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Unread 12-17-2021, 08:51 AM   #1
MesaTileworks
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Joist Reinforcement Question

Howdy folks, I’m seeking a little advice Helping a friend tile their 6’x6’ bathroom. Corner shower, walls and tile on the floor. The tile is a mix of stone and ceramic—ceramic field tile but with a stone curb-topper, border, and baseboard tile.

Joist specs:
-2x6 with a span of 6’
-All are 16” OC except the first two as you enter the room which are 20” OC (probably laid out that way originally for plumbing reasons).

According to the deflectometer these specs are OK for ceramic but not for natural stone, so I’m thinking we will install a beam running under/perpendicular to the joists and support it on jack posts with concrete blocks underneath. (Not looking forward to working in the crawlspace but here we are… I maybe cried a little inside when I measured that 20” span ).

Three questions:

1) is 4x4 sufficient for the cross-beam, or should it be a 2x6 set on the vertical?
2) For the jack posts to sit on, can we use blocks like what are sold as “cap blocks” for topping a block wall? Or is there a more appropriate type of material to foot the posts?
3) Does the cross-beam need to extend the whole length or only order the two joists that are 20” apart? (Deflecto says 16” ones ate fine, but my instinct is to run it the whole width or most of it since we will already be crawling around down there, but if it’s difficult to maneuver a piece that long, could we get away with a partial span?

Link to the blocks and posts:

4-in x 8-in x 16-in Cap Concrete Block https://www.lowes.com/pd/Cap-Concret...625-in/3608974

34-in Adjustable Jack Post https://www.lowes.com/pd/Akron-34-in...k-Post/3201381

Thanks for any advice!

-matt
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Unread 12-17-2021, 09:17 AM   #2
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I’m reading back over some old threads here on deflection and joist reinforcement and also wondering if we should be concerned about deflection *between* those two joists. If so, would some cross-pieces running perpendicular and screwed into the joists be a sufficient fix? If not, I won’t obsess about that.

The toilet waste line is at one end of that bay, so I don’t think running a joist lengthwise is an option but I’ll look today when I get under there and see what’s going on.

The subfloor is two layers: 3/4” boards plus 3/4” BCX ply, so maybe that mitigates the between-joist deflection issue?
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Unread 12-17-2021, 09:18 AM   #3
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Without seeing a drawing of the structure and an actual layout of your various tiles, I'd say you have essentially a ceramic tile installation and I'd not be much concerned with the joist structure.

You don't give any indication of the subfloor material, but a second layer of nominal half-inch plywood over what you've got would likely result in a very good subfloor for your tile installation.

My opinion; worth price charged.
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Unread 12-17-2021, 09:56 AM   #4
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Hi CX,
The subfloor is two layers: 3/4” boards with 3/4” BCX ply on top. I’ll make a drawing that shows the position of the shower relative to the 20” span and where the stone border runs.
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Unread 12-17-2021, 10:34 AM   #5
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Here’s a rough drawing showing the position of the fixtures and orientation of the joists. Not perfectly to scale but hopefully enough to show the situation.

Three joists are 16” apart and then there is a 20” OC between the two that are on either side of the door in the toilet.

The floor tile is 3” hex ceramic, and if that were all I would feel fine about it but there is also going to be a strip of 6x12 marble tiles running around the perimeter (as a border for the hex).

Mostly I’m concerned about the floor and the top of the curb which is also going to be stone. There will be stone baseboard tile running around the wall but I’m not worried about that because well, it’s the wall and not the floor.
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Unread 12-17-2021, 10:46 AM   #6
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Here’s an idea of the proposed layout showing the stone curb top and stone border. The hashed lines are the hexagonal 3” ceramic.
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Unread 12-17-2021, 11:08 AM   #7
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I'm concerned with your joist span measurement, Matt. You're using the room dimension rather than the unsupported span of the joists, perhaps?

That subfloor should be fine over the 20" between-joist span.

My opinion; worth price charged.
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Unread 12-17-2021, 12:34 PM   #8
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Good question, CX. If I measure from drywall to drywall (interior room-footprint) it is more like a square that is 5’ 8” on a side. When I measured the joists in the crawlspace, I think I took the tape beyond the edge of the plates that the joists sit on and got a length of 6’ for joists in total. If I understand your question correctly not all of that 6’ length is “unsupported“ because that 6’ extends onto the plates on both ends. If I should be measuring differently (only the underside of the joist that is not supported) it’s possible that the deflectometer would spit out a different result? I will climb back in there and measure from plate to plate to get the exact length of the unsupported portion of the joists.
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Unread 12-17-2021, 01:53 PM   #9
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OK, I remeasured the joists and the unsupported span (not including the ends of the joists that sit on the sill) is 5’ 6”. I think when I put in 6’ I was measuring the whole thing end to end.

Reducing my numbers accordingly, the Deflecto says that at 5’ 6” I’m good for stone if SYP or Douglass fir, but not if I put on “unknown wood in good condition.”

I don’t actually know what the joists are. They are in good condition and to my eye just look like “normal” floor joist material but I don’t see a stamp on them. They are slightly orange-reddish in color.

Anyone have thoughts? Having reduced the span number I’m using and getting the partial thumbs up, I’m tempted to assume that the framing members are Syp or Douglas Fir and just proceed without the extra bracing, but not knowing for sure what the wood is gives me pause.
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Unread 12-17-2021, 02:58 PM   #10
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I was questioning whether you were actually measuring between joist supports, Matt, or just guessing by the room size.
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Unread 12-17-2021, 03:03 PM   #11
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Quote:
Originally Posted by cx View Post
I was questioning whether you were actually measuring between joist supports, Matt, or just guessing by the room size.

Ah, gotcha. No, those are the real measurements.

Turns out I was wrong about the 3” hex floor tile being ceramic. They are in fact stone.

… So I definitely want to make sure I’m within acceptable specs.
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Unread 12-20-2021, 11:38 AM   #12
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Pinging to stay up in the feed. Since I don’t know for sure what the joist material is (and the deflecto says no good for stone if the material is unknown) I’m leaning toward putting a 4x6 under the joists perpendicular to it, and supporting it on jacks footed in concrete cap blocks. Anyone have a take on whether this is good insurance or total overkill? Is there any overkill margin already built into the deflecto calculator?
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Unread 12-21-2021, 10:53 AM   #13
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I have read here, Matt, that the Deflecto tool is conservative. Just how conservative I don't know. Chances are that your joists are SYP but hard to say without seeing them. Can you get a photo of them fairly close up, but not too close, so we can see the grain? Are they full of knots and splits or mostly free of them?

Certainly a 4X6 under them would be good insurance in light of the unknowns. A couple of 2X6's would also work, assuming the span isn't too long, or that you have enough posts. You mentioned setting the posts on blocks; what would those blocks be sitting on?
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Unread 12-21-2021, 11:12 AM   #14
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With 3" hex tiles, Matt, or similar mosaic tiles, I personally don't think it makes a whole lotta difference whether they're ceramic or natural stone. What you've got is a bed of thinset mortar and grout with some small pieces of tile pressed in. When things start to move, chances are the grout joints are what's gonna fail first, regardless the composition of the tiles. But that's just:

My opinion; worth price charged.

The Deflecto uses a 50psf live load for its calculations, rather than the customary 40psf live and 10psf dead load. The intent was to compensate for the greater variation of conditions in remodeling work as opposed to new construction. If you compare the Deflectometer results with the results from a common span table, you'll see a substantial difference.
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Unread 12-21-2021, 11:18 AM   #15
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OK, thanks for the info, gentlemen. I’ll get a pic of the joists to post.

To answer the question about footing the posts I had to thought to sit them on 4x8x16 concrete cap blocks like they sell for topping a block wall.

Good point a lot the smaller tile, CX, that makes sense. The other variable in terms of size are the larger (6x12) stone tiles that will border the field tile.
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