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Old 12-28-2017, 03:29 AM   #1
cngodfather1
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subfloor questions

Hi!

I am doing a tile job that spans 1000 sq/ft of my home. I will be setting 6x36" porcelain tiles. At the longest point, my 2x12 joists are 16' long and on 16" centers. The deflecto meter gives a value of L-444. The current subfloor is 3/4" OSB. I was going to add a layer of plywood over the OSB and then 1/4"Hardi-backer on top of that. The OSB is laid parallel to the joists and I plan on laying the plywood perpendicular to the joists. My questions are:

1. Is the 1/2" plywood overkill for what I plan to do?
2. Will staples be sufficient to attach the plywood to the OSB?
3. What is a good length of staples (if possible) would you use?
4. Should I add construction adhesive between the plywood and OSB?
4a. If yes to construction adhesive, how should it be applied?
5. Do the orientation of the tiles matter relative to the direction of the joists?

Thanks
Chris
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Old 12-28-2017, 07:50 AM   #2
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1. The subfloor should never be laid parallel to the joists. Is this new construction? If so the GC needs to be held accountable for this. All layers should be ran perpendicular to joists. Minimum of 3/8" ply for 2nd layer.
2. Some use crown staples, I prefer screws.
3. Someone that staples may weigh in
4. No
5. No
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Old 12-28-2017, 08:39 AM   #3
cngodfather1
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thanks

Cool, thank you! The house is about 6 years old. I am the second owner.
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Old 12-28-2017, 09:41 AM   #4
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Hi Christopher,

The "strands" in OSB are oriented (generally) in the long direction, so the board should be laid perpendicular to the joists. The strength of OSB v. plywood is the same if the OSB has not been soaked with rain.

Definitely screws instead of staples.
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Old 12-28-2017, 12:41 PM   #5
cngodfather1
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Thanks John.

I may be wrong with the direction of the first layer. I was going off of my memory. I hope it is laid perpendicular to the joists.

For my second layer, should I stick with plywood, or would I be ok with
1/2" OSB?
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Old 12-28-2017, 06:30 PM   #6
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also, how long of screws would you recommend? for the second layer of subfloor?
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Old 12-29-2017, 10:29 AM   #7
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My understanding is that mortar adheres to plywood better than it does to OSB. Screw wise, if you're laying 1/2" and going into 3/4" material then 1.5" screws will be fine. If you're screwing down the existing 3/4" into the joists you'll want 2.25" minimum. 2.5 is probably better.
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Old 12-30-2017, 06:12 AM   #8
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I was incorrect. My first layer of OSB subfloor is laying the proper orientation. (Which I am happy about)

With 1.5" screws, will I need to worry about missing my joists so that I don't screw into them?
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Old 12-30-2017, 09:19 AM   #9
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I would avoid hitting the joists. Its not too hard if you just measure before you install the next layer.
Have you checked into how the 2nd layer should be situated over the 1st, and the suggested fastener pattern?
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Old 12-30-2017, 09:11 PM   #10
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Only difference I'd do: we always glue between sheets of plywood or OSB. The glue if 100% applied is stronger than mechanical attachments like screws, mails or staples. What holds sheets of plywood together? Only glue. Ever try pulling the plies apart by hand?

Maybe I'm missing some valid reason for not gluing. If no one has a good explanation, I'd screw and glue it mucho heavy, then use the cement board on top with mortar under the board and mechanical attachments per mfr recommended schedule.



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Old 12-30-2017, 09:29 PM   #11
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If done with the proper glue and full spread there's nothing wrong with doing it. The problem is people hear glue the sheets together and think that running lines of liquid nail, then sticking the next layer down is the same thing.
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Old 12-31-2017, 01:17 AM   #12
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I always try to overkill on strength with my projects. what kind of glue do you use?

Thank you
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Old 12-31-2017, 01:15 PM   #13
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Check out titebond II, that seems to be a common suggestion when it comes to glueing subfloor layers together.
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Old 12-31-2017, 02:01 PM   #14
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Gluing subfloor layers could be considered a exercise in futility. Many would think it's a belt and suspenders approach, I'd beg to differ. Plywood is manufactured under controlled conditions and pressure that is not realistically achievable in the field.

Consider this:

The most appropriate glue would be something designed for a strong, thin full coverage glue line, like wood glue. Titebond I and II have an open time of 5 minutes, Titebond III is 10. You'd ideally want 100% coverage. You could do this with a small notched trowel designed for contact adhesive or sheet flooring. Here's where it gets harder. Imagine dumping the glue from a gallon container on your vacuumed subfloor, spreading consistently over a 4x8 area, dropping the sheet into your glue, screwing down according to fastening schedule (something like 60+ screws) in five minutes. If you can do that sheet after sheet, you're a better man than me.

All that and you don't really gain any relevant structural advantage vs screwing only. There's probably a time an place for it, but I've yet to encounter it. The more is better philosophy just doesn't hold true in every case and could potentially be a detriment in this case.
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Old 01-07-2018, 11:15 AM   #15
cngodfather1
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Lotsa Tile

Hi

I will be tiling most of the rooms in my house with 6 x 36 porcelain tile. I was wondering if I could do this with one continuous tile layout. the largest area is about 30 x 20. Will I need to put a separation at doorways to prevent cracks? The large area is the living space in the center. Coming out from the living area is the kitchen, a hallway, a pantry, a dining room, and a bathroom.

Thanks
Chris
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