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Unread 04-04-2020, 08:54 AM   #1
whatmemory?
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Retiling bath, underlayment and plan of attack questions

Hi All, first time posting, and about to tackle my first tile job. Renovating a full bath ~40 s/f. Subfloor is 3/4" plywood (not T&G from what I can see). Existing underlayment is 3/8" plywood, existing ceramic tile laid directly on thinset on the plywood. Some of the original tiles (~8) in main field between vanity and toilet had cracked I assume, because a previous homeowner did a hack replacement job - tiles slightly different variation, plenty of visible lippage and sloppy grout work (can also see in pics that they used a different thinset). The replacement tiles, not surprisingly, cracked as well. Sadly I already started chipping out most of the cracked tiles (easy starting point) before I found this forum, but the pics show at least a couple w/ lippage & cracks in the main field and one under the right side of toilet that's cracked as well.

Ran deflectolator = L/442. (Joists unknown lumber, 9" x 1.5", span of 11.5'. Joist spacing varies, one bay 15.5" OC, another 16.5" and another 16". I split the difference and ran calc w/ 16" OC, faulty logic I'm sure.

Ideally I want to lay a rectangular / fairly large format porcelain tile (undecided on size, maybe 12 x 24 or 6 x 20 maybe). Definitely will not be laying stone. Intend to use Ditra.

Questions:
1) Any concerns with this plan that jump out?
2) Could the original tiles have cracked simply because tile was laid on plywood rather than on backer or an uncoupling membrane? Original tile job looked ok to my untrained eye (no real visible lippage, grout lines consistent).
3) Should I definitely replace existing underlayment? If so am I ok to replace w/ fresh piece of 3/8" plywood (saw some posts stating exterior grade required, CCX or BCX)? If new underlayment needed I will read posts on placement/screwing requirements.
Note, existing tile is flush with hardwood in adjacent hallway, and unless my math is off I don't think I can keep it that way if I'm using Ditra (thinset above & below)....but I'd like to keep it as low as possible to avoid having trim too much (entry door, bifold door, trim).

Thanks in advance, the info on this forum is a huge help.
John
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Unread 04-04-2020, 09:55 AM   #2
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Welcome, John.

Your first problem, if your assessment of your subfloor is accurate, is the lack of center-matched edges on those plywood panels. How did you determine that the long edges were not T&G style?

While there is an industry accepted method of installing ceramic tile directly to a plywood underlayment, there are requirements that your existing nominal 3/8ths" plywood does not meet. That second layer would need to be a minimum of nominal 1/2" exterior glue plywood (there is no exterior grade) with no face of grade lower than C and it would need to be properly installed over the subfloor. Your slightly varied joist spacing would have no meaningful effect on such a floor.

The cracking tiles would generally indicate movement of some kind. If your first layer of subflooring is actually square edged, that could ammount to some of the problem. The nominal 3/8ths" plywood, if not sufficiently flat and well fastened could be another. The lack of sufficient thinset mortar coverage on the backs of the tiles could be still another.

Several ways to approach the new installation, but I'd first want verification of the type and condition of the first layer of subflooring. Do you have access to the subfloor from below?

My opinion; worth price charged.
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Unread 04-08-2020, 07:25 PM   #3
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Appreciate the reply CX. Just getting my head above water.

Yes I do have access to the subfloor from below, unfinished basement. Bad news is a lot of ductwork in the way, and most of the subfloor seams are obscured by blocking, whether by design or by chance I don't know. Fyi house built in early 80's for what that's worth, so I'm pretty confident subfloor laid back then.

1st pic underneath tub, see seam...doesn't appear to be T&G to me? I could be wrong.
2nd pic - Odd angle, but at bottom of pic you can see someone added a 2x4 under subfloor, I assume as extra support. While it might be in the bay where some of the cracking happened, I think it's on the wrong side (still underneath tub, not tile field). Would a 2x4 even be large enough to add integrity?
3rd pic -Plywood stamp. Unsure if anything in code would denote T&G ?
4th - A seam I found, was able to fit a piece of card stock almost 1" up (again subfloor 3/4"), seam while not super tight was tight enough that if there was a tongue there I'd have assumed I'd hit it half way up.

Regarding your point on the thinset coverage, I was able to get a few of the tiles to chip/pop out in tact and coverage looked a bit uneven. Granted these tiles weren't in the problem area but I'd think still lends some credence to your theory.

Let me know your thoughts.

Thanks,
John
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Unread 04-11-2020, 05:14 PM   #4
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Hi All,

Just kicking this thread back up to follow up.

Thanks
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Unread 04-11-2020, 05:45 PM   #5
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Difficult to get perspective in those photos sometimes, John, but that plywood edge in your tub drain photo does not look like 3/4" to me. You're sure your first layer is that thick?

I can't read your grade stamp very well, but if it's marked as T&G you would expect to find that in the area where it says it's Sized For Spacing. I'm not at all sure it's a marking requirement, though.

I can't tell from your photos if you have T&G or not. If you're sure it's not, that's a problem. 'Specially if it's not actually 23/32nds" thick. I would recommend you drill a hole in the center of a joint, preferably with a Forstner bit, and see if you can determine if the panels are T&G style. But your card trick should be sufficient to say they're not.

I don't see any indication that a second layer of plywood subflooring has been installed at all. There should be many fastener points visible in the field between joists in your photos.

The 2x4 you mention could be fastened securely to the joist to which it's adjacent and might be installed as a "nailer." If so, that should not be a factor.

My opinion; worth price charged.
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Unread 04-12-2020, 03:01 PM   #6
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We had similar challenges with a small full bathroom renovation many years ago. It was a small bath: 4 x 10.5, with 36x42 shower. They wanted all natural stone.

We started with a full gut. The floor joists were awful. It was a mix of 2x3 and 2x5 over a crawlspace, with minimal support.

This would not do at all - we needed a VERY stiff floor to avoid cracked tiles (expensive 12x12 natural stone, 3/8" thick).

We used a brute force approach. IMO, the best way to decrease deflection (if one has access) is under the plywood. We used 2x6 and added double sisters to all floor joists. Then, added more blocking throughout. Then, installed new plywood. For minimal cost, there is no deflection at all.

IMO, I would forgo Ditra. There is no need to add the height or cost, as there will be no deflection.

In our case, we installed under floor electric heat, embedded in a thin layer of SLC. Material cost about $250 and took maybe half a day.

This was some 10 years ago. We have done several more projects in that house since then. There has been no tile cracking at all.

I hope that this helps.
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Unread 04-12-2020, 03:31 PM   #7
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John doesn't have a joist deflection issue here, JS, his problems are primarily in his subflooring. Adding joist sisters won't correct that.
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Unread 04-12-2020, 03:42 PM   #8
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With reinforcement of the joists he would not need a 2nd layer of plywood or cement board, etc. He seems concerned about no adding height, so this is an easy way to make that floor stiff. Naturally, he should replace the plywood regardless.
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Unread 04-14-2020, 07:25 PM   #9
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CX, JS thanks both.

JS - I'm not so sure it's a deflection issue but I'm not opposed to go extra mile and install some sisters & blocks just in case. But a lot of ducts and pipes in the way, not sure I can get full pieces of lumber in there from sil to main beam. Do the sisters need to be that long? Need to be contiguous? Any magic to installing them, or just push flush to underside of subfloor and nail in place? One 2 x 6 on both sides of every joist I assume right?

CX - While I see your point that there aren't any/many visable screws or nails in pics of joist bays....there definitely is an underlayment / 2nd layer of plywood. See latest pics, I pulled floor register/diffuser off in bathroom...can clearly see the subfloor (3/4" actual, it's thick) and 11/32" plywood. I do see drywall screws roughly 5.5-7" apart around the perimeter, and others in the small amount of field I've exposed (more randomly spaced). I wonder if they only used 1" or 1.25" screws...would explain why we don't see them from below, haven't removed one yet to confirm. Regardless I doubt they followed recommended fastener placement guidelines to a T.
Per your suggestion I drilled a hole w/ a forstner bit in a seam of the subfloor (directly under tub, so no underlayment present there). Looks like all we can see is just layers of the plywood, no T&G.

1 - I'm getting that I'd be best off replacing the underlayment, correct? Do you think I could get away with replacing old underlayment w/ fresh layer of 11/32" and going w/ Ditra (not afraid of the cost) w/ proper fastener placement, or am I just asking for trouble sticking w/ 11/32?

2 - If I'm willing to deal w/ the added height, would pulling old underlayment and replacing with 1/2" plywood suffice, and/or is there anything else I'd need to do?

Thanks
John
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Unread 04-14-2020, 08:52 PM   #10
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Quote:
Originally Posted by JS
With reinforcement of the joists he would not need a 2nd layer of plywood or cement board, etc.
You'll need to explain that for me, JS. The OP has square-edge plywood as his first layer. How is your joist sistering going to correct that deficiency?

Good photo of that drilled hole, John. I sure don't see any evidence of any center-match there, either. Regardless what else you do, you'll want to correct that. You don't need any structural framing members to do that, you just need something to tie the two sheets of subflooring together at each seam between joists. I usually use 5 or 6-inch rips of 3/4" plywood for that when required, each cut just long enough to fit between joists without touching at either end. Those blocks are then centered across the seam with construction adhesive and screws securing each edge of the subfloor to the blocking.

If you've removed your top layer of plywood (recommended in your case) you could put just a couple screws or nails from below to hold the block in place while you go topside and screw the subfloor edges permanently. Or you could simply screw the blocks in place permanently from below and then cut off the screws on top using an angle grinder with a thin cut-off wheel. You don't care what it looks like, eh?

Once that's done, you can decide what you want to use for your tiling substrate. Again, if you elect to tile directly to plywood (not what I would recommend), you must use a minimum of nominal 1/2" material as the second layer. Adding that layer would make you a better subfloor for whatever tiling substrate you elect to use, but it's not required for a CBU or membrane of some type.

That nominal 3/8ths" plywood you have currently, installed with the incorrect type and length and possibly schedule of fasteners could also have contributed to your tiles cracking. Unless it happens to be glued down with a full spread of wood glue, it could very well have voids under it in places that allowed vertical movement. Of course, if it does happen to be glued down, you have this other problem.

I think the most important step here is to fix that first layer of subflooring and get rid of the second. Then you can give further thought to how you want to prepare for your new tile.

My opinion; worth price charged.
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Unread 04-15-2020, 04:43 PM   #11
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CX - full disclosure there's blocking under almost all the subfloor seams right now, I assume that would not accomplish the same thing as the glued and screwed plywood strips correct? (certainly wouldn't prevent lateral/horizontal movement).

As to removing the 2nd layer of plywood (we'll assume for now it's not glued...Murphy's law would say otherwise) - most/all of the screw heads are embedded w/ thinset. I'm sure this is the norm. Should I just try to pry the plywood up with a demo bar? Or is there another approach. I thought of multitooling the screw heads but that sounds awfully tedious.
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Unread 04-15-2020, 05:26 PM   #12
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Continuing on previous post.....just setting circ saw to depth of underlayment and cutting sections out would work well enough I assume right?
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Unread 04-16-2020, 06:35 AM   #13
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John,

If you can find the screw heads there's a good chance you'll be able to force the driver bit through the mortar in the screw head and remove the screw. The use of an impact drill/driver might help. Removing them would be preferable to having the heads snap off - which drywall screws might do in this situation, leaving a bunch of drywall screw nubs that you'll have to deal with.

Yes, just set the blade depth to be the same as the subfloor thickness. Use a blade you don't care about.
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Unread 04-16-2020, 03:51 PM   #14
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Thanks SS, appreciated. Was just able to get a couple out by scraping out the mortar with a 5 in 1 first and then impact driver.
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Unread 04-16-2020, 09:49 PM   #15
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@whatmemory

I would absolutely rip out the current subfloor. 3/8 is simply too thin and has too much inherent flex.

Agree with CX - min 1/2. Better yet at least 5/8. I would recommend "high tech" moisture resistant subfloor like Advantech or similar. It's great stuff. Lowes seems to carry it now? When we originally started using it, it was only available at specialty supply houses. Unlike OSB/Plywood it seems impervious to water. There are some great youtube videos on it, that you may find interesting.

Also, by tearing out the existing subfloor, you maximize access to install sisters and blocking for those joists. It is the easiest and least expensive way to make that floor very solid. This will also give you the most room to add longer pieces, without the other stuff in your way. Ideally, use the longest pieces that you can. Preferably 2x6 but if you have to notch or use 2x4, you will still end up with a very solid structure.

Once you clamp the sister in place then shoot a few nails to hold it - then I would secure the rest using structural screws. An impact drill makes this easier.

Personally, I would skip the Ditra. The new subfloor will not allow any movement / deflection (so no cracking). There is nothing "wrong" with Ditra but here it will add height and not provide a benefit - and you expressed concern over matching that room to the next.

All the best and hope this helps!
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