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Old 09-28-2017, 07:55 AM   #1
snugglez
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subfloor/underlayment joist blocking alignment

Hello.

I've tried to read all the posts/links I can on this site on this subject, but I want to make sure my plan is the best one possible before I proceed.

- bathroom remodel, 72" by 90"
- I wish to install ceramic or porcelain tile and a large shower (34x60) using either full mud job or CI shower base
- joists run the 72" length.
- joists are (full dimension) 2x6 supported on both ends by load-bearing walls. I have sistered 2x8s to each joist using 5/16 construction screws and nails, glued with PL Premium. The sistered joists were used to level the floor, but I'm assuming they will reduce deflection as well. I wish I could have fit 2x10s, but no room for those.
- joist spacing varies from ~24" to 19" (old house)

plan:
- I have advantech 3/4 T&G sheets, which I plan to lay 2 layers of, all perpendicular to the joists (that is, the T&G will be perpendicular to the joists).
- Since the space is exactly 72" I will likely just lay the first layer 1 full sheet, and then one half sheet, then use the remaining half sheet on top of the first full sheet and finish off with a final full sheet covering the rest, thus the overlap seams will be 2' away from each other.
- I will install full-height blocking around the perimeter of the space
- I will glue and screw the first layer to the joists, including that blocking
- I will not glue the second to the first, and i will screw it only to the first layer, not the joists or blocking

Question:
- would it be better to add blocking in the middle of this floor as well?
- if I do add mid-span blocking, should I align the first layer seam above that blocking and screw it in? I think the answer is "no, have all T&G seams break away from the blocking," but that just seems so anti-intuitive that I have to double-check. Note that I know I should not put the *second* layer seam above the blocking, just not sure about the first...

Much obliged, folks. I hope I have included enough details to make this comprehensible, but please lmk if additional info is needed.

THANK YOU!!

Last edited by snugglez; 09-28-2017 at 09:41 AM.
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Old 09-28-2017, 10:02 AM   #2
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Welcome, SZ.

What is the unsupported span of these joists?

What is the intended purpose of the blocking you intend to install?
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Old 09-28-2017, 12:38 PM   #3
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Hello!

Unsupported span is 6' (72 inches). Both ends of each joist are supported.

blocking around the perimeter would be for:

- screwing first layer to
- supporting edges of the sheets
- the current framing has no blocking at all, so I'm just putting some in here while I have the opportunity.

I'm assuming the perimeter blocking sounds ok to you, but please lmk if not--I haven't installed it yet.

the blocking I'm asking about in the middle of the span is (I'm assuming, but that's why I'm asking here) to further stiffen floor and prevent deflection.

This came up because when I first was planning the project, I hadn't read this site, and I was going to run blocking down the middle in order to put the first layer of subfloor T&G above it and screw through it. My understanding now is that putting plywood seams directly above the joists/blocking increases the chances of tile failure. So I am now offsetting the first layer of plywood from the centerline of the span to give the seam for the second layer as much space as possible.

But now the question is: do I even need the blocking at all? If I'm not going to use it to screw the T&G seam into, is it worth installing?

Does that make sense? Please lmk if I can provide more/better details.
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Old 09-28-2017, 03:01 PM   #4
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If your unsupported joist span is only six feet you have no deflection issue with your 2x6" joists. If you're sistering to flatten or level you subfloor, that's a different consideration. And yes, you'll require blocking around the perimeter to permit fastening of your subflooring.

So long as you install your subflooring perpendicular to the joists, you will have no problem with the joints so long as you overlap the joints on the first layer with the second layer as you've indicated.

And you require no other blocking if you use T&G material for your first layer.

My opinion; worth price charged.
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Old 09-28-2017, 04:07 PM   #5
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Blocking causes the joists to remain vertical on edge and not twist when a load is applied. It is most critical before the subflooring is installed. Once the subflooring is installed, most of that twisting under load is controlled, especially if there's something on the bottom of the joists as well...the subflooring, because of its stiffness, helps by sharing the load between joists. Blocking can change the resonant frequency of the floor, but doesn't really improve the deflection rating. It can help with a point load by sharing the load with adjacent joists, but does NOT improve the overall strength on a distributed load. You're going to be adding a significant dead load to your joists with two 3/4" layers of Advantec, then more blocking just adds more dead load. Once you've added your tile, yet more dead load. Unless you need or want the added height, the second layer of ply is optional when installing ceramic tile (porcelain is one type of ceramic). A second layer certainly does improve the between the joist deflection rating, but in most cases, isn't needed with your joist spacing when doing ceramic tile. Advantec tests out to be about 10% stiffer than the same thickness plywood, and plywood is sufficient. Adventec does lay flatter, but boy is it heavy!
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Old 09-28-2017, 06:20 PM   #6
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Thank you CX and jadnashua!

I suspected that the blocking in the middle was not needed at all once I understood I shouldn't be "supporting" the t&G joints with it. But I wanted to double-check with you knowledgable folks.

It's a good point about the weight, I just assumed that the joist spacing was enough that *something* on top of the first layer of advantech would be needed, and since I was already going to have the extra half sheet... I could use it up quite nicely by using it as part of a second layer.

I'll sketch out the weight estimates and see where that puts my dead load.
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Old 09-28-2017, 06:30 PM   #7
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Quote:
Originally Posted by SZ
- joist spacing varies from ~24" to 19" (old house)
Quote:
Originally Posted by jadnashua
Unless you need or want the added height, the second layer of ply is optional when installing ceramic tile (porcelain is one type of ceramic).
Not sure I can agree with Jim with that joist spacing. You certainly don't need a second layer of nominal 3/4" plywood, but you need a second layer. Nominal 1/2" would be fine if you don't want the added height of the 3/4" plywood.
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Old 09-28-2017, 08:34 PM   #8
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Sistered joists narrow the spacing of the original. Some tiling methods allow up to 24", some don't, so depending on what you use, a second layer can be useful. I don't think I'd use Advantec unless I needed the height of the second layer. Driving screws through it is a pain and it's heavy. I think it would be quite easy to get 'jacking' from the screw not immediately starting on the second layer unless you predrilled the first layer, making more of a pain in the install.
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Old 09-30-2017, 07:41 PM   #9
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So the joist spacing I mentioned (between 19 and 24") is after the sistering (!), so... pretty far.

I have plenty of vertical room, so I was just going to use the advantech for the second layer, but I'm rethinking that.

Aside from 1) the weight and 2) the difficulty of working with it/screwing it, is there some reason to not use the second layer of advantech based on the materials interaction with thinset?

I plan on putting unmodified thinset on the underlayment, then ditra (heat), then thinset and tile. Does that sound ok on top of advantech, or is there some reason to use (real) plywood as the underlayment/2nd layer?

Again, thanks so very much, and have a good one.
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Old 09-30-2017, 07:52 PM   #10
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Schluter requires a MODIFIED thinset to bond Ditra to any wood. Some thinsets do not bond well with OSB, but if you use one on Advantec, it should be fine if they specify use on an OSB.

FWIW, a second layer of ply will improve the strength of the between the joists and is required if the gap is 24", so to be safe, it is your best bet. Advantec should work.

I highly suggest you drill through holes in that second layer when attaching it to the base layer and offset the sheets so no seams extend from the base up through the top.
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Old 09-30-2017, 08:23 PM   #11
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Quote:
Schluter requires a MODIFIED thinset to bond Ditra to any wood.
derp. I better re-read those instructions, make sure I didn't miss any other obvious stuff. Thanks!

Quote:
I highly suggest you drill through holes in that second layer when attaching it to the base layer and offset the sheets so no seams extend from the base up through the top.
Will do. It's a small area so I don't mind taking the time. Thanks again.
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Old 10-12-2017, 06:11 AM   #12
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Hi! Just a follow up:

- I'm planning to use the advantech for both layers. Assuming I use the correct modified thinset, any other special prep or application tricks I should use to attach to it? IE: sanding it before, or using a v-notch trowel to "burn" in the thinset?

- Correct/best (from HD--they don't carry a large variety of brands) thinset for between advantech and ditra heat is "Custom Building Products FlexBond White 50 lb. Crack Prevention Mortar," correct? Any reason to skip the "crack prevention" bit? I'm unclear on if that is simply insurance, or if it would be worse in my application.

- again, I'm only using advantech because I've already got an extra 1/2 sheet. But I could just get 2 sheets of BC 3/8" or 1/2" plywood if that would improve adhesion to the thinset--ie if using the advantech will require a bunch of sanding, I might just use plywood instead Also, I like the idea of saving weight and height.

Thanks very much!
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Old 10-12-2017, 08:52 AM   #13
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Quote:
Any reason to skip the "crack prevention" bit?
Sorry, I've re-examined the choices, and it looks like ALL FlexBond is "crack prevention," so this part of the question is silly. But I still just want to confirm that FlexBond is both ok to use between advantech and ditraheat, and that there isn't another type or brand that HD stocks that would be a better choice. Cost is not an issue, just prefer to use the best material.
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Old 02-17-2018, 08:44 AM   #14
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Hello again!

Thank you all for the help so far. I'm now at the point where I'll be affixing the kerdi-board to the shower framing, and I have a some questions.

Current shower situation:

- 36x48 alcove newly framed: fairly flat, plumb, and square.

- shower base is cast-iron. It is level.

- shower height is ~90"

- this is an attic, so one of the walls hits the rafters at about 66" and follows up to the ceiling at an angle

- have attached blocking around perimeter of shower base

I am planning to attach kerdi board, two kerdi alcoves, and tile using traditional subway tile


questions:
1. There is a slightly larger gap than I intended around the shower base, meaning that in order to get the kerdi board past the lip of the base, I'll likely be furring out using 3/4 strapping. Might get away with something thinner, but I'm not counting on it. If this is the case, is there any advantage to running the strapping horizontally? It's unclear to me if the orientation of the structure makes any difference.

2. Given the gap between the base and the framing, should I put some shims between it and the framing? The base is heavy, but I figure sticking some composite shims around the framing couldn't hurt, right? To be clear, the gap is not 3/4, it's like 1/4 to 3/8, but then I also have to clear the lip itself, so that's why I think I'll end up furring out using 3/4 strapping.

3. regardless of orientation, should I affix blocking at all points where the kerdi board butts against another board? I obviously have blocking at the top, and bottom, but given the height of the shower, I'll have butt joints along the walls. I can install blocking now if that would significantly improve the job. But I just am not clear if that's really needed.

4. I have two kerdi shower "niches" which I was planning on installing at the far end of the shower--opposite the shower head. I assume that ideally a niche would not have the shower head pointing at it, but it's just the best place for them. However, I can try to fit them along the "side" wall--that is on the wall not facing the shower head, but perpendicular to it--if that would significantly reduce the chance of a leak for some reason. I mean, I just don't know how vulnerable these niches are to leak--I'm assuming "more than a flat wall."

5. My plan for the tiling is to do the first row on one day and let it sit for 24h, then come back and do the rest. That way I don't need to screw through the kerdi with a ledger board, but I still have an absolutely solid "base" for the rest of the tile. Just want to confirm that this is a reasonable plan. I realize a pro would never do this, since it "wastes" a day, but I have other things I can do in the meantime... I'd prefer to make zero holes in the kerdi way down at the bottom if I can help it--is that just being TOO careful?

I'm working on pictures to attach asap. But any thoughts or suggestions would be very welcome. Thanks so much!
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