Porcelain on wood subfloor questions

pnoskoDecember 14, 2010

Hi. I'm a newie here. I'm wanting to install porcelain tiles and want to make sure my floor can support it. Any guidance would be greatly appreciated.

My floor has 2x10 joints spaced 16" that span 13'8" each way from a center steel I-beam. The subfloor has a stamp I can barely read, but I can make out the following.

Rated Sturd-I-Floor, 20oc, 19/32" Exposure 1.

The website for HardieBacker says the subfloor needs to be 23/32 OSB and engineered not to exceed the L/360 deflection criteria. I saw some posts that said sturd-i-floor is better than OSB, but it looks like OSB. I don't know how to evaluate the deflection criteria.

1) Will my floor support porcelain tile?

2) Do I need to add to the subfloor before installing 1/4" hardiebacker, and if so, what should I consider?

I do notice a few weak spots and squeaking on the existing floor (luan plus two layers of vinyl tile). It might be the luan that is squeaking.


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I'd recommend adding a layer of 5/8" thick underlayment over your subfloor. Offset the seams of that layer from the seams of your existing subfloor. Screw it down, fastening it to the existing subfloor, but not to the joists.

When you put down the 1/4" hardie, use thinset between the hardie and the new underlayment. You can use unmodified thinset (less expensive), you're not looking for bond strength, just a bedding material that will eliminate any vertical movement due to micro-gaps between the hardie and the underlayment. You'll also screw or nail the hardie down, that will be the bond strength so to speak.

    Bookmark   December 15, 2010 at 10:31AM
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1/360 works for smaller tiles, but the allowable deflection goes down for larger tiles.

1/360 is mosaic tile, 1/540 and even 1/720 are needed for larger tiles.

    Bookmark   December 15, 2010 at 2:57PM
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