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Unread 12-31-2020, 05:34 PM   #1
ScullyM
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Old Subfloor Quandary

Our house was built in 1926. Due to a mold problem in the unfinished basement and a small leak from the toilet in the bathroom above it, the floor of the bathroom had to be torn out to the subfloor (3/4" thick diagonal boards). On top of the subfloor, there had been some sort of hard impervious composite material that also covered the front of the tub and 7" up the walls in one solid unit (no seams). When we bought the house in 2000, we tiled this surface with sheets of 1" hex mosaic tiles and had no problems with it over the years. We want to tile again with the same type of tile. The question is how to adequately support the installation.

The overall bathroom floor is 7' x 5 1/2'. The area to be tiled is 4 1/2' x 5 1/2'. The finished floor in the hallway is 15/16" from the top of the subfloor. Ideally, the finished bathroom flooring would be at the same height. The subfloor boards are not rotten, but have holes and cracks and metal plates reinforcing them in places. All are otherwise intact except one that is cracked partway through along its width where it had been flexing. The joists are in great shape, although the one directly next to the toilet flange is deeply notched. They are 1.5" x 7.25" and spaced 24" apart. There is a beam supporting the joists about a foot into the bathroom floor from the wall with all the plumbing.
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Unread 12-31-2020, 07:28 PM   #2
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Welcome, Scully.

While it's quite common for folks to assume that since their floor had ceramic tile before it is certainly suitable for ceramic tile again, just the way it is. I recommend against that mindset.

You need to know the longest unsupported span of the joists under the area to be tiled. Nothing to do with the size of the floor to be tiled, you need the unsupported span of the joist. The type, species, grade, spacing and condition of the joists, along with that unsupported span will tell you if the joist structure meets the ceramic tile industry deflection requirement, which is also the residential building code requirement. You'll see a Deflectometer link in the dark blue bar near the top of the page that can be of substantial help in making that determination.

Your joist spacing is very wide. That, combined with the diagonal orientation of your board subfloor, is problematic from the standpoint of your between-joist deflection, which is very important. The fact that it consists of sawn boards makes it unsuitable for use with any of the known tile installation substrates other than a reinforced mortar bed, but even that requires a joist spacing of no more than 16 inches on center.

I've no clue what the "composite" material you previously had as a tile substrate might have been. I wish you had at least a photo or two of it. The only think I can think of is a traditional hot-mop sort of coating, but I've never heard of that in anything but a shower application.

There is an accepted method for tile over sawn board subflooring that also requires 16" joist spacing, but instead of the mortar bed you must install a minimum of nominal 1/2" exterior glue plywood and a suitable tile substrate. That method also presumes that the board subflooring is T&G style and oriented perpendicular to the joists.

Bottom line? You obviously must sell the house and move to a tileable structure.

If it were my house, and if I really wanted tile on that floor, and if I found my joist structure to meet the deflection requirement, I would install at least a nominal 5/8ths" plywood over what I have (after re-fastening, using screws, all the existing boards), then install a suitable tile substrate and tile. See my warranty information below.

My opinion; worth price charged.
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Unread 01-14-2021, 02:56 PM   #3
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Thanks for your quick reply, CX!

To clarify, the diagonal boards of the subfloor are tongue-in-groove.

The joists look like old growth CVG doug fir in great condition except for the notch next to the toilet flange. The longest unsupported span under the bathroom and adjacent bedroom is about 13’. According to the superfantastic Deflect-O-Lator, the deflection is L/163 (0.958 inches). Can this be solved without adding beams and new footings? We don’t want to sell our house, but obviously we MUST have tile! ????

Not sure if it would be possible to get them in place, but would it solve the deflection problem to add two or three joists midway between the existing ones under the area of the bathroom to be tiled? Or would sistering the joists do the trick? If so, how many sisters and how long would they need to be (i.e. just under the bathroom or along the full span)?

In case the deflection problem can be adequately addressed, I have been exploring using exterior grade ply over the existing subfloor with something like the Schluter-Ditra uncoupling substrate, but I’m not sure I can use it with the 1” hex tiles. The instructions specify a 2”x2” minimum. The 1”x1” hex come in 12”x24” sheets. Does that make a difference? Another option would be Tavy Thin-Skin, but then the question is would that system need waterproofing and if so, how? This is for the bathroom floor outside the tub/shower, not a wet installation.

Thanks for the help!
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Unread 01-14-2021, 03:16 PM   #4
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Wow! I would expect that floor to feel much like a stiff trampoline with that joist system, Scully.

Even adding full length 2x8 joists between the joists you have would not get you to the required L/360 deflection for your ceramic tile. That composite material you had on the floor under your previous tile installation must have had some magic in it, think I.

On paper you might be able to install full length 2x10 joists between the existing joists and get to the necessary stiffness, but it's not really correct in the real world.

I really think the only reasonable way to stiffen that joist system would be to install a mid-span wall or beam under the joists to reduce the unsupported span. You'd need to reduce the span to no more than about 9 feet to make it work. Chance you could install a beam in that area of one end or the other of the Joists?

No waterproofing is generally required on a bathroom floor, but if you think your floor will get wet, you should install some waterproofing.

My opinion; worth price charged.
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Unread 01-14-2021, 03:45 PM   #5
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Even on a connected sheet, the 1x1" tile won't work well over Ditra...the goal is to have each tile supported by enough thinset towers so a point load won't cause it to tip into one of the plastic ridges in the membrane. NOrmally, outside of the shower, a bathroom floor isn't considered a wet area and while you normally do want something between the subfloor and the tile, in your case, with your preferred tile, Ditra won't work. CBU, or a different sheet waterproofing membrane could, or use a painted on liquid waterproofing material could, though.
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Unread 01-14-2021, 07:01 PM   #6
ScullyM
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Jadnashua, thanks for the very clear explanation. Now I understand the 2"x2" minimum tile size.

CX, I'll have to look into what it would take to add a beam. In the meantime, just for fun, I've included a picture of the mystery composite material. Perhaps someone on this forum can identify it. I found some small (about 1" long) pieces wedged between strips of lath on the walls near the floor. It was definitely applied on site - there are places on the material where it took an imprint of the wood and was colored by the tannins. The substance itself is a pale yellow with sparkly white pieces of what looks like quartz. It's very hard and dense, like stone, and was 1/2" to 3/4" thick in different areas. Any ideas? Wish we could put it back in!
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Unread 01-15-2021, 04:01 PM   #7
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In lieu of adding a beam, it looks like two additional 2x8 joists between the current joists would put us within the maximum deflection limits. Do you see any problems with this approach?
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Unread 01-16-2021, 07:50 AM   #8
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Scully, see CX's post, #4 above.
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Unread 01-17-2021, 10:29 AM   #9
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Ss3964spd, Thanks for your response. To clarify my most recent post, I am proposing to add two full-length 2x8 joists evenly spaced between each pair of existing joists, which would reduce the span between joists from 24" to 8". I would be adding 5 or 6 new joists in total under the area to be tiled.
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Unread 01-17-2021, 11:47 AM   #10
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Scully, while that would solve both your joist deflection and between joist deflection problems, it's far from an easy solution judging by what we see in your photos. I think trying to work around all that plumbing and wiring will not be a simple task. You have a plan?
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Unread 01-17-2021, 01:30 PM   #11
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Smile

Hi CX, we are thinking about taking down the OSB ceiling on the other side of the beam and putting the new joists in from that direction - more room for maneuvering and just a couple wires to reroute. Hopefully, we won't discover any obstructions when we open the ceiling. There is insulation in the cavities, so I can only see so far into them. Previous owners did some strange "repairs" to this house over the years! We're going to get a couple bids for the work. Hopefully it will be reasonably affordable and we can have our tile! Thanks so much for all of your help!
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