In an effort to save $ I would like to prep my entire main level for 18x18x1/4" porcelain install. I would like to keep the floor height to a minimum so as not to create tripping hazards w/ 2 stairways, and also because I have raised my cabinets 3/4" already and am out of room to raise them further.

The area in question is approx 13x37' 23/32" OSB over 2x10's 16" OC w/ 2 Lam beams @ 12' intervals. (I understand the importance of an expansion gap.)

Had lots of flex. I installed 2 additional Lams @ 6' intervals. This made the floor much stiffer.

Have read lots of info on the web about prep. Most call for additional 3/4" or 1/2" external grade plywood w/ 1/4" backer, but height is an issue here.

Given the additional floor supports, can I forego an additional layer of plywood?

Was advised by an installer to just screw down 1/2" HardiBacker WITHOUT thinset over the OSB and WITHOUT 1/8" gaps. Claims 1/2" thickness will take care of any flatness issues (which are mostly under 1/8") and that gaps are not necessary.

Is he right?

Also, the kitchen, front door, closet, and garage entrance are part of this install. Installer claims the HardiBacker acts as a moisture barrier.

Do I need to install a moisture barrier over the OSB? If so, what do you recommend?

This tile is $8.63/sq. ft... lots of money to me! I would like to do this right. Please advise.

Welcome to the board
There are 2 areas influenced by up and down movement (deflection) within a floor's construction. One is the span of the joists and the other is the plywood between the joists. The beams you've installed may have solved the deflection of the joists.

Hell no! In fact your floor would fail and have no warranty. HardiBacker should be installed as they advise with a bed of mortar under their panels. Gaps and taping/mudding panel seams are also essential.

HardiBacker isn't a moisture barrier ... and no moisture barrier is required.

If you have completely removed the flex of your joists, then you should reinforce the OSB to the joists ... sand the OSB seams if swollen or peaked and install the HardiBacker as HardiBacker recommends.

Dita membrane is another option instead of HardiBacker.

I'd recommend at least 1 control joint somewhere central of the floor within the 37' length.

Thanks for the info, Harry!

I'm not sure what the deflection is between joists, but I could rig up something to test it. I am getting closer to 300 lbs...

If that deflection is out of spec, install 2x10 blocking between joists? How far apart?

Would there be any advantage to 1/2" vs. 1/4" HardiBacker if everything else is OK? Or is Ditra the way to go?

HardiBacker specs both mortar or modified thinset. Which will adhere to the OSB better? Any brand recommendations?

Again thanks for the post!

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Frank

Update: Deflection between joists @ 3' from any beam, middle of room = 0.035" just standing on it, 0.045-0.060" when I bounce on it. (I weigh 240 lbs.) Am I OK with those numbers?

I normally use a modified thinset ... but any cheap brand will do fine, even unmodified. It isn't for bonding capabilities as much as it is to simply fill in the voids under the board.

I don't think you have to worry about the deflection between the joists ... but I recommend you using a good quality mortar for bonding those big porcelain tiles. Something pricey with lots of latex.

Great! Thanks for all the valuable info, Harry!

Been reading all the threads related to OSB. This site is excellent (especially for me). As with anything in life, there's a lot more to learn once you start diggin'!

Do you prefer Ditra over HardiBacker... Ditra sounds a lot faster and easier? From what I've read, everybody loves the stuff.

If not, is there ANY reason to use 1/2" as I was advised by my installer? Mfr. calls for 1/4" on floors (unless there's a transition issue, which I don't have).

Thank God for professionals who'll take the time to teach others. You guys haved saved my butt! Peace.

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Frank

Yes ... I prefer Ditra.

1/2" or 1/4" cement board/fiber board is all the same .. the only purpose for the 1/2" board is for walls where it meets up with 1/2" drywall panels. Either for your floor will be fine.

I'll give 'er a whirl.

Sorry if this wasn't the right spot for my questions... I just really needed some advice.


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Frank

I thought that OSB wasn't a satisfactory base for tiling.? I have always put plywood over OSB.

Schluter is one membrane manufacturer that says it's OK to set its product with modified thinset onto OSB.