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Tile Subfloor underlayment mkurley 11-02-2006
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Posted by on November 2, 2006, 8:52 am


I plan on installing 13x13 porcelain tile in my entranceway/hallway,
bathroom, and kitchen. I have a question about the subfloor and
underlayment.

The floor joists are 16" OC 2x10 joist (Douglas Fir) with a span of 12'
and 13'. I look good on the deflection with L/458 and L/558.

My current floor is 23/32" plywood, then 1/4" Luan, and then vinyl
flooring. I plan to tear out the vinyl flooring and Luan. I have been
told that I should not put Hardibacker on Luan since it has a greater
tendency to warp.

After I pull out the Luan I have about 1/2" to use for building the
underlayment since I will run into problems with height restrictions
namely two exterior doors that can't be cut down.

How should I build my underlayment on top of the 23/32" plywood? I
want to have a layer of hardibacker and it looks like 1/4" is actually
stronger than 1/2" ? Will 1/4" hardibacker on top of the 23/32"
plywood be enough support? Could I do this and then add blocking to
the joist under the floor to increase the support of the floor?


Thanks for the help.


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Posted by RayV on November 2, 2006, 9:54 am



mkurley@gmail.com wrote:
> I plan on installing 13x13 porcelain tile in my entranceway/hallway,
> bathroom, and kitchen. I have a question about the subfloor and
> underlayment.
>
> The floor joists are 16" OC 2x10 joist (Douglas Fir) with a span of 12'
> and 13'. I look good on the deflection with L/458 and L/558.
>
> My current floor is 23/32" plywood, then 1/4" Luan, and then vinyl
> flooring. I plan to tear out the vinyl flooring and Luan. I have been
> told that I should not put Hardibacker on Luan since it has a greater
> tendency to warp.
>
> After I pull out the Luan I have about 1/2" to use for building the
> underlayment since I will run into problems with height restrictions
> namely two exterior doors that can't be cut down.
>
> How should I build my underlayment on top of the 23/32" plywood? I
> want to have a layer of hardibacker and it looks like 1/4" is actually
> stronger than 1/2" ? Will 1/4" hardibacker on top of the 23/32"
> plywood be enough support? Could I do this and then add blocking to
> the joist under the floor to increase the support of the floor?
>
>
> Thanks for the help.

You could also use 1/2" cement board. I used 5/16" cement board on top
of 1/2" plywood and haven't had a problem yet (1 year) YMMV.


Posted by maurice on November 2, 2006, 6:48 pm


Have you considered using "Ditra" instead of a more traditional
underlayment? Check with your tile supply shop.

This is a fantastic product. I've used it under tiles, heated and
unheated, and eliminated the need for thicker substrates. Even in
applications where there are some movement problems with foundation or
the like, I've never so much as had a crack in the grout when I've
properly applied this product. I've used it under 18"x18" slate, right
down to mosaic tile. Always works well.

It takes a fraction of the time to put down (all you need is some
thinset and a knife), and doesn't generate the noise and mess you get
from plywood.

Here's the website:

http://www.schluter.com/english/products/2002/sectionf/ditra/601-index.html

It was written up last year in Fine Homebuilding, I can get the issue
date if you need it.

good luck!


Posted by on November 3, 2006, 7:47 am


I was also thinking about Ditra but haven't decided between the Ditra
or Hardibacker. Would I still have to add the layer of 3/8 plywood on
top of the 23/32" subfloor if I go with Ditra?

I just need to figure out if I have to add the 3/8 plywood to make the
floor stronger before I add the Ditra or 1/4 Hardibacker?

I am a little confused because some are saying I need 1-1/4" subfloor
before I lay hardibacker or ditra. Then on Hardie's website they say "
For all floors: Use minimum 5/8" exterior grade plywood or 23/32"
exterior grade OSB, complying with local building codes and ANSI
A108.11. Joists' spacing not to exceed a maximum of 19.2" on center.
"

So according to hardie's site my 23/32" subfloor is enough and I can
lay 1/4" on top.

Does anyone know if I would need to add the 3/8 plywood? If I don't
add it and run into problems after the installation would I be able to
block my joist under the floor to improve the strength?

Thanks
-mark


maurice wrote:
> Have you considered using "Ditra" instead of a more traditional
> underlayment? Check with your tile supply shop.
>
> This is a fantastic product. I've used it under tiles, heated and
> unheated, and eliminated the need for thicker substrates. Even in
> applications where there are some movement problems with foundation or
> the like, I've never so much as had a crack in the grout when I've
> properly applied this product. I've used it under 18"x18" slate, right
> down to mosaic tile. Always works well.
>
> It takes a fraction of the time to put down (all you need is some
> thinset and a knife), and doesn't generate the noise and mess you get
> from plywood.
>
> Here's the website:
>
> http://www.schluter.com/english/products/2002/sectionf/ditra/601-index.html
>
> It was written up last year in Fine Homebuilding, I can get the issue
> date if you need it.
>
> good luck!


Posted by maurice on November 3, 2006, 11:33 am


I don't think you'd have any trouble using Ditra over the 23/32"
subfloor. According to their technical data, you can use them over
19/32" tongue / groove subfloor, with 1/8" spacing between the boards.

I've put Ditra directly over the tongue/groove subfloor with no
problems. Some differential movement is okay under this product. Read
their installation manual to get more information:

http://www.schluter.com/english/products/2002/pdf/ditra_handbook.pdf

Make sure you follow their directions regarding the type of thinset to
use - you should use modified thinset to put the Ditra down, but
unmodified to lay the tile.

If you're using really small tiles, Ditra's not as good an option - I
would still use it, but I'd opt for a thicker substrate.

What's really nice about it is its ease of use - you can prepare 1000
square feet for tile in a few hours, and start tiling immediately if
you want.

I don't know much about hardibacker...

good luck!


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