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Unread 08-16-2021, 09:23 AM   #1
DAfromNJ
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Correct alternative to NJ Mud Job

Thank you for creating such a great resource. I have spent several hours researching on this forum in the past 2 days, but would like advice on my specific situation.

We are having a 500 sq ft 12x24 unglazed rectified 10.5mm porcelain tile floor installed in a space that spans our old 1955 house and a new addition. The structural details spec out at L426 on the Deflectometer - if I did it right.

Joists 2x10, 16”OC, length in original house 12’, length in addition 10’. Full depth basement under entire structure.

Subfloor in old house 3/4” diagonal tongue and groove (looks like pine). Subfloor in new area 3/4” Advantech. Our contractor nailed 3/8” plywood over all to prepare for the tile.

The tile contractor installed lath and scratch (aka Jersey Mud Job) on Friday. I had no idea what those meant on Friday, but I do now. My husband and I started researching on Friday night because we though the job looked shoddy. Multiple sources (including your informative forum) indicated that the whole approach did not meet TCNA standards. This morning, the tile guy (we are in NJ…) gave us the usual Jersey Mud story (doing it for years, stronger than CBU, better than Ditra). But both agreed to rip up down to the subfloor and put down “something we would be comfortable with”.

The question is, what is correct. New (B/C glued) plywood over existing subfloor, but what thickness? Same plywood for CBU or Ditra? We are leaning toward Ditra because this floor is adjacent to a 3/4” hardwood floor, so there is already a level difference and Ditra would keep it to a minimum.

A second question relates to expansion joints. The space is not rectangular. Maximum span in one direction is 21 feet. In the other direction, there is a 3 foot wide section that spans 29 feet. 22 feet to a doorway and then an additional 7 feet in the laundry room. Are any expansion joints needed? In the 21 foot direction there is not a natural point for one, so I hope not. In the other direction, one in the doorway would not be problem (if it’s OK to do that).

Any help would be greatly appreciated.
DA
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Unread 08-16-2021, 10:21 AM   #2
jadnashua
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Over plywood, the minimum thickness of additional ply recognized by the TCNA is 3/8" as long as all faces are 'C' or better, but in reality, it's hard to find 3/8" ply that will lay flat, so most people tend to use 1/2". Over planks, I think, they call for a minimum of 1/2" ply.

Over two layers of ply, there is a TCNA procedure to bond directly to the plywood, but that also requires a special thinset. Most pros much prefer to use something in between, and that is either a cbu or a membrane.

The TCNA guidance on expansion joints has been changing, and I've not got the latest year's handbook...if that long section gets any natural light from direct windows or there are heating ducts running underneath, yes, I'd want an expansion joint. Expansion joints can be any one of three things:
- an actual gap (usually possible underneath molding along the wall)
- an elastomeric filled joint (often a silicone, but could be urethane or some other similar material often available in grout colors)
- an engineered joint (Schluter has the largest selection and was the company that introduced them to the industry decades ago)
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Unread 08-16-2021, 08:32 PM   #3
DAfromNJ
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Thank you Jim.

We are going with 1/2” plywood and Ditra. Seems like the best combination. As to expansion joints, the entire perimeter will have the trim applied above the tile, so that should help with expansion. Direct sun is on the hardwood floor, not the tile. Heating is all hot water baseboard. Reading the Ditra install, I think we will be OK with adding a silicone joint in the one doorway that is on the 29 foot axis.
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Unread 08-17-2021, 08:22 AM   #4
cx
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Quote:
Originally Posted by Jim
Over plywood, the minimum thickness of additional ply recognized by the TCNA is 3/8"
Quote:
Originally Posted by Jim
Over two layers of ply, there is a TCNA procedure to bond directly to the plywood, but that also requires a special thinset.
While both statements are correct, let me point out that they would not be correct together. The minimum underlayment thickness when bonding tile directly to plywood is nominal 1/2 material.
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