# what sub floor for slate tile?



## gk02 (Jan 22, 2009)

I am trying to figure out the correct sub floor for putting down slate tile in my kitchen. It is an area less than 150 sq ft. 

When I pulled up the old ceramic tile it was just 1/4" hardibacker screwed down to the sub floor. 

The house is about 70 years old so the sub floor is wood planking. 

After reading some of the other posts on here I am unsure about what I will need to have an adequate base for the tile. 

I can access the basement and add additional cross support below the original planking if necessary. 

I would like to avoid putting an additional sheet of plywood down over the planking to keep from raising the floor anymore than I have to. The threshold to the wood floor in the living room will already be an issue with just hardibacker and tile. 

I have been advised by some to adhere the hardibacker to the original planking and then screw it down and then I have had someone tell me to do nothing but screw it down like it was when I pulled it up. 

Any advice would be appreciated.


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## JazMan (Feb 17, 2007)

Hi,

Neither of those people told you the right way to do this.

First of all you should know there are two factors to consider about floor deflection or stiffness. The first is the strength of the joists system. The second is the subfloor layers which gives the strength between the joists. 

Another factor is having the proper top layer of underlayment, but that is the same no matter what the other factors are.

Before you go much further, you need to know that natural stone tiles require a much stiffer floor than does ceramic or porcelain tiles. The basic maximum deflection (minimum quality), allowed for modern homes is a factor of L360. (1/360). That is also the max deflection recommendation for these classes of floor tile. Natural stone should be installed on floors that meet L720 deflection. It is hard to find a house that meets this if it was built without the intention of installing stone. This applies to both joists and subfloor/underlayment stiffness. These are two different things.

Lets us know about your joists system, what type and size are the joists, spacing from one the next, the unsupported span? Also if possible the species and grade. Are they in good condition, lots of holes for pipes and wires? How does the floor feel now? We will consult a span chart to see what you should do.

Also let us know about the subfloor layers. Is there only this single layer of planks? Are they t&g, how thick, size etc. 

You should never install a tile backer over planks, there should have been a layer of plywood over your old sub, then the backer board and tiles. Maybe you got lucky?

With ALL natural stone you need a two layer system. You should install 1/2" or thicker ply, then either backer or a membrane such as Ditra or one from the Noble Co.

Jaz


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## gk02 (Jan 22, 2009)

Ok, thanks for the response. This is what I was afraid of. 

I will measure everything involved this evening and post back.


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## gk02 (Jan 22, 2009)

Ok so I have taken all the measurements.

The floor joists are 2x8's on 16" centers. There is a section in the middle of the floor where the spacing is decreased to like 6" on center for about 4 joists. They are in good condition with minimal holes for wiring. I cannot identify the species but definitely something other than your standard pine. Seems much more dense when I have had to drill in it in the past. Its also quite a bit darker in color than pine. 

The joists also have crossing x braces run between each one of them in the middle of the floor

The span is 11ft unsupported. 

The current sub-floor is one layer of 7/8" thick by 3.25" wide t&g. It is solid and is not soft. I have been walking on it for the last couple weeks since I ripped out the old floor. 


Thanks for the help.


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## gk02 (Jan 22, 2009)

I also didnt mention that this is a single story house.


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## JazMan (Feb 17, 2007)

OK. First about the joists system. You are barely OK for ceramic and porcelain tiles. If everything is perfect and the joists were Southern Pine (usually the stiffest used), grade #2, you would be at L360 at 11'5". To meet L720 for slate your span would have to be at or under 9' 6". So, no good. You need to build a supporting wall under that area, or sister all the joists. You could get away with sistering the mid-2/3 of the span.

After you stiffen the joists, you add at least 1/2" underlayment grade ply fastened to the planks. Then install a CBU or Ditra, then slate. 

All that including gauged slate adds about an inch or so. If the slate is not gauged, it'll be higher.

Alternatively you could go with a nice porcelain that mimics slate. It would be easier to install, and must easier to maintain. 

Jaz


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## gk02 (Jan 22, 2009)

ok, thanks for the help. I must have found a similiar calculator for floor deflection and came to the same discouraging results. 

Its wonderful how the floor place is happy to show you around and sell you whatever you want without even a thought to mention that if you want to install slate tile in your 70 year old house it might be a problem. I would assume a lot of people would just go ahead and lay it down and not even think about it. 

I will have to see what the wife has in mind since she has already purchased the slate. I can access the floor from below and could add a support wall to cut the span in half. This would not be ideal in the least in terms of the amount of work for myself but appears to be do-able. 

I would probably have to drill and pour footers I would assume in the existing basement floor for the support?


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## gk02 (Jan 22, 2009)

So I am thinking I am going to build a wall to support the floor as my wife really wants to use this tile. The intention is in the long run to build a bathroom in that area anyways so I am thinking I will build a temporary support wall using 2x4's to support the floor until I can come back and build the wall exactly where I need it for the bathroom. One of the new bathroom walls will be exactly in the middle of the floor. 

According to the calculator I have found this should increase the floor stiffness more than enough for slate because it will cut my span in half. 

Does this sound reasonable or am I missing something? 

Thanks


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## HABSFAN2006 (Feb 25, 2009)

gk02 said:


> ok, thanks for the help. I must have found a similiar calculator for floor deflection and came to the same discouraging results.
> 
> Its wonderful how the floor place is happy to show you around and sell you whatever you want without even a thought to mention that if you want to install slate tile in your 70 year old house it might be a problem. I would assume a lot of people would just go ahead and lay it down and not even think about it.
> 
> ...


 
I would say make sure you build this supporting wall, and it does not move there after, it stays permanently after the tile has been installed.
Also, there is no need to pour footers.

Use vapor barior between cement and bottom 2 x 4 plate, drill every 16" and on ends of walls, then screw in 4" cement tapcons. Double up your top plate, studs on 16" ctrs, and cross piece/fire stop at 48" from bottom.

For a premium result, add PL Premium between floor joists and top plates.


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## gk02 (Jan 22, 2009)

ok, thanks for the advice. I believe that is the way I will go. I will just need to map out the position of this wall carefully before I start as it will need to have a doorway in it to access the hvac equipment for the house. 

thanks


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## HABSFAN2006 (Feb 25, 2009)

gk02 said:


> ok, thanks for the advice. I believe that is the way I will go. I will just need to map out the position of this wall carefully before I start as it will need to have a doorway in it to access the hvac equipment for the house.
> 
> thanks


great. don't forget to double up your header above the door, 
and double your 2x4 on each side of the door.


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