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Unread 03-26-2021, 09:30 AM   #1
SiRDOHC
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Borderline Deflection Figures for porcelain tile install.

Hi guys,

I have read many posts like this but I have some specific questions pertaining to my particular tile install.

House is a 1960s bungalow. 1 support beam running down the middle in-between a fireplace. Unsupported span is 11.5'. Most of the tile work is to be done within one side of the beam a very small amount of tile near the thresholds would be on the other side of the beam.

2x8's Joists (1.5x7.5) 16 OC for the most part but it varies and is tighter in some places with joists doubled up here and there (done by builder, I think in relation to staircase and fireplace).

All layers of underlayment/asbestos/vinyl removed down to original plywood which is 5ply but measures 1/2". I have screwed down this original plywood with a bucket of floor screws. I am blocking the plywood edges because it is not tongue and groove and weak in these spots.

What else should I be doing for a successful tile install? How much plywood do I need to add? Of course sandwich height is a consideration, I don't mind angled thresholds, but I have to figure something out at the staircase nose, cant create a tripping hazard. Read a lot about Ditra, is it recommended here?

Tile I'd like to have installed or install myself is a 60x120x8.5mm commercial grade tile.

Thanks for your help with the most challenging part of this renovation.
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Unread 03-26-2021, 01:43 PM   #2
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Hi Jon, welcome to the forum!

What is the span of the joists? Your two options are doubling / sistering the joist or placing a support wall to reduce the unsupported span to less than 11 feet for 2x8 with 16" spacing.

The subfloor you have is typically considered 5/8" nominal and is minimally compliant for tile. Adding a second 1/2" layer will significantly improve the between joist deflections.
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Unread 03-27-2021, 07:58 AM   #3
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So the unsupported span from ledger board to beam is 11.5 Ft. The deflecto score is L / 337, when entering 2x8 x 11.5'. 16"o.c.

There is a furnace room below the kitchen and dining room. Part of it was a wall built by the builder inline with the staircase and it was built floor to Joist shown in green and seems to be bearing load well. The rest of the room shown with a blue line was built by a basement renovator in the 60s rather poorly, and the walls don't seem to be taking too much load as they have a hollow feel when you bang on the studs. So I am guessing they do very little to reduce my unsupported span. It can be rebuilt floor to joist but that changes the scope of this project considerably.

Furthermore underneath the last 3 feet of the dining room to be tiled can not have a partition wall as it is a hallway to the rest of the basement and has cold air returns running through it and main ductwork below it, ; sistering is not an option for this section. I have attached pictures showing the walls.

Thanks for your on going help. Trying to figure out if we should change course to a non tile flooring option upstairs. The porcelain could be repurposed in the garage or basement I suppose.
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Unread 03-27-2021, 08:15 AM   #4
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Welcome, Jon.

What I'm not seeing in your drawing is an indication of the location of the floor to be tiled.

And I'm not sure what you're showing in the two photos in your second group. If you have walls below the joists in question, it should not be particularly difficult to make them load-bearing if they're not already. That presumes you have a suitable floor below them to support such load-bearing.

But it sounds as though you're saying your potential support walls do not extend below the entire floor to be tiled. Keep in mind that your support walls or beams do not need to be in the center of your joist span, they need only reduce the unsupported span to an acceptable length.

My opinion; worth price charged.
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Unread 03-27-2021, 08:27 AM   #5
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The area to be tiled is the Dining Room, Kitchen and the little carport entrance in front of the stairs. I've attached the builder's floorplan to add clarity.

I realize that the wall in the basement is actually in a decent spot to reduce the unsupported span, My concern is that the way it was built with it's top plate 3/4" shy of the joists sort of tacked to 1x3 strapping. Was trying to show that shoddy construction in the photos also the hallway that has ductwork below and through the joists making sistering impossible.
Can we give a pass to this little section where I have no way to reduce the span because building a wall there is impossible?
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Unread 03-27-2021, 08:38 AM   #6
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So, essentially you're tiling everything in your drawing, yes?

Without seeing the basement it's difficult to guess from over here how you might be able to correct the deflection on the west side of the dining room, which appears to be a high traffic area.

You can, of course, leave it as is and tile it and take your chances, but that might not be a good idea.

My opinion; worth price charged.
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Unread 03-27-2021, 11:51 AM   #7
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Yes, tiling everything in the areas where I have shown the joist spacing.
I've made a little video showing the run under the kitchen and the wall that is built there. Should help show you what I am up against:

https://www.youtube.com/watch?v=1Kii8rXAqhg
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Unread 03-27-2021, 12:39 PM   #8
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OK. The "little piece of plywood" is all I ever use to block between joist seams in plywood subflooring. Some construction adhesive and screws makes it much better than would a customary T&G joint. I keep the block short enough that it can't touch either joist to prevent squeaks. The joist-size blocking is no better at the job and can actually be of less value.

Place I don't see in your video is the wall behind you when you're showing the doorway that prevents your putting a support wall in under that part of the dining area. Again, you need only shorten the joist span enough to reduce the design deflection to a suitable level. Probably only a foot or so in your case.

My opinion; worth price charged.
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Unread 03-27-2021, 12:40 PM   #9
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So you really just need to get the unsupported span down to 11’ for 2x8 joists. I didn’t see the foundation walls in your video or missed them anyway. You can add a wall along one side to reduce over span. In your situation I’d be inclined to just put a support beam along one wall and support it with post every X feet based on the size of your beam. That way you really wouldn’t lose much more space and if you were creative you could turn it into an architectural feature in that space
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Unread 03-27-2021, 01:39 PM   #10
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Hehhe, Ok I will stop with the solid blocking, and just put in the strips. Would be a lot easier for sure!

So let me get this straight, I'd need to install a beam something like the one pictured anywhere within the 11.5" foot span? a regular wall built tight up against the joists does not cut it?

Thanks for your quick help guys, I really appreciate it.
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Unread 03-27-2021, 05:56 PM   #11
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Either a load bearing wall or a beam will work. Just remember, you only need to reduce the span by 6” to get L/377 so reworking those walls to be load bearing will for the trick. One pretty simple thing to try would be to place 2x4 flat against the existing wall centered/shimmed under each joist so the load is carried down to the slab. You could then drywall over the 2x4.

As CX mentioned previously you can go with what you have, no guarantees of either success of failure when you are that close to the pass or fail margin.
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Unread 03-27-2021, 09:00 PM   #12
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Quote:
Originally Posted by PC
One pretty simple thing to try would be to place 2x4 flat against the existing wall centered/shimmed under each joist so the load is carried down to the slab.
I hope Jon understands that, PC, on accounta I don't.
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Unread 03-28-2021, 06:41 AM   #13
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I’ll haft’a attach a drawing!
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Unread 03-29-2021, 07:52 PM   #14
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Here you go, quick sketch up to show inclusion of 2x4 under joist to reduce unsupported span. 2x4 are attached to existing non load bearing wall to prevent bowing of the 2x4 under load.

Name:  Joist supports.PNG
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Unread 04-01-2021, 11:40 AM   #15
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Wow, thanks for the sketch. I will see if I can do this to the existing walls I have down there.

In the meantime I have taken CX's advice and put in 3/4" ply strips in between the subfloor sheets. It feels quite solid!

An experienced tile guy is supposed to come check the job out next week. I'm planning to add 1/2" nominal ply but we'll see what he thinks before I go through the trouble of setting up that ply delivery.
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