Bowed OSB Sub-Floor Tile Question

Flyboy Mach II · January 3, 2009, 07:30 AM

I am preparing to install marble tile in a receiving room butting up to oak hardwood, but after pulling up my carpet, I found my sub-floor bowed. The room is 13'6" x 14' and from wall to wall has about a 1" rise in the middle. I had planned to install 19/32 plwood to match surrounding flooring, but am concerned over the bow. If i level around the bow, It may raise the flooring so that it is higher than adjoining hardwood and tile. I'm assuming replacement of the OSD sub-flooring is the best fix, but wanted to get other opinions.

Bud Cline · January 3, 2009, 08:13 AM

Even though a surface match is desirable the greater concern is whether or not the floor structure is suitable for a stone tile. Stone tiles require twice the subfloor strength of that required for a ceramic installation. The minimum allowable deflection for a stone tile floor is 1/720 and most floor structures don't meet that qualification.

In some cases the addition of plywood is all that is necessary but of course that will elevate your stone-area beyond what you desire.

If we had the structural criteria of your floor we could better estimate your position.

What size are the floor joists?
What is the species of the wood?
What is the spacing of the floor joists?
What is the unsupported span of the floor joists?
What is the thickness of the present subfloor?

Flyboy Mach II · January 3, 2009, 08:40 AM

2x10 Joists (Pine)

16" Centers

16"x16" Concrete Block pillars as supports every 8' (2 - 2x10"s strapped together rest as beams atop each of these pillars the length of the house) 3 rows total of beams and block supports. Also there are several rows of 2x10" cross supports between the joists at places.

Current Subfloor appears to be 3/4" +/- OSB.

Flyboy Mach II · January 3, 2009, 08:53 AM

Also, the tiled area that joins the room has 19/32 plywood on top of the subfloor with 18x18 ceramic tile on it. The hardwood is 3/4" oak with on top 1/4" plywood on felt on top of the OSB subfloor.

Kman · January 3, 2009, 12:29 PM

You would need a minimum of 1/2" ply on top of the 3/4" subfloor to install marble. Then you would also need some type of tile backer on top of the 1/2" ply. With the 1/2" ply, backer, thinset, and marble, you're probably going to add approximately 1 1/8" to 1 1/4" to the height of the floor. This is, of course, assuming that you get the bowed section taken care of. I would probably cut out that section and replace it.

What is the longest unsupported span of the joists in the area where you're wanting to tile? If it is no more than eight feet as your post suggests, you have an extremely strong floor. Haven't seen many houses built with the joist system that strong.

tony lamar · January 3, 2009, 06:11 PM

"Bowed" OSB probably = a severly crowned joist(s) underneath. I'd pull up some OSB and investigate further. May need to plane off a joist or two.

Professional flooring I · January 3, 2009, 07:56 PM

I leave structure to the framers - carpenters .

Ken · January 3, 2009, 09:08 PM

If your going to do it right and pull the sublfoor to do the joists you could drop the new subfloor to keep the height closer to the existing floor

tony lamar · January 4, 2009, 04:15 AM

Professional flooring I said

I leave structure to the framers - carpenters .

"Framers - carpenters" caused the problem to start with.

I find most of them to be less competent than me at getting the floor to the state that I need it to be.

Flyboy Mach II · January 4, 2009, 06:28 AM

Yes the 16" square cement block supports are every 8' in either direction. Yes, it could be the joists that need planing, but once I was out of the country and my ex had the carpet cleaned by some yahoos that left it far too wet. I suspect that could be the issue, but really won't know I guess until I pull a sheet. If i do have to replace the subfloor, would it be better to go back with the original OSB or Plywood (regular or T&Groove)?

Kman · January 4, 2009, 04:44 PM

Plywood is probably better, but I would still replace it with whatever is there, unless you're replacing the whole thing. One small section of plywood is really going to help the whole installation.