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Unread 04-23-2018, 08:56 AM   #1
bacon
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Check My Work - Designing for 24OC Truss Floor

I've read lots of posts and articles on here, TCNA, APA, etc and I think I have my design figured out, but wanted some of the knowledgeable folks on here to give it a once over first.

Background:
Planning to replace linoleum in master bath with some form of tile - yet to be determined. House is 4 years old with 24oc open web trusses that are 18" tall - unknown manufacturer or span, but I could guess. I havent pulled the linoleum up to see the exact grade of OSB, but I'm assuming it is the same as what's on the main level which is 23/32 Exp 1 APA Sturd I Floor OSB.

Dilemma:
I want to minimize height difference to the carpet in the adjoining bedroom. I believe there is 1/4" particle under the linoleum which I will tear out. I was going to follow TCNA F155 which is the design for ceramic over OSB with 24oc trusses which calls for 1/2" ply over the OSB with an optional membrane before the tile. However, I also read that TCNA recommends an L/360 of 250psf for residential use on tile floors. Looking at the APA Q255 load span tables, I come up with 130psf for my current subfloor and see that 1/2" sanded ply over 24oc only gives me an additional 61psf. That only puts me at 191psf - it looks like I would need another sheet of 23/32 to achieve the recommended 250psf

Questions
- Do I really need 250psf? Or is that only for "heavy objects"?
- I have two young kids that run/jump a lot and I notice some bounce to the floor. At 191, will that go away? Will their youthfullness still be a problem for my tile?
- Should the two layers of ply be glued together?
- Should I use ring shank nails or screws?
- Some documents say to use lots of screws, but in one TCNA document it stated "minimum fastening according to recommendations in APA Engineered Wood Construction Guide, Form E30"
- Do the thin membranes like Noble, Mapei, or Greenstreak actually provide much benefit? Kerdi just adds so much thickness...


Thanks for reading my novel and for any insight.
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Unread 04-23-2018, 02:47 PM   #2
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Quote:
Originally Posted by Matt
...but I'm assuming it is the same as what's on the main level which is 23/32 Exp 1 APA Sturd I Floor OSB.
I would recommend you not make such assumptions. F155 requires a minimum of nominal 3/4" plywood or OSB subflooring.
Quote:
Originally Posted by Matt
I also read that TCNA recommends an L/360 of 250psf for residential use on tile floors.
'Fraid I'm not at all sure what you might mean there. The TCNA recommends a joist deflection of L360 or better for a ceramic tile installation, but that would normally be calculated at a residential loading of 10psf dead and 40psf live loads or calculated using whatever other loading you're designing the floor to handle. The subflooring and underlayment requirements are spelled out in the various applications in the TCNA Handbook. As I read F155 the subfloor minimum is 23/32nds and the underlayment plywood minimum is 19/32nds. That's nominal 3/4" and nominal 5/8ths" in real world terms. With your 3 1/2" top chords on your engineered joists you may get by with nominal 1/2" plywood underlayment, but then you're not installing per F155.

I would also have to recommend you not install your tile directly to a plywood underlayment. While it is an industry recognized installation method, it requires very critical attention to the installation of the subfloor and the underlayment and the tiles and I'd recommend you use some other underlayment. Your nominal 1/2" plywood would be adequate if you then install a suitable underlayment CBU or membrane. If you use Schluter's Ditra as your underlayment, you can, technically, use nominal 3/8ths" plywood over your subfloor, but I also don't recommend that because of the poor quality of 3/8ths" plywood I'm accustomed to seeing locally. There are also products on the market that would allow you to install them over your subfloor as it exists. I would not do that.

Make your floor suitable for a ceramic tile installation. Make transitions to other floor coverings as necessary.

My opinion; worth price charged.
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Unread 04-23-2018, 04:04 PM   #3
bacon
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Thanks Cx. It's entirely possible I was looking at an out of date publication, but I sure thought it was current and I'm almost positive it said 1/2 ply. I can't find the same sheets back right now, but I still have them open at work and will check them again in the morning and report back. Any input on the other generic questions, load ratings aside?
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Unread 04-23-2018, 04:21 PM   #4
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I don't have the current TCNA manual either, my most recent being 2015.

1. I don't know what you mean by the 250psf.
2. See #1.
3. No.
4. Yes.
5. TCNA is recommending you follow at least the minimum requirements of the product manufacturer as is always their position.
6. Some membranes can provide a very substantial benefit. I've never heard of "Greenstreak." If perhaps you meant Greenskin, I can't comment. Kerdi is not thicker than the Noble membrane you mentioned, but is also not meant for the purpose you describe. Perhaps you meant Ditra, which is for the purpose mentioned and is only about 1/8th" thick. Don't know what MAPEI membrane you might have in mind.

My opinion; worth price charged.
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Unread 04-24-2018, 06:55 AM   #5
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Turns out I did have an out of date publication, but the numbers were the same as yours...not at all sure where I got that 1/2" number from.

So 5/8" ply over my 23/32 OSB - no glue between.

Will ring shanks in a nail gun be ok, or do I need to do screws (it's a fairly good sized floor).

What are the thoughts on the Nobel CIS or similar? Seems like the CIS plus the Nobelbond EXT would be thinner than thinset plus Ditra (incorrectly said Kerdi before).
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Unread 04-24-2018, 07:44 AM   #6
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Matt, if it were me I'd run screws through the existing subfloor into the joists, and then screw the 1/2" underlayment to the subfloor - avoiding the joists.

Use 1.25" or 1.5" construction star drive screws that have a smooth shank of about 3/8" to 1/2" ish under the head. Those will prevent jacking. Using screws will pull the two sheets of ply together, permanently.

JMHO.
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Unread 04-24-2018, 07:50 AM   #7
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Keep in mind that you need the 5/8ths plywood only if you're actually using it as your tile underlayment. There are other methods in your Handbook that require thinner plywood for the second layer if you're using a membrane as your underlayment (disregard their calling every second layer of subfloor and underlayment. Don't know why they do that. The second layer of subflooring is only an underlayment if you're bonding your tile directly to it).

I recommend screws for all parts of any subfloor application, but ring-shank nails are usually an approved method for attachment.

I think NobleSeal with EXT is an excellent tile underlayment.

My opinion; worth price charged.
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