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Unread 09-22-2019, 09:46 PM   #16
wwhitney
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The first place to look is the prescriptive tables given in the International Residential Code. My state code is based on the 2015 IRC, and in it table R602.7(2) is titled Girder Spans and Header Span for Interior Bearing Walls.

Assuming the girder carries only the floor joists, the table tells you that the allowable span for 3-2x12 is 10' 2" for a 20' wide building, and 8' 10" for a 28' wide building. The building width determines the tributary width of the girder, (half the building width), so that determines the load on the girder. Interpolation is allowed, so with a building width of 23', the allowable span would be 9' 8".

That doesn't look so good, but those tables are very conservative. Check your girder for any grade and species stamp and let us know what you find.

Also, that 12' span, are there any splices in the 3 plies within that span? And what is happening girder wise beyond the 12' span, at each end?

Cheers, Wayne
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Unread 09-22-2019, 09:53 PM   #17
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BTW, you can use the Deflecto for a girder with a little post processing.

What you want to enter is (3) 2x12s, so 4.5" wide by 11.25", with a 12' span (the girder span) and with a 11' 6" spacing (the tributary width of the girder). The spacing options only go up to 24", so use that. 11' 6" / 24" = 5.75.

The deflection is proportional to the spacing input (you can verify it for yourself). So the deflection you get will be 1/5.75 as high as it would be for your girder. In other words, the denominator you get needs to be divided by 5.75 to get the actual deflection denominator for the increased spacing.

Cheers, Wayne
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Unread 10-05-2019, 12:07 PM   #18
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(Picture) New tile over two types of sub floor

I am building a new bathroom. I have removed subfloor and beefed up the joists by sistering.

Subfloor prior was slats. Now is 3/4 T&G. Planning to install ditra prior to 12x24 tiles. I am going to have to pour self leveling compound on the subfloor before ditra as the joists were not level and I didn’t want to buy a hand planer or mess with furring up the low spots.

I am moving the wall a little further out than initial plans. It’s only adding an extra 8” or so in the bathroom tile area. I decided to move the wall AFTER sistering joists and replacing subfloor. This means there is about 8” of old slat subfloor butting up to the new subfloor before the vertical wall will be built. The new subfloor edge sits on new sistering joist, old subfloor edge sits on old joist.

My questions are;

1) Is it necessary to extend new subfloor beyond where the tile will be installed?
2) If not necessary, there is about 1/8 height difference in the new subfloor and old subfloor. Can I seal the old subfloor between slats, and use self leveling compound to bring the new floor level up to where the ditra will bridge from the new subfloor to the new subfloor?

I *feel* like this is not the proper way to do it, but it would sure save a ton of effort. And there will only be about half a tile laying over ditra on the old subfloor so I don’t believe weight is an issue. Just concerned about stability issues crossing two types of subfloor, even if it is a small portion of the floor.
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Unread 10-05-2019, 12:08 PM   #19
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Sorry for sideways pics.

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Unread 10-05-2019, 12:09 PM   #20
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New wall location is that 4.5” open particle board area. Bottom plate is sitting there.
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Unread 10-05-2019, 12:21 PM   #21
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Brendon, it'll help if you'll keep all the project questions on one thread so folks can see what you're working on and what's been previously asked and answered. A moderator can give it a more generic title any time you'd like to suggest one.

If I understand the plan, so long as you're willing to accept the very high likelihood of a crack in your tile installation at the junction of the plywood and board subfloors, your plan should work. Your Ditra is not likely to protect you from the differential movement between those two subfloors, even without foot traffic in that immediate area.

My opinion; worth price charged.
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Unread 10-05-2019, 12:29 PM   #22
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Will do. Figured it would be better a separate thread since it’s a different topic someone might search titles only for.

It is not a high traffic area, except for a 30” section which is the entrance to the bathroom. Definitely don’t want a crack. If you’re saying it will definitely crack, I need to figure something else out.
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Unread 10-05-2019, 12:47 PM   #23
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What if I pour the self leveling compound, then another layer of 1/2”, then the ditra?
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Unread 10-05-2019, 07:44 PM   #24
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No go on that either. If you want to add a second layer of subflooring (good idea) you must do that first and then do any leveling or flattening before you install the Ditra.

My opinion; worth price charged.
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Unread 10-05-2019, 08:04 PM   #25
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Ok, so best bet is to go ahead and redo floor where slats are as well, then second layer of subfloor across the entire first layer of subfloor, opposite direction to first layer I assume. Then self leveling compound, then ditra. Correct?

Would 3/8 or 1/2” be ok for second layer? Does it also need to be T&G?
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Unread 10-05-2019, 08:10 PM   #26
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Brendon, all layers of structural subflooring must be installed with the strength axis perpendicular to the joist structure. You do not want the second layer opposite the first.

Schluter accepts nominal 3/8ths" plywood as the second layer when required, but due to the low quality I usually find in that thickness I would not use anything less than nominal half-inch plywood in your application. Up to you.

If you replace the board subflooring we see in your photo and replace it with plywood you would not technically need the second layer with your joist spacing according to Schluter. Again, up to you.

My opinion; worth price charged.
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