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How thick must subfloor be?

Posted by elizawhyza (My Page) on
Tue, Feb 28, 12 at 11:49

We are doing an extensive remodel. During the demo, I recall seeing the guys removing a lot of plywood, which I assumed would be put back. The flooring contractor was just here, and they noted that our subfloor is only 1/2" on the first floor hall. It used to be tiled, so I wonder if when they broke up the tile, they removed that layer of subfloor. The living room subfloor is 1/2 higher than the hall.

In the upstairs, there is a large area of subfloor that is 1", but then there is a large area that is down 1/2" from that level, and you can tell that there used be more wood there.

Does code call for at least 3/4"? Right now we are at only 1/2" in the area of new construction in our front hall and entire master suite, the upstairs hall, and two upstairs bathrooms. The biggest problem is that we have brand new stairs, and I believe the calculations for the risers was done without the 1/2 inch. I don't want saddles between rooms if I don't have to have them, and more importantly, I don't want only 1/2" of subfloor, especially if code is 3/4".

Does anyone know what you have to have?

Many thanks!

Follow-Up Postings:

RE: How thick must subfloor be?

Code is dependent on your location - state, county, city, municipality. Code aside, I would not accept a subfloor less than 3/4 inch, and ideally screwed into the joists to avoid squeaking over the long term. 3/4" results in a quieter, more solid floor, and a better base for any tile. And if you are doing a major remodel, this is your *one chance* to easily do this.

RE: How thick must subfloor be?

Thanks, Vate. I appreciate the help.

RE: How thick must subfloor be?

A quick check at the Home Depot web site finds OSB tongue and groove subflooring for $12.47 per 4X8 sheet for 3/4" (technically 23/32). Tongue and groove will also help with stability and strength as the pieces lock together on their edges.

1/2" OSB is $6.97. So sure, that's six bucks per sheet less, so on a 3200 square foot house, an extra $600. Probably a drop in the bucket to your overall remodel cost, but a way to trim the pricing for a contractor. Of course, the effort to lift the thicker sheets is more, and you have to cut more carefully with T&G as placement is critical (tongue has to align with groove), so there is more effort to installing the heavier board.

RE: How thick must subfloor be?

Thanks for looking that price difference! In the areas with the 1/2" difference, can we just add a layer of 1/2"? It looks like we had 2 layers of subfloor before that added up to one inch, and some of it was removed.

Also, in one area, we had tile before. I think when they ripped up the tile, they also removed the plywood.

RE: How thick must subfloor be?

Subfloor thickness can also depend on what type of flooring you are putting down. The type of surface you put over the subfloor also depends on the flooring. Normally you have three layers, subfloor, underlayment and flooring.

If you had tile, it is almost certain they removed the underlayment (Top layer of the subfloor) as the thinset/mastic would be stuck to down, and paying someone to scrap it off would cost way more than buying new material to put in place.

I am currently scraping thinset off plywood to change out some tiles - not a favorite thing to do and not something I would want to pay someone else to do.

RE: How thick must subfloor be?

Thanks for the info on the underlayment. If they removed the underlayment from the tile, (and I think you are right), and if that area is now to get hardwood floor, it seems that the GC have to bring the subfloor/underlayment back to its original height.

The contract specifies that plywood is to be 3/4" at flooring.

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