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Unread 11-27-2019, 10:55 PM   #1
Madumi
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21" Joists (old house) - suggestions

I'm re-doing a bathroom (9'x5') on the 2nd floor of an old house (circa 1900). Earlier today, I discovered that the floor joists are 19-21" apart. walls are offset between the 1st and 2nd floors, so placing new floor joists isn't an option.

The tile I'm thinking of using is approx 20"x10." If I use a decoupling layer above whatever floor board I put up there, what would be the minimum thickness/type of board I would need for the floor... or is there anything else I can do to stabilize the floor?

thanks!

ps. I've really appreciated reading the forum & the varied but helpful opinions expressed here
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Unread 11-27-2019, 11:09 PM   #2
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Welcome, Bert.

I don't understand this part:
Quote:
Originally Posted by Bert
...walls are offset between the 1st and 2nd floors, so placing new floor joists isn't an option.
Have you evaluated your joist structure using our handy Deflectolator in the dark blue bar near the top of the page?

And do I understand you currently have no subflooring installed?
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Unread 11-27-2019, 11:26 PM   #3
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Thanks for the reply & sorry for the ambiguity.

Yes, deflection is L / 652

By offset walls, I mean that the 2nd story walls are not lined up with the 1st story walls.

and yes, subfloor is being torn out--too many "edits" in it to be serviceable for tile...
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Unread 11-27-2019, 11:37 PM   #4
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Welcome to the forum, Bert.

Is this ceramic tile you're installing, or natural stone?

If it's ceramic, I would suggest a layer of 3/4" tongue and groove exterior glue (exposure 1) plywood, followed by a layer of the same plywood in 1/2" (not tongue and groove).

Normally you could get by with the single layer of 3/4" ply, but because your joist spacing is wider than normal, I would suggest the layer of 1/2" ply on top.

Some manufacturers of tile underlayments allow for their product to be used over a single layer of ply with the joists at 24" on center. I would not suggest that to anyone. And you might get by with a single layer on your joists since they're even less than that.

But if you have the height to allow for an extra 1/2" of plywood, I would certainly do it. The extra two sheets won't cost much money, and you could have it cut and in place in just a few minutes. It makes for a much stronger floor.
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Unread 11-27-2019, 11:48 PM   #5
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Thanks again for the reply & welcome.

Yes, it's ceramic tile we're considering.

I forgot to mention, we're trying to minimize the subfloor thickness (but not to the point of making it too weak)... just so the step up from the carpet outside isn't too excessive.

Are there other viable alternatives that would keep the subfloor thickness down?
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Unread 11-28-2019, 12:10 AM   #6
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While not an official method, some people have installed cleats on the sides of the joists, then installed ply between the joists on the cleats such that the top of it is aligned with the tops of the joists. With your sistered joists, that would be problematic to some amount since the new joists may not be at the same level as the old ones, leaving a gap in the support of the second layer (but it would probably be okay). But, if it did work, you could then add that second layer on top of the joists, and anchored like the second layer as described in the article in the 'Liberry', that would give you the additional strength between the joists and the decoupling effect of having two layers. You'd want to keep the grain running between the joists, so you'd end up with 4' wide pieces.

Personally, with new sistered joists and new subflooring, I wouldn't have any qualms installing DitraXL then tile, but I don't have as much experience as the pros here. If the bits under were not new, I'd definitely want that second layer. Those old joists are probably true 2" thick, so adding new nominal 2" material (1.5" thick) will cut that 21" gap down, approaching the nominal 19.2" spacing that some consider equivalent to the 16" OC in some situations. I'd talk to Schluter and get their blessing on what you have. You'll have less room for errors with one layer, but if you do it all properly, it should work. Depends on how much you trust yourself and your skill level. Think of the second layer as insurance...you may not need it, but will be happier if you do and it's there.
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Unread 11-28-2019, 02:34 AM   #7
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There are a couple of other less labor-intensive options.

You could use 3/8" ply as your top layer over the 3/4", for a total of 1 1/8".

You could also use 5/8" ply as the bottom layer and 3/8" on top, giving you a total of 1". The only problem with 3/8" ply is that it doesn't like to lay flat, so it's a little tougher to deal with. If you can find a sheet or two that are flat, go for it. Once it's secured properly, it won't be a problem.
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Unread 11-29-2019, 11:15 PM   #8
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Thanks again for replies.

I'm a little unclear about suggestions.
On the suggestions of two layers of sub-floor, is this for decoupling... or is this for strength?

Again, a couple more n00b Q's:

1) If I use joint sistering, what is the minimum thickness subfloor I could get away with?

2) If my subfloor is plywood, what would be the thinnest uncoupling layer I can use below the tile?

(I'm trying to avoid having too much of a step up from the carpeted area outside the bathroom going into the tiles of the bathroom).
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Unread 11-29-2019, 11:38 PM   #9
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A second layer of ply reinforces the seams of the first layer of subfloor (so as to reduce the bending stress at the seams) and stiffens the plywood layer in general so that there is less ‘between joist deflection’. In short, it’s cutting down the bending stress that’s exposed to the brittle layer of tile.

1) Minimum subfloor thickness is independent of joist deflection. The minimum thickness will depend on what method of install you’re choosing.

2) If you want uncoupling, the thinnest is 1/8” thick.

About the adjoining carpeting: Have you considered laying cedar shake shingles under the carpeting to very gently ‘ramp’ the carpeting up to meet the tile height? You can’t go crazy on this...but you can probably increase the height by at least 1/4” without being noticeable or a trip hazard. Just some food for thought.

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Unread 11-30-2019, 03:11 PM   #10
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thanks again...

Hmmm, I'm guessing I did the deflecto calculation wrong. How do I do it for a bathroom where the supporting wall underneath is close to the middle of the floor (1/3 of the way from one wall)?

Going out from this supporting wall, the entire joist length spans 14 feet to the sides of the house. The bathroom itself however, occupies only 19" of the joist on one side away from the supporting wall, and 43" of the joist on the other side away from the supporting wall.
I was also wrong about the joist width. They're 1-5/8, not as small as dimensional, but not nominal & 7-1/4 wide.

Any direction on how to do the calculation would be hugely appreciated, thx!
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Unread 11-30-2019, 03:53 PM   #11
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Quote:
Originally Posted by Madumi
Hmmm, I'm guessing I did the deflecto calculation wrong. How do I do it for a bathroom where the supporting wall underneath is close to the middle of the floor (1/3 of the way from one wall)?
It makes no difference where the bathroom is in relationship to the span. The curvature of the joist is the same along the entire length.

As far as your question about thicker joists: The width of the joist is cumulative to the deflection rating. So, if you had a 1 1/2” joist of a certain height, span, and O.C. joist spacing that equaled L/360....but you compared it to the same scenario with joists that were 8.3% wider at 1 5/8”, you’d have a deflection rating of 8.3% more at L/390.

Can you tell us all the exact joist measurements?

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Unread 11-30-2019, 06:03 PM   #12
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Sure. It's a pretty small bathroom space. Joists running from the outside walls, overlap over the middle support.

I've included dimensions on a sketch, viewed from the front & just the bathroom viewed from the top. The two lines sketched on the Joist are the Joist overlap.

Again, Joists are 1-5/8 x 7 1/4
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Unread 11-30-2019, 10:53 PM   #13
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Bert, are you depicting the supports below your joists in the bathroom area, or the walls above the floor?

The 2x8's that are spanning the 13.5 feet are pretty substantially overspanned even of they are of particularly good grade and species. Is that the actual support under the bathroom area as well, with the overlapped joists spanning 11 and 13.5 feet?
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Unread 12-01-2019, 08:25 AM   #14
Madumi
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The left hand drawing is depicting the walls below and above the floor supporting the bathroom area (62" wide area), and the right hand drawing is just the bathroom area, looking from above.

Surprisingly, the floors have always felt solid in the house. I don't know if it has made a difference, but the floors and walls are all covered with 3/4" boards, top and bottom, T&G on top, shiplap on the bottom.

Does that make more sense?
thanks!
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Unread 12-01-2019, 09:53 AM   #15
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Quote:
Originally Posted by Bert
...walls are offset between the 1st and 2nd floors, so placing new floor joists isn't an option.
Quote:
The left hand drawing is depicting the walls below and above the floor supporting the bathroom area...
Nope, that just makes it a bit more confusing to at least one of us, Bert.

We need to know the unsupported span of the joists that are under the subfloor of the area to be tiled. I can't tell from your drawings and descriptions what that might be.
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