Proper way to install backer board

[Kurt M.]

Good morning. I just found this forum and I am excited. I hope that some of my question can be answered her.

I am fixing to redo our entrance hall. I plan on putting a hardwood floor border (about 10 inches) around the outlines of the hall. Then I plan on laying marble to fill in the remaining space. Of course I will be laying the marble first and then the hardwood border.

I am pretty handy with many aspects of DYI, but never worked with marble or tile. I spent tons of hours researching and learning as much as I can before I start my little project. Of course, my wife wants to know why it isn’t done yet.

Our subfloor, 5/8 plywood, is in pretty good shape. I plan on laying down a cement board where the marble goes. I have read many references to hardi boards to use instead of cement board. What is a better material to use, cement board or hardi board?

I am also not sure regarding the spacing between the tiles? I was told by some not to leave any space between the tiles and others told me to leave 1/8 in. When I visited a model home I saw space between tiles of ¼ in. What is the recommendation? Also, if I butt the tiles together there wouldn’t be any space for grout lines and I would think grout actually makes it more compact and rigid. Am I wrong?

Last question for now. What is the recommended thinset thickness? Seems that ¼ in is common, but is this true for marble as well? Once dry, will it remain ¼ thick?

Thanks, this is truly a great forum and I think it’s great that many of you professional pass on your experience and tips

[bbcamp]

Welcome to the forum, Kurt.

5/8" subfloor is not thick enough for marble. You will need 1-1/4" (not counting your backerboard). We will also need to look at your floor joists to see if they are up to a marble installation. Marble, like all natural stones, needs a floor that is twice as strong as a ceramic tile floor, and that's pretty strong in itself!

As far as the backerboards go, use whichever one is available, they're all about the same.

I'd rethink the order of installation. You will be nailing down the hardwood which shakes the floor. Tiles don't like that. You can finish the hardwood, then protect it with rosin paper and painter's tape during tile installation.

Don't ever butt tile together, leave a grout gap. For marble, it can be small, 1/16" or so. In any case you want the grout line less than 1/8" so you can use unsanded grout. Sanded grout will scratch the marble tiles, both during installation, and during regular cleaning and maintenance.

Thinset is thin. Probably 1/8" to 3/32". However, if your tiles are large, then you will be using a medium set mortar, so the thickness may be greater. The notch size is not the final thickness, the thinset smushes down when you set the tile, so there are no voids under the tile. It doesn't shrink noticeable when it cures.

So, do you need to tell the wife that you'll need more research time?

Bob

[Kurt M.]

Thanks Bob, for the reply.

I was affraid that the subfloor would not be strong/thick enough to support the marble.

The actual area that would see marble is only 12 x 5 ft. The rest would be hardwood flooring. Still need the 1-1/4 subfloor? I hate to see a great difference in floor height between the hall and other rooms.

BTW, thanks for the reco to install the wood floor first. I can see the marble tiles shaking loose when I air nail the wood.

[jadnashua]

I read in one sequence that if you cut out the subflooring, you could build it up between the joists, with additional flooring on top of the joists. This may help you reach the desired thickness and keep the entire height similar to the other rooms.

Someone else may give you the details of how this is done.

[Kurt M.]

In preparation to install marble floor I need to lay down ½ in backer board (i.e. Hardi or Durarock) and was wondering what the proper way of preparing the subfloor is? I read somewhere to staple polyethylene plastic on top of the subfloor and the backer board on top. Is the backer board only held in place by screws? What is the proper spacing for the screws (3-4 in)?

I would really appreciate if someone could tell me the proper way/method. Thanks Kurt

[Mike2]

Hi Kurt. Glad to have you with us.

Your homework assignment for the day will be to visit each one of these two links containing detailed instructions on how to install either product.

Hardibacker
Durock

Then come right back here and ask any clarifying questions you may have


P.S. Are you sure your floor is strong enough to adequately support a marble tiled surface? Our engineer bbcamp will be glad to help you with that question but first he will need some more information like: What size are your joists, how far apart are they spaced, and what is the longest unsupported span? Additionally, a description of your sub-floor would be most helpful.

[Kurt M.]

Thanks for the directions Mike. In a previous post I already received much of the info I need.

I plan on redoing our entrance hall. Basically laying down a perimeter of about 1 ft of hardwood flooring all around and filling the middle with marble. The hall is about 13ft x 5.5 ft. Not really a large space.

Joists are 8in and spaced 16in. One thing that does not help is that the joists go in the same directions as the hall space adding to the flexing of the subfloor. The subfloor is 3/4in and in very good condition. I plan on using ½ in backer board to come to the recommended thickness of 1 ¼ in. before putting the marble down.

I think your link to the Hardibacker installation instructions answered my questions.

[cx]

OK, Kurt, you gotta come clean here.

I merged your former thread in here, just stay with this same thread until this project is complete or we'll not know what you've been told and have told us.

Now, if you'll look back a few posts you'll see that Injineer Bob done already tole you the following:

5/8" subfloor is not thick enough for marble. You will need 1-1/4" (not counting your backerboard). We will also need to look at your floor joists to see if they are up to a marble installation. Marble, like all natural stones, needs a floor that is twice as strong as a ceramic tile floor, and that's pretty strong in itself!

So, first of all, which do you have, 5/8ths or 3/4 inch subflooring?

Second, you gotta be specific about the size of your joists and tell us the actual unsupported span of the joists. If, in fact, you have only 2x8 joists spanning 13 feet, we're not gonna recommend you install any kind of tile until you correct that situation, and certainly not natural stone.

Out with the information or my next call is to the Tile Ranger and I don't make that threat lightly.

No sense spending the time and money on an installation that's not likely to survive, eh?

[bbcamp]

Kurt, your joists are undersized for marble or ceramic tiles. If there were some way to reduce the span of the joists to something less than 8 feet, then you could make a go of it.


Also, you don't count the backerboard for subfloor thickness. You can, however, add plywood under the existing subfloor. You can glue and screw sections of plywood (face grain perpendicular to the joists) to the bottom side of the subfloor, then add cleats to the sides of the joists to support the ends of the new pieces.

I measured and re-measured. The joists are 10in and not 8 as I indicated previously. I also checked from the basement and never even realized that the floor is supported by a wall, as well as an I-beam about 7ft down the hall-length of 13 ft.

The wall was put in by me and uses 6in studs (leads to a wine cellar). So, while perhaps not designed to hold a load, it certainly helps as well.

I jumped up and down on the floor and there is absolutely no bounce whatsoever.

Please tell me to go ahead with my plans.

I dry fitted all pieces last night, including the backerboard, the marble, plywood to raise the floor for the hardwood flooring around the tile and it looks fantastics. Having to raise the floor doesn't look very nice where it transitions into other rooms, but since I will be laying down hardwood flooring there also, the uneven transition will go away later.

[cx]

Joist support sounds fine, Kurt.

I'm a little confused by some of this:

I dry fitted all pieces last night, including the backerboard, the marble, plywood to raise the floor for the hardwood flooring around the tile and it looks fantastics. Having to raise the floor doesn't look very nice where it transitions into other rooms, but since I will be laying down hardwood flooring there also, the uneven transition will go away later.

You do understand that you must add another layer of plywood (you still haven't clarified whether you have 5/8 or 3/4 now) over the entire area to be tiled?

And you further understand that it's not necessary to use 1/2 inch CBU if height is a concern? Quarter inch is fine, and Ditra would save you an additional 1/8 if you use it in lieu of CBU.



And you further understand that I actually have the Tile Ranger's number in my speed dial?

Let's see if I can be a little more specific. The floor is 3/4 plywood. THe funny stuff made from a gazillion little pieces. I purchased 1/2 backerboard to raise/support the floor to, what I understood, 1 1/4 in thickness.


I m now confused however, why would I need another layer of plywood?

[cx]

OK. That's not plywood, Kurt, that's OSB, Oriented Strand Board. Store that in your memory bank, it'll come up again.

On top of that subfloor you must add a minimum of 1/2 inch plywood (not OSB) in a grade of C or better, like AC or BC or CC plugged (which you ain't likely to find. You can add thicker ply, but not thinner. This will give you the recommended inch and a quarter subfloor, which does not count the CBU. CBU adds nothing in the way of structure to the floor, it's only there to provide a good tiling substrate. It can be 1/4 inch thick, or you can replace it with Ditra which is only 1/8th inch thick.

What's your pleasure?

Makes a ton of sense what you are saying CX. I will exchange my 1/2 board and go with your recommendation. Adding another 1/2 plywood and using Ditra.

I like to do it right the first time.

You can now dump the Tile Ranger's number.

[cx]

Ooooh, no, no, no, never know when we'll need to call the ol' Ranger. Get some pretty stubborn people hereabouts sometimes.