Plywood Subfloor Seams

[Richard Tunison]

LOL

[Tiger Mountain Tile Inc]

I've started using duct tape to tape the seams when going over it with cbu, but I don't use anything when covering it with Ditra. I'm uncomfortable with an 1.5" unbonded strip running through a Ditra installation.

I have to admit that Tom makes a lot of sense. I never really thought about the science behind this.

For the sake of argument: If the unprotected end seams is what we are worried about, wouldn't it be beneficial to duct tape the seams to protect them from the moisture in the thinset when we go over them?

Let's get a real backerboard scientist on here to explain what they are so worried about in regards to the plywood seams. I bet less than 1% of the tile installs over plywood have seam tape on them.

[Higher Standard Tile]

And what percentage of cbu floors actually have thinset under the board?

This thread has been an interesting read. I don't see the harm in preventing the plywood joints from getting filled with thinset.

I have an upcoming renovation project where the GC removed the old tile and cbu.

New spec is 1/4" cbu over existing double layer ply. Top layer has no 1/8 gaps.

I've asked GC to cut in 1/8" joints in top layer. I'm going to cover plywood joints with duct tape.

I can hear the GC now if I didn't, " You have me cut the joints and then you go fill them with thinset? "

[DRBlais]

It sounds to me like a load of bunk. I have never had a crack telegraph through the substrate and subsequently through the tile in all the years that I have been installing. Let me qualify that statement by saying that I only use Kerabond/Keralastic.....Or Probond/Plus to bond the CBU to the subfloor. Today I used Proquick and Plus to do my Fiberock, but never with out the Bonding agent. It adds strength, flexibility and tenacious bond, I am of the mindset that I do NOT want the CBU moving independent of the substrate.On the top of the substrate for the tile I use Proquick half water half agent and same with the Probond. I prep all of my floors by screwing the seams with Torx 2 1/2 decking screws every 4 inches. My CBU is fastened every 4 inches and 3 inches on the seams . I also grind all the advantec seams to make sure they are not peaked especially the end seams falling on a joist. I don't know how this stacks up here, But I think that it is just one more layer of protection for the big manufactures to shift the blame from Material to Installer responsibility. That being said, I say we use schluter and do decorative borders around each individual sheet to insure proper expansion......I swear. Ridiculousness.

[Lump]

Here is where the problem lies, Cement board was introduced in 1969, fiberock in 2002? Advantech ? I joists etc. etc.

Do any of us truly know if today's modern building practices will last the 50 years that tile is meant to last?

Denny you are right, it comes back to the last guy who touched the floor....the installer.......that's why we caulk the joints/sometimes tape.

[DRBlais]

I guess that I can not really wrap my mind around there being enough difference keeping the joints clear of mortar, Even in contraction I would think that the rigid cement would have more of a tendency to cause the substrate to compress in those areas without causing any pressure on the tile above it, only testing the flexibility of the mortar used. However, using a siliconized caulk, I would think it may have a tendency to force the "gasket" up and down possibly causing failure if it was a big enough crack. I do not know how many thousand sheets of underlayment I have installed and never once sealing the seams in the recommended manner and Never, and I mean NEVER had so much as a cracked Tile! I really don't know, But it certainly is a good question... ...Dealing with Theoretical Tile 101. LOL

[Lump]

The more people involved in this discussion the better.

I have seen 4x8 subfloor joints telegraph through fiberock and tile structure......was it too much moisture in the OSB?, poor construction? Wrong glue from OSB to joists? Bottom line is I was the last guy to touch the floor so I paid to repair. Good discussion

[DRBlais]

depends on the movement, If its lateral movement, you can compensate for that. Not much under the canopy of heaven will stop vertical movement in a solid substrate. That opens up another can of worms for me......I dont trust Unmodified on kerdi or Ditra.

[Topspin]

To gap or not to gap, that is the question. So lets ask the sheriff, or at least his deputy - the NTCA. They've got to have an up to date opinion in this. I'll call Gerald monday to see what they have to add.

[Lump]

Kevin, I swear in the last year I saw Gerold taping subfloor joints in a video tile tip for Iwantmytiletv. Do you think for the life of me I can google that video? I also may be losing my marbles.?.

[Lump]

Here is one article I found that touches on protecting joints.http://www.ehow.com/way_5404060_requ...amic-tile.html

Again the article just touches on the subject.

[DRBlais]

So what do you do as an installer when you get to the job and Subflooring is rammed right tight together?

[Lump]

I wish it was only "rammed" together. Typically it's peaked up so high that you damn near trip over it.

[Topspin]

Halex Underlayment says "C. Leave a 1/8 inch gap between all panels and between the panels and the walls. Do not force panels together"

Georgia Pacific Document "Installation and Preparation of Plywood Underlayment" says:

[Lump]

Kevin, Post#39 has a link that talks about the same thing. Vinyl type coverings is what I feel they are covering or materials that can telegraph.

Telegraphing subfloor joints in vinyl (resilient) flooring is a huge problem. Why does the industry not see it as much anymore? Because today's vinyl floors no longer have a high sheen, they are also textured and have depth. They also went from a felt type back to a super thick fiberglass backing to hide some subfloor imperfections.

If we can install tile directly to plywood, which is risky in itself (I would wager most would agree) then how can we thinset down backer boards to the substrate fasten the heck out of it, then direct bond tile to the cement board? Does/will the backer board absorb some of inplane movement.?

Note: a fair amount of people heat with wood (very low RH this time of year) and a fair amount do not have a/c (very high moisture in summer months)

If all homes had perfect climate control (temp.andRH) year round then I would install direct to another layer of premium 1/2" plywood.

Just adding to the discussion