# Subfloor thickness issue



## Curb Feeler (Jan 27, 2009)

The subfloor in my project house appears to be 1/2 OSB. There was a layer of 1/2 MDF on top of this, but it has all been removed. So now, the only thing between me and my crawl space is 1/2 OSB. 

Question: Should I rip up the entire floor and relay 3/4", or can I lay a second layer of 1/2"? 

This is one of questions where you kinda already know the answer before you ask it, but this is my first encounter with subfloors. I'd hate to take the hard road and find out the easier path is acceptable. Help. 

-Bryan


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## Bob Mariani (Dec 1, 2008)

What are your joist size, span and spacing? What finished floor are you installing? 3/4" over it will be fine. Do not match the joints. Leave 1/8" gaps at all joints.Resin paper or felt between the two sheets and use 2 1/2" screws into the joists.


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## Curb Feeler (Jan 27, 2009)

I didn't measure the joists, but I believe they're 2x10. The spacing is 16". Not sure about the span yet. I'll be sure to get that info next time I work on the house, which is two weeks from now...the house is 400 miles away.


The finish will be tile and carpet depending on location. I assume all standard rules apply when it comes to laying those down (backer board and carpet padding respectively).

Thanks for the response.


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## Bob Mariani (Dec 1, 2008)

You need a much stronger floor for tile. Use the 3/4 for carpet use the 1/2


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## ccarlisle (Jul 2, 2008)

I agree about the thickness for tiles being 3/4" as a minimum depending on a lot of other things but am not sure whether half-inch under carpeting is sufficient. May well be 'sufficient' but IMO is the _bare minimum by code_ and probably not what you'd want in the long run. The half-inch was put there by the contractor as a means of keeping you from falling through the floor, not to put anything on top of it.

A question remains: do you put 3/4" on top of the 1/2" - or the reverse...one would be easier but am not qualified to answer that. Go to the 'Construction' forum for that...:yes:


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## drtbk4ever (Dec 29, 2008)

Could you just glue and screw the two sheets together?

I imagine that would really stiffen up the floor, but may cause a bit of a headache for the next homeowner wanting to remove a layer.


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## Bob Mariani (Dec 1, 2008)

Yes that is the sub flooring we would do in normal spec homes. If we do custom homes I would 1 1/4" flooring under tile or a mud base. I also use 3/4 advantec for my first sheet which is more tolerant of getting wet.


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## JazMan (Feb 17, 2007)

Many years ago it was standard practice to use 1/2" ply as the first layer of the "subfloor-underlayment" system", then follow with a sheet of 5/8" in the rooms that received vinyl. However I have not seen where the first layer was 1/2" OSB. That would be really bad. Are you sure it's not 5/8" at least? 

I would probably install 5/8" underlayment where tile is being installed, then install a cement backer or Ditra. 

You cannot install anything including carpeting on a single 1/2" sheet of plywood or OSB etc. 

I think Mariani misspoke when he said; *(You need a much stronger floor for tile. Use the 3/4 for carpet use the 1/2.)* 

We're waiting to hear about the joists, also see if you find markings that tell you the species and grade too.

Jaz


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## Bob Mariani (Dec 1, 2008)

I said use 3/4 for tile because he already has 1/2". The 1 1/4" should be enough for tile. His question was to use 1/2 or 3/4 over the existing 1/2". So what is it that you disagree with here?


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## ccarlisle (Jul 2, 2008)

Well, I hope it is 5/8"-something - not 1/2" - let's wait and see. 

I just can't see a contractor saving a few dollars and putting down 1/2"-something instead of 3/4"-something, knowing the chances are almost 100% that it would have to be replaced eventually for carpeting or tiles. Because I think a flooring contractor may insist on 3/4" first, then whatever else, but not 3/4" on top of something. 

But I'm not sure about this.


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## Curb Feeler (Jan 27, 2009)

ccarlisle said:


> Well, I hope it is 5/8"-something - not 1/2" - let's wait and see.
> 
> I just can't see a contractor saving a few dollars and putting down 1/2"-something instead of 3/4"-something, knowing the chances are almost 100% that it would have to be replaced eventually for carpeting or tiles. Because I think a flooring contractor may insist on 3/4" first, then whatever else, but not 3/4" on top of something.
> 
> But I'm not sure about this.


 
I won't be able to measure the joists for another few weeks, but here's a video for all you guys with calibrated eyeballs... The intro is a joke. 

You can see the joists in the upstairs bathroom shots. 

http://www.youtube.com/watch?v=2OMPF5iPDzs


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## JazMan (Feb 17, 2007)

We'll have to wait and see the rest of the specs.

Bob, 

I knew what you meant to say, but, you had a problem with periods (.) and new sentences. So to a DIY'er the answer isn't clear, and could be misleading. Read it again.:thumbsup:

Curb,

Was that a cast iron tub you removed? You did it the hard way. Usually they smash it with a sledge and carry out the pieces. 

Jaz


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## Curb Feeler (Jan 27, 2009)

JazMan said:


> We'll have to wait and see the rest of the specs.
> 
> Bob,
> 
> ...




NOW you tell me. I'll stash that golden nugget of info away for future use. 

Thanks.


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