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Andrew
Subfloor Thickness
Andrew

Hello,

I have ripped up old carpet and the particle board underlay that was beneath it. Doing so I have discovered the subfloor is only 1/2" thick. If I lay down 3/4" hardwood flooring, will I have to add more to the subfloor first, or will this be sufficient?
Thanks!

A. Spruce
Re: Subfloor Thickness
A. Spruce

1/2" subfloor sounds a bit thin, should be 5/8" or 3/4". If your current subfloor has much for deflection between the joists, then you may want to consider adding another layer. If you are not noticing much for deflection, then just install the flooring.

HoustonRemodeler
Re: Subfloor Thickness
HoustonRemodeler

Is this 16 inch on center joist spacing ?

Andrew
Re: Subfloor Thickness
Andrew

Yes the joists are 16" on center. So I guess I should just better be safe than sorry and glue down another 1/4" before I install the flooring.

Sombreuil_mongrel
Re: Subfloor Thickness
Sombreuil_mongrel

No, Adding 1/4" plywood on top of 1/2" plywood does not give you the rigidity and nail-holding capacity of 3/4" plywood.
If your hardwood floor was to be room-length solid wood boards nailed into the joists, you would be fine.

Mastercarpentry
Re: Subfloor Thickness
Mastercarpentry

Code today requires a glued and nailed 3/4" material rated for use as subflooring with the finish flooring laid on top of that. But many old houses built on the cheap had just T&G flooring nailed directly to the joists- not optimum but it works. You could lay on top the 1/2" but I wouldn't. !/2" adds no structure and lacks nail-holding ability so I'd remove it and use either 3/4" T&G plywood or Advantek, followed by the finish flooring of your choice. Or you can lay that on top the 1/2" but you'll have problems with height where other floors meet and doors having to be cut shorter.

Phil

Bill
Re: Subfloor Thickness
Bill

What is the problem we need to prevent?  Inadequately supported hardwood deflecting and breaking tongues.

In case anyone else runs into this, here is another way of doing it.  Avoid using hardwood lengths under 16".  Mark the joist locations on the existing subfloor.  Run the hardwood perpendicular to the joists.  Nail hardwood thru subfloor into joists.  Stagger lengths and stagger end joints, and make sure every piece gets at least one nail into a joist.  Those 4-foot lengths picking up 3 joists will stiffen things, and the tongue joints will transfer from one piece to the next.

If you absolutely can't run the hardwood perpendicular to the joists you need to stiffen the subfloor.  

You will get additional stiffness from adding 1/2" ply if you stagger the joints.  Adding short screws or glue to laminate the layers will help further.  This will probably end up stronger than 1 layer of 3/4.  But is you like new and want to avoid the extra thickness, nothing wrong with new 3/4.

 

James
Re: Subfloor Thickness
James

A related question. I'm building a container house. Instead of building a sub floor with joist then decking, I'm using the cut out walls and ceilings as the structural decking over the bare steel beamed floor of the container. This will save me on waist and the cost of wood for the framed subfloor. That being the case, when I deck it, what thickness of plywood should I use?

Structurally, I don't think 7/8 or 3/4 is necessary, as the floor will be made of steel on top of steel, but it's not flat. So, putting some decking (or other?) down makes sense. I was thinking 1/2, but could I go 1/4? They currogated steel is 12" on center for the ceiling panels and 8" on center for the wall panels. I just need a flat surface to put the floor down on.

Bill
Re: Subfloor Thickness
Bill

1/4" seems risky, but 1/2" will work and won't cost much more.

Ed
Re: Subfloor Thickness
Ed

Bill (comment #7),

Are you saying you can add the hardwood directly to the 1/2 subfloor if you follow what you say or are you saying to add 1/2 first then do what you say??  I have the same issue with 1/2 subfloor under 5/8 particle board so I am removing the particle.

Thanks,
Ed

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