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Unread 07-26-2022, 07:41 PM   #1
ninamarie8253
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How do I lay tile down on 5/8" subfloor with possible deflection issues?

We just moved into a split level house built in the 60s. The previous kitchen floor (which is on the 2nd floor over a finished basement) had 12x12" tile that cracked in 7 different random tiles. Next to that is 0.75" high hardwood floors.

Our subfloors are 5/8" plywood with 2x8" joists 16" OC with 12ft length (from what I can tell) between beams. And the center of the house dips down about 0.75" right there, so leveling will bring us nearly to the height of the hardwood floors anyways. But if all of that is right, then I think we have too much deflection for tile.

We want to add 12x24" porcelain tiles. But we are trying to figure out the best method to do so. Our contractor wants to do the old school wire/mesh route, leveling it with sand+cement mix and then doing the mud job. I was thinking Ditra XL - but not sure if I need to add 0.5" plywood boards to thicken the rigitidy of the floor?

Even so, now I'm worried about deflection, and if we should cut open the finished basement ceiling to add more joist support?
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Unread 07-26-2022, 08:09 PM   #2
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Hi Nina, welcome to the forum.
You’ve got the classic problem of a floor with over-spanned joists. I ran information you provided through the Deflecto tool in the blue bar above with the following results.

Quote:
Thank you for using the John Bridge Forums Deflect-O-Lator :-)

For joists that are SYP or Douglas Fir, in good condition, 7.25 inches tall, 1.5 inches wide, 16 inches on center, and 12 feet long between supports, the deflection calculated is 0.478 inches.

This translates to a deflection of L / 302.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.
The best way to address this situation is to either sister (double) the joists or add a support beam to reduce the unsupported span to less than 11’. Both of these approaches require access to the floor from below but adding the support may be the better option since you have a finished ceiling.
Is it possible to add a support wall along one side of the room?
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Unread 07-26-2022, 08:12 PM   #3
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Thank you! I'm glad to be here.

I agree, I'll speak with the contractor tomorrow morning about it, not sure what he'll say, as it's extra work for him. The floor below the kitchen is a mostly opened finished basement, which means we'll need to remove and re-do the sheetrock. Hope he'll be up to the task if thats truly what it needs. Any other way of doing it without accessing below?

Also my Q is, should I let him do the mud + wire job he recommended doing, or Ditra XL? And if so, should we add 0.5" plywood board overtop of our 5/8" subfloor, or is that not necessary? The floor also still needs leveling before we do anything.

Thank you!
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Unread 07-26-2022, 08:15 PM   #4
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If you add a support wall about 2 foot off of one side of the basement you could turn that into a storage closet. You really only have to move drywall from the ceiling in the area where the new wall will be located.

Once I felt the floor structure was stiff and flat enough to meet the requirements for large porcelain tile, I would install a second layer of BC plywood and then standard Ditra. You may be able to get away without the second layer of plywood if you use Ditra XL but I really have no experience with the XL version of the product.

Follow up question: When you say you have a dip, is the dip in the center of the floor span where you want to install the tile? Meaning the joist are dipped? Or is over the beam the low spot on the floor?

Tile floors don’t necessarily have to be perfectly level but they do need to be very flat. Is the floor flat within 1/8 inch over 4 feet? I’m not sure of the exact TCNA flatness requirements for 12 x 24 tile but that’s a good first check.

Last edited by PC7060; 07-26-2022 at 08:24 PM.
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Unread 07-26-2022, 08:38 PM   #5
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I wish it was that easy, but on one side is a mechanical room enterance and to the right of that a bathroom, on the otherside is a fireplace. So there isnt a good spot to do that, unless we add a post in the middle of the room which I dont want to do. So I'd rather sister it - is that easy enough to do once we remove sheetrock on the ceiling? Can we sister it after installing or no because thats too much shaking and hammering for porcelain tile? I have extra pressure treated 2x4's, what wood do I need to use to sister the joists?

The dip is in the center of the house, where the kitchen ends (our house is about 24ft wide, so the kitchen is 12ft span just about). and yes over the beam is now the low spot in the house it seems, my contractor thinks they installed the beam after building the house. idk though

What is BC plywood? When I called Schluter they seemd to say I could use the Ditra XL over my existing subfloor, but I didnt tell them it was unlevel (I'll call back tomorrow to do that). Dont know how much I can trust their customer service or if they really know what they are talking about.

I'll say this, from 12ft one side to the other, its 0.75" unlevel at most from what I remember the contractor saying. Thank you so much for helping me.
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Unread 07-26-2022, 09:09 PM   #6
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Welcome, Nina Marie.

Your subfloor doesn't need to be level at all, but it needs to be very flat. The industry standard for tiles your size is no deviation from intended plane of more than 1/8th" in ten feet nor 1/16th" in two feet. That's a very flat floor and difficult to achieve.

I can't telly what your contractor might mean by using "do the old school wire/mesh route, leveling it with sand+cement mix and then doing the mud job." There are two methods of using "dry-pack", or "deck mud" forms of mortar over a wood framed floor and your subfloor qualifies for only one of them, which requires a mortar bed of a minimum 1 1/4" thickness with welded wire mesh in the vertical center. That adds a good deal of dead load to a floor structure that is already deficient and would not be a good idea.

The mortar bed, properly executed, would provide the best possible substrate for your tile, but it doesn't sound like you have the vertical space for it and certainly don't have the required joist structure.

If you decide to sister your joists, you might consider removing the existing subfloor (should be jail time for any contractor who installs nominal 5/8ths" plywood as a subfloor), sistering the joists such that the tops are in plane and level if you want it level, then install a minimum of nominal 3/4" plywood as your first subfloor layer. On top of that you could install the tile substrate of your choice.

BC is a type of structural plywood with a face grade of B on one side and C on the other. Almost always an exterior glue plywood, which is what you need. And I'm aware that Schluter indicates their Ditra for use over nominal 5/8ths" plywood over 16" joist centers, but I wouldn't do that on a bet. Consider that the testing for that installation is done using new material, in perfect condition, near perfectly installed over joists with zero deflection, and it needs pass the test only once.

Entirely up to you, of course.

My opinion; worth price charged.
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Unread 07-26-2022, 09:20 PM   #7
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Thank you so so much I was so excited to see that you responded to my issue here!

So I agree, adding extra heavy cement sounds like a not great idea when the joists aren't as beefed up as they are in new construction using 2x10s vs our 2x8s. Is that what you are also saying?

Old houses were all built with 5/8" plywood subfloors, sucks very much. But the contractor says to cut the subfloors is not a great idea because its providing some structural integrity to the exteror walls of the house - so he recommends adding 3/8" or 0.5" plywood if I choose to ontop.

Do you think Ditra XL could be a solution to any of this? And I think he was talking about the traditional mortar bed with the wire mesh, but I'm not positive I understand the difference.

I'm going to talk to him about sistering the joists tomorrow - can I use scrap 2x4x8ft pressure treated boards to do that or does it need to be the same size as the joists? Thats going to get pricy. And again I Dont think hes gona want to remove the subfloor that's providing some support to the ingegrity of the house, but rather just add ontop, and maybe level it out another way?

All of this is sounding like we should just go luxury vinyl and save ourselves the headache. Or else we risk trying it and maybe it works or fails over some years
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Unread 07-26-2022, 09:31 PM   #8
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Quote:
Originally Posted by Nina
And I think he was talking about the traditional mortar bed with the wire mesh, but I'm not positive I understand the difference.
What I described above is the only "traditional" mortar bed that is suitable for use over your existing subfloor (or was when the subfloor was new). There is a thinner mortar bed over expanded metal lath that is only 3/4" thick as a minimum, but that requires at least a nominal 3/4" subfloor. And both, of course, require a suitable joist structure.

Adding plywood on top of what you've got is certainly an option, but I'd want to see you correct your joist structure before you did that.

I think it unlikely that removing and replacing the existing subfloor would significantly effect the structural stability of your house, but I'm not there to see all the ramifications of that.

I've seen some very nice looking LVT out there.

My opinion; worth price charged.
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Unread 07-26-2022, 09:41 PM   #9
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Someone else I was speaking to gave me this idea, what do you think of it? Does this sound structurally sound enough to provide the added stability for the contractors mud-n-mesh way of doing things that you described (that im hoping the contractor meant when he told me 'mud job')?


'Additionally, I'd go under the house and put in additional supports, at least in the mid span, if not two rows of support, eliminating the need for the additional layer of subfloor. Mid-span would be the center of the joist length, two supports would cut the overall span into 3 sections, thereby eliminating any possibility for deflection and cracking of the new floor.'



Hahahaha Im seriously considering LVT now too, but it lacks that luxury shine that high gloss porcelain has...
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Unread 07-26-2022, 09:58 PM   #10
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The additional support need not be in the center of the span at all, Nina Marie. Go back to PC's post #2 and #4. You need only reduce the unsupported span to about 10 feet in order to meet the required L/360 deflection minimum.

You could, of course, put as many additional supports as you want, but once you've reached the required deflection, you're not really gaining anything of value.
Quote:
Originally Posted by Your Friend
I'd go under the house and put in additional supports, at least in the mid span, if not two rows of support, eliminating the need for the additional layer of subfloor.
The additional joist supports would have nothing at all to do with the subfloor requirements. Two completely different categories of deflection: One is the joist deflection, the other is the between-joist deflection of the subfloor. Correcting one, does not correct the other.

My opinion; worth price charged.
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Unread 07-26-2022, 10:05 PM   #11
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Okay perfect, thank you so much for clarifying. I'll run this by the contractor tomorrow - but it sounds like if we do that, will it Then meet the requirements to add the thick mud bed that the contractor wants to add (and forgo Ditra XL + extra plywood subfloor support)?

Also the guy who said that last idea also said this:
'I wouldn't sister joists, I'd add piers and a support header under the joists to carry the mid-span weight. Both methods (sisters or piers) have their installation issues, so I don't really see one being more expensive than the other. I DO see piers and header being the better way to go.'

Just trying to understand this all, thank you very much.

ALSO - if my calculations are wrong and the deflection is indeed OK - do we proceed with the mud bed or is that still to heavy etc? or will the mud bed (needing to be 1" thick atleast" be too high to saddle along the hardwood (he says after he levels the floors, one side will be 0" leveling and the other side 0.75" leveling, nearly butting up against the hardwood). so it looks like you cant achieve a 1+" mud without that much of a saddle difference. IF we do Ditra XL over our existing plywood subfloors + leveler, then we're raising it 0.65" which is better...
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Unread 07-27-2022, 07:26 AM   #12
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The mud bed is not appropriate for this application as CX described in post 6 above. I also agree with CX recommendation to remove the subfloor, sister the joist such that you have a flat floor (not level but flat) and then install 3/4” Advantech, Plytanium or similar T&G subfloor. Then you will be ready for Ditra and the tile.

Or you can take the chance the floor structure is good as and use DitraXL. Your call, it’s not our house and I can’t see your floor very well from here.
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Unread 07-27-2022, 07:46 AM   #13
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Nina Marie, the fellow suggesting piers and columns and headers is saying basically the same thing as PC mentioned above regarding adding a support wall. Both will reduce the joist span. The piers/columns/header approach will, of course, add support columns in the basement - which you previously mentioned you'd rather avoid.

Sistering the joist will resolve the joist deflection issue, but sistering could still be a challenge because of existing duct work, piping, and electrical lines running between and through the existing joists. Not impossible to do but would add cost. IMO, if you decide to sister them, I'd rather remove the drywall from the ceiling in the basement than remove the subfloor. But that's just me.

If you decide to add a mud bed on top of the existing 5/8" subfloor, the height of the mud bed plus the thickness of the thinset mortar and tile, will put you will over the height of the adjoining hardwood.

Just some food for thought...
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