Self Leveller over 5/8 ply before Ditra Heat [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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05-21-2019, 10:24 AM
Long time information gatherer, 1st time posting a question.

New construction done in 2017, I'm just getting to my master bathroom now.
I'm pretty strong with the DIY just don't have the experience with the smaller details.

Subfloor is 5/8 ply, 16oc, open web joists. (2nd floor)
Schluter says this is suitable for Ditra Heat installation.
In a perfect world I would add 1/2 ply over top to make me sleep better but that will mess with two thresholds which are 3/4 hardwood (therefore not really an option).

Over the Ditra Heat I'm installing 12x24in porcelain tiles using Schluter All-Set (also used for Ditra Heat).

Issue: Almost right down the middle of the floor is a ridge, which is right above a supporting wall underneath resting on foundation.
Floor dips on either side. Shorter Right side Dips 1/8th to 3/16 and the left side dips 3/16.
Span from the ridge to the lowest part of the dip is 4-6ft.
TCNA says it should not be more than 1/8th over 10ft for large format tile.

1) Is self leveller effective on 5/8 ply at 3/16 thicknesses feathered out? (will it crack due deflection, 5/8 ply not being thick enough?)

2) Is a primer needed?

3) should I prime the self leveller patch before using Schluter All-Set to lay the Ditra Heat?

4) Could I patch with modified mortar as an alternative? (Mapei Ultraflex 2, I've got a full bag lying around)

Thanks for the assistance!

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05-21-2019, 10:52 AM
SLC over plywood is a different beast.

Most SLC's I'm familiar with require the use of expanded metal lath secured to the plywood.

Additionally, some MFR's will spell out ASTM standards or TCNA specs for min. plywood thickness under self leveler. Many others require minimally the use of 3/4" T&G plywood. If you don't have T&G your joints should be back blocked to mitigate deflection at seams.

1)my suggestion is find a locally available SLC and work towards meeting what the MFR requires and if there are minimum thicknesses required for installation over plywood.

2)maybe, depends on the MFR. Most require expanded metal lath secured to the plywood.

3) Typically no need to prime, but check , but SLC is thirsty. Make sure you appropriately saturate it before combing your thin set.

4) Can't help you on that one. How old is the bag? Take a look at Mapei Planipatch, might be more appropriate.

clifton clowers
05-21-2019, 11:29 AM
Ardex and/or Henry Liquid Backerboard does not require lath. It does require primer.

05-22-2019, 08:59 AM
Thanks houndzilla.
5/8 ply is T/G but the seam at the problamatic ridge is butted jointed as the ply changes direction to flow with the change in joist direction in the floor.

I'm looking at my options over the next few days.

Per clifton clowers recommendation I'm looking at Ardex but can't find suppliers in my area yet.(Montreal QC)

Thx to both of you.

05-22-2019, 10:16 AM
Note that the liquid backerboards that Clifton suggests require 3/4" T&G in their cutsheets.

As long as the structure is solid under the butted square edge seam, you should be fine, that's technically the same as being back blocked. The T&G is there to support the seam when it occurs perpendicular to the structure below whe

Really?! 5/8t" T&G plywood. Interesting, my engineers in the States have told me it doesn't exist. Maybe it's a Canada thing.

05-22-2019, 10:36 AM
George, nominal 5/8ths" T&G plywood is common in the USofA. Thinner than that usually exists only in square-edge panels.

My opinion; worth price charged.

05-22-2019, 10:44 AM
wait... my bad this was 15/32" (1/2") that I was thinking about.

Thanks for the correction.