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Old 09-04-2014, 01:28 AM   #1
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About Plywood Over Subfloors and Ceiling Joists?


I plan on laying 3/4” plywood over all of the old, rough, wooden slat sub flooring in an old house that I just bought.

I’m doing this so that I have an even, firm surface upon which to install various kinds of “finished” floors on top of in the future.

The information that I’ve received about doing this has left quite a few things unclear to me.

1. Do I have to put down some sort of kraft paper or plastic before laying down the plywood sheets?

2. Do I butt each plywood panel up against one another on all sides, or do I leave a 1/8” gap around ALL sides of each panel?

3. I plan on installing the same 3/4” panels on top of the ceiling joists to create a loft/upstairs floor.
If it turns out that I am supposed to leave a 1/8” space around ALL sides of each plywood panel on the bottom floor, do I also follow this rule when securing the plywood panels to the tops of the ceiling joists?

Thanks!
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Old 09-04-2014, 12:38 PM   #2
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Use tongue and groove,
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Old 09-04-2014, 01:05 PM   #3
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Yes, I looked into tongue and groove, but that's not going to work for me. I'm stripping this house down to it's studs pretty much. I'm going for as non-toxic, indoor air quality preserving as I can do.

I'm using all non-toxic, zero-voc paints, sealers, glues, drywall, insulation, etc. I've chosen not to use any traditional plywood/osb/or mdf on the interior of the home, as they are loaded with formaldehyde that off gasses for years. None of the tongue and groove stuff that I've looked at is air quality/health friendly.

I plan on using a high quality, hardwood, cabinet grade plywood that is constructed using no toxic formaldehyde resins/glues.

This product only comes in regular 4x8 sheets. So... do I leave space around all four sides of each sheet, or do I just butt all sides of one sheet up against the other sheet?

Thanks!
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Old 09-04-2014, 01:09 PM   #4
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Hi Highmarks,

Quote:
I plan on laying 3/4” plywood over all of the old, rough, wooden slat sub flooring in an old house that I just bought.
I probably would not do that. I'd have to know more, but doesn't sound like a plan to me. The reason is your next statement which of course is what we always need in a floor.

Quote:
I’m doing this so that I have an even, firm surface upon which to install various kinds of “finished” floors on top of in the future.
Quote:
The information that I’ve received about doing this has left quite a few things unclear to me.
That's very common.

Quote:
1. Do I have to put down some sort of kraft paper or plastic before laying down the plywood sheets?
I can't think of one situation where you'd wanna do that.

Quote:
2. Do I butt each plywood panel up against one another on all sides, or do I leave a 1/8” gap around ALL sides of each panel?
Structural ply is always spaced about 1/8". Subfloor grade (t&g) is installed according to the instructions of the maker. There's usually a 'built-in' gap when installed. Also depends whether there's 2 layers or just one. Need more info on which method you're gonna go with.

Quote:
3. I plan on installing the same 3/4” panels on top of the ceiling joists to create a loft/upstairs floor.
If it turns out that I am supposed to leave a 1/8” space around ALL sides of each plywood panel on the bottom floor, do I also follow this rule when securing the plywood panels to the tops of the ceiling joists?
When it's the subfloor you glue and fasten to the joists and must leave a gap of 1/8" per directions. Again, can't be specific without know more. If the subfloor is installed after the walls, you also need 1/4" at perimeter and blocking under to support any unsupported ends. (on the short ends of the sheets).

**The additional info above kinda changes a lot of what I just said. No t&g means you need two layers or blocking under.

Your first decision is if you're gonna keep
Quote:
all of the old, rough, wooden slat sub flooring
From there the answers vary accordingly.

Jaz
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Last edited by JazMan; 09-04-2014 at 01:16 PM.
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Old 09-04-2014, 03:39 PM   #5
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highmarcs, if you are going to insist on adhering to all of those tree-hugging agendas you are going to shoot yourself in the foot right off the bat with your underlayment proposal.

The wood slats you are covering are always going to be somewhat unstable in that they will move within themselves during every climate change your area experiences. To counteract this movement you should at the very least use t&g plywood for your underlayment in this case. Then you can go on hugging your trees without any worries about your basic foundation.

It would be nice to know what final floorcoverings you propose to use throughout the project that may have different basic requirements, and cause recommendations to change or vary.
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Old 09-04-2014, 09:02 PM   #6
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Nothing wrong with being a tree hugger. IMO
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Old 09-04-2014, 09:05 PM   #7
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Quote:
Originally Posted by Bud Cline View Post
highmarcs, if you are going to insist on adhering to all of those tree-hugging agendas you are going to shoot yourself in the foot right off the bat with your underlayment proposal.

The wood slats you are covering are always going to be somewhat unstable in that they will move within themselves during every climate change your area experiences. To counteract this movement you should at the very least use t&g plywood for your underlayment in this case. Then you can go on hugging your trees without any worries about your basic foundation.

Hmm... I simply want to live in a house with less toxic crap and that makes me a "tree hugger"? More like a hugger of myself and my family.

Apparently, other folks also think it's best to choose non-toxic or less toxic building materials in their homes, as there are more and more products on the market catering to those of us who care about such things. In fact, I even found a tongue and groove flooring at Home Depot that is free of formaldehyde and other toxic chemicals and helps to block formaldehyde from the framing lumber.

http://www.homedepot.com/p/Eco-Build...SFTG/203909109

Forgive me, but some of us actually care about our indoor air quality and would simply like to have ourselves and our family live among less POISON where it's at all possible.

Sadly, that gets me condescension and name calling. Perhaps this is not the forum for me.

Last edited by highmarcs; 09-04-2014 at 09:08 PM.
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Old 09-04-2014, 10:50 PM   #8
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The old slat subfloor should be removed. Unless it's not as bad as I envision by your description.

If you can't find a suitable t&g subfloor, you will need to add a second layer of ply over the first. The subfloor should be 3/4", min 5/8", and the underlayment should be min. 3/8" with 3/4" or 1/2" with 5/8" sub. That's min.

If you wanna go over what you now have we need more info and for you to determine how flat the floor is. We'll have you add an underlayment but how thick we don't know yet. You need to tell us what you're planning to use as the flooring.

I recall many years ago I did a few jobs for a family whose son had allergies. Apparently he was sensitive to everything. They used a portable air cleaner when riding in the car, etc. Of course the first thing is to get rid of carpeting and as much fabric as possible, no plastic laminates, (Formica) and I don't know what they did for cabinets and furniture. I could not use latex modified thinset and most concrete backers. Yikes.

She had a book that must have been 600-700 pages and was a member of a national help-group. All lawn spray companies and insecticide companies had to call her with an advance warning and she made the school district change the products they used in the schools. It was a full time job for the lady.

Jaz
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Old 09-04-2014, 11:04 PM   #9
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Quote:
Originally Posted by highmarcs View Post
Hmm... I simply want to live in a house with less toxic crap and that makes me a "tree hugger"? More like a hugger of myself and my family.

Apparently, other folks also think it's best to choose non-toxic or less toxic building materials in their homes, as there are more and more products on the market catering to those of us who care about such things. In fact, I even found a tongue and groove flooring at Home Depot that is free of formaldehyde and other toxic chemicals and helps to block formaldehyde from the framing lumber.

http://www.homedepot.com/p/Eco-Build...SFTG/203909109

Forgive me, but some of us actually care about our indoor air quality and would simply like to have ourselves and our family live among less POISON where it's at all possible.

Sadly, that gets me condescension and name calling. Perhaps this is not the forum for me.
Oh lightin' up a little Michelle. There is nothing wrong with what you want to do but if you are going to save the world single handed then be prepared for some criticism from time to time.

I'm out - enjoy.
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Old 09-08-2014, 07:04 AM   #10
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You guys are so careless when it comes to methods foreign from your own.
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Old 09-08-2014, 09:07 AM   #11
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A subfloor of cabinet grade plywood is going to cost a very pretty penny ($50+ per sheet vs $30 around here). The tongue and grove is necessary to keep one edge from lifting up from the other, something you would see telescope through the finished floor layer. I imagine that installing blocking between the joists to screw the non-tongue and groove edges to might work. Either that or route the edges to add a tongue and groove.

Knowing the type of finished floor would be helpful.

What size are your ceiling joists and what is the span? Too small for the length and you can crack your ceiling drywall from the flex of you walking on top of it. Plus generally being unsafe for regular usage and/or storage.

Last edited by ccbs1; 09-08-2014 at 09:11 AM.
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