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Unread 10-30-2019, 05:37 PM   #1
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Preparing an Advantech subfloor

Hi! Planning my first tiling job for the 16’ by 24’ 4” first floor of an early 1900s house. I’ve been through a lot of posts here and I’ve learned a lot. But I still have a few questions!

I’ve reinforced my floor assembly with the help of a structural engineer. Here are the stats:
Joists run 16’ long with OC spacing running between 19” and 25”.
Joists are the original 2 by 6s with new 2 by 8s sistered on. (Only the 2 by 8 makes contact with the subfloor.)
A 6 by 6 beam supports all joists midspan and is supported at each end by the foundation walls and there are three evenly spaced lally columns on concrete pads supporting the center of the beam.
There are two runs of solid blocking 4’ from either joist ends.
I have used subfloor adhesive and ring shank nails to attach ¾” Advantech t&g panels to the joists.

The Deflection calculator (with 8’ joist length and a single Douglas fir joist 1.5” by 7.25” entered) gives me a rating of L / 545 so I think deflection along the length of the joists is more than adequate for porcelain tile.

With my 19”-25” joist spacing Advantech says I should have used their 1 1/8” panel if planning to install tile. Schluter says I could use Ditra XL over my current assembly and be fine. From reading posts here on John Bridge I think screwing down at least 3/8” plywood (per the Woeste and Nielsen article) over the Advantech would be preferred.

I plan to install 12” by 24” porcelain tiles and then install kitchen cabinets over the tile. (Although I did see some good looking 30 by 30 tiles recently…)

My questions:

If the addition of 3/8 ply underlayment is indeed recommended, I could then use regular 1/8” Ditra followed by the tile?

The majority of the subfloor is within the 1/8” or less deviation within 10’. However there are a couple of dips (1/8”-3/16”). Should I fill these dips before or after I screw down the 3/8” plywood underlayment? What type of product could I use? I don't think a SLU over the whole floor is needed, just looking to fill the two or three spots.

Whether I use cement board or Ditra, thinset will go into the 1/8” gaps I leave between underlayment panels. One cement board manufacturer (Wonder board) says to lay a 1 inch “Bond Breaker Tape” over the seams but they offer no suggestions as what tape to use nor do they sell such tape. What is the general practice here? Fill those gaps with thinset or tape over them?

I will install ¼”+ movement joints at the perimeter of the room and at the door sill. With my 16’ by 24’ room do I need a movement joint mid-field? I do have one south facing window in this room.

Thanks for reading this long post, Scott
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Unread 10-30-2019, 06:44 PM   #2
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Finding 3/8" ply that doesn't closely resemble a pretzel or a Pringle chip is hard, although it is the minimum thickness recommended by the TCNA. But, yes, if you can find some that is actually flat (and is made of all plies C or better), it would work. Keep in mind that Advantec is really dense, and screw jacking is a big risk unless you predrill a pilot hole, or the screw has a long enough smooth part to ensure the two sheets get clamped tightly together.

Personally, I'd probably use some floor patching material to fill the low spots and then use the Ditra XL. Your overall floor buildup would be lower, and it would be easier. Not all patching materials are designed for use under tile. If you choose to use the ply, you'll probably find nominal 1/2" material will work better and, you would flatten the surface AFTER you install the ply, as screwing through it could cause it to crack and delaminate.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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Unread 10-30-2019, 07:21 PM   #3
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I wouldn't bother with anything thinner than nominal 1/2" exterior glue plywood for a second layer of subflooring for the reasons Jim cited. And I'd want a second layer. I know Schluter likes their Ditra XL over single layer subfloor over 24" joist centers, but I wouldn't be comfortable with that. A second layer and regular Ditra would suit me much better.

As for movement accommodation, the industry standards call for joints every 20 to 25 feet for interior installations not subjected to temperature changes of more than 20 degrees F or exposed to moisture, but specify the exterior standard of every 8 to 12 feet for interior installations exposed to direct sunlight. Judgement call on your part.

As for taping the plywood joints, I don't really know if Schluter has a recommendation under Ditra, but for the most part the substrate community seems to think doing nothing at all about those joints on interior installations is acceptable. Don't try to fill them with mortar, don't try not to. A little incongruous among all the other requirements, but we don't seem to hear of problems resulting from the intentional neglect. Certainly no reason not to use bond-breaker tape if it makes you more comfortable. There is such tape out there, but you're not gonna find a lot of pros who can tell you where to get some.

My opinion; worth price charged.

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Unread 10-30-2019, 10:54 PM   #4
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I wouldn't worry about the plywood joints either, but if I did, I'd fill them with some cheap latex caulk. You just want something to keep the mortar out.

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Unread 10-31-2019, 05:20 PM   #5
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Thanks for the quick reply and the advice. Much appreciated!

Regarding the "floor patching material" for the dips in my subfloor. Once I've installed plywood underlayment I would apply this material. Something like Ardex Feather Finish or similar?

Thanks again,
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Unread 11-01-2019, 05:46 AM   #6
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Read the instructions though, because some of those fillers require using their matching primer.
If I recall correctly my memory is excellent, but my ability to access it is intermittent.
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