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DrDave 12-16-2007 10:50 AM

Granite or Marble Tile on wood flooring
I just removed carpeting from our upstairs guest bathroom. I plan to install either granite or marble tiles (12 X 12 or larger). I have Hardie Backer on my BBQ Islands and since it is available at my favorite HD, I plan on using it.

The big question is: Do I need to go to 1/2" or will the 1/4" be fine. Using HB screws to hold it down, or should I use adhesive.

Next, whatever the thickness including tile what is the recommended method of reseating the toilet now that everything is higher? Is there a riser or do I use longer screws and an extra high wax ring?:huh:

redline 12-16-2007 01:35 PM

How thick is the sub-floor?

JazMan 12-16-2007 01:43 PM

Hi Dr. Dave,

Interesting island you've got there. I guess you didn't realize you can NOT use Hardi backer outdoors though? It is only for interior use.

As for your bath floor, you should use the 1/4" unless you need the extra height for a smoother transition. However, granite and marble tiles require not only a double subfloor under the backer, but the joists system has to be much stiffer than most houses are initially built. The standard max deflection for ceramic and porcelain is L360, where stone has to be no more than L720. This is for both the joists and the subfloor.

It is my experience that most houses are only built to meet the min required spec of L360, although some are L480, still to much deflection for stone tiles.

Please tell use about your subfloor system, get this info; the size, type, spacing, species, grade and unsupported span of the joists. Then the type, thickness, number of layers of the subfloor. With that info I can check my span chart and let you know.


DrDave 12-16-2007 02:32 PM

The existing sub floor is 5/8".

Thanks for the compliment, I think... It was designed with the available space and access in mind. I had to make it easy to get in and out of, keep it near the kitchen yet out of the walkway. It has been duplicated over 300 times that I know of.
My BBQ Island is protected with stucco and has been outside in direct sunlight (108 d last summer), freezing cold (most winters), driving rain and hail for the past 8 1/2 years and still looks new.
I have a test bare piece of HB that has been part of a long term endurance test and it still looks very good with no protection for the same time period.
There are thousands of BBQ Islands built every year this way and no-one that I know of has had any problems. My designs are in use all over the world and in every climate known to man.

Back to my floor...
The subfloor joists are on 16" centers and as previously mentioned 5/8" plywood. I can't tell if there is another under it.

The HB site says to use thinset under the 1/4" in prep for tile. This is a very small bathroom that sees guests or users only a couple times a year. The floor size to be tiled is only 63" X 74". No heavyweights in my family or kids to jump up and down on it.
I think the granite tiles at HD are 3/8" thick, there may be thicker, I haven't checked. I have only experience with granite slab as seen on my BBQ Island.

Thanks in advance for your time.

JazMan 12-16-2007 03:53 PM

All I can tell you is that James Hardie Company does not want it used outdoors. and that is fails the tests for that purpose. Maybe your results would be different if there were tiles over it instead of slabs? I think the main problem is that it swells and shrinks a lot?

Unless your house was built a long time ago, chances are you only have the one subfloor. If so, 5/8 is bare bones minimum even for carpeting. You need at least another 1/2" plywood, then your favorite CBU, if you want natural stone. That is as long as your joists meet L720 deflection at least. The size of the room has nothing to do with it.

Thinset under all CBU's, nothing special there.

Granite and marble tiles are usually 1 cm. thick, it's standard. Every piece is different and one reason they are more likely to crack under even normal tresses.


DrDave 12-16-2007 05:34 PM

Thanks Jaz
Mine had tiles on it for 8 years. Some did come loose just before I got tired of them and decided to go with the granite slab. I might have had better success with 1/2" HB. Maybe not, now that I think about it since they were the vertical ones by the corners that came off first.

Back to the bathroom...
1 cm as a standard for granite and marble tile, got it. They like that metric method don't they? Thier standard for slabs are 2 and 3 cm. You mentioned cracking. Is that only when used over wood flooring or does it happen over concrete slab. Our Master Bath is probably next, it's on a slab.

This may seem crazy, but since this room sees only adults several times a year and they are typically very carefull. What do you think about laminate flooring? Isn't it completly synthetic and impervious to water damage? I wanted to do this to start with, but my wife is insistant on granite tile. I figure a couple of bathroom rugs will take care of anyone stepping out of the the shower.

JazMan 12-16-2007 06:35 PM

You're right! Laminate? .............................. I think I'm gonna be sick!!:laughing:


DrDave 12-16-2007 06:47 PM

Well you have to admit it has to be better than the blue carpet I took out.:laughing:

So, tiles it is. What tiles are better than others when it comes to cracking? Should I re-consider the granite and go with another tile less likely to break?

JazMan 12-16-2007 08:14 PM

Everything is better than carpet, OMG you had carpeting in a bathroom! That is unsanitary, you're a Dr. I'm sure you know that? Even in a living room carpet is awful. OK, I have lots of carpet too.:(

If you go with ceramic or porcelain you're likely to have a better chance. You're still at minimum though with that subfloor, and we still don't know the other factors.


DrDave 12-16-2007 08:30 PM

The first thing I did was remove the carpet from the Master Bathroom when I bought this place. This unused bath has never been a higher priority than heating, roof, septic, appliances and so on. Seems like there is always something.:yes:

So ceramic or porcelain it is. :thumbup: Now I need to find one to match the granite vanity we want. Oh well, the beat goes on.

I guess a toilet flange extension kit is the answer to the other part of the original question.

Thanks for the help:thumbsup:

JazMan 12-16-2007 09:11 PM

They also make shims for the flange, rings. All depends how much you need to raise it. You can also go with 2 wax rings and longer bolts if about 1/2" low. The bottom of the flange is supposed to be at the level of the finished floor. It's OK if it's up to 1/4" low though. Just make sure the toilet compresses the wax about 1/2".


DrDave 12-16-2007 09:16 PM

Got it thanks again!

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