08-06-2015, 12:00 PM   #8
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I don’t think so that what you are going for will work perfectly. Building a floor on sleepers is surely not a recommended solution. Remember, natural stone floors need a double layer subfloor greater than the one required for ceramic tiles. Also ensure that the plies of the underlayment are laminated.
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Old 08-06-2015, 01:17 PM   #9
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5.25" ( 5-1/4" ) amounts to rip cut if using 2 by 4 and two layers of 3/4 plywood.
3.5 + 1.5 = 5" give or take..

Sleepers on concrete should be placed on their side..
If it were me.....
I would lay the moisture barrier, then 2x6 sleepers flat at 16" on centers intervals. Using a nail set with blasting caps nail the sleepers enough to keep them from moving ( every 2' ). Add a second layer of sleepers ( just nailed) in the opposite direction then the 3/4 plywood sub floor.
NOTE: I wouldn't use OSB board in this installation.


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