Underlay Help!! [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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10-27-2009, 07:44 AM
I need help deciding what type of underlay to use in my kitchen. I have a large area that I need tiled about 14'x20' the joists are on 16"centers with 5/8 plywood subfloor screwed and I believe glued down. There is also a large patio door at one end it looks like it was leaking in the past because the subfloor is stained but in good condition around the window. I will be resealing the window to make sure the leak has been addressed.
Now for the underlay, I keep hearing different things as follows:
-3/8 plywood screwed down 6" in the field and 3-4" on perimeter.
-3/8 plywood screwed and glued down
-3/8 fiberboard screwed and thin-set with taped seams
-Ditra straight on the subfloor.

They guy's at the tile place recommend the fiberboard it apparently looks like drywall but instead of gypsum it’s a fiberglass material “its not concrete board”. Apparently more stable then plywood. I have not seen any information about this product online so I only have the word of the tile store.

I really want this floor to last any suggestions would really help.

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10-27-2009, 07:56 AM

Welcome to the forum!

You happen to have you joist size and their max. free, unsupported span factored in as well?

And what kinda tile you planning to use? ceramic? stone?

w/ 5/8" existing, you probably want to add 1/2" new ply (total of 1 1/8" min = goodness) and then a suitable substrate, like 1/4" Hardie or Ditra.

You don't wanna typically tile right to the plywood. Might work, but isn't very forgiving.

Do count on issues w/ that door, should any leakage or condensation issues continue to cause moisture to reach the plywood near it. If it gets wet and swells, the tile and grout will fail in short order there :(

That all answering your initial q's? hope it helps! :)

10-27-2009, 01:04 PM
The subfloor sits on 2X8' and the free span is about 14' X 10 to 12' i also found the site for the fiberboard it is actually called fiberock

the tile is going to be 18" ceramic

10-27-2009, 01:13 PM
...and the free span is about 14' X 10 to 12'What does this mean? What we need is the distance between whatever structure that supports the 2x8 joists. It's not the room size. And, with your joists, you do need to be accurate in your measurement. Measure inside to inside between supports.

10-27-2009, 02:25 PM
after the joist situation is figured out i would go with hardi or ditra, fiberock from all comparisons i've heard is not as good. i really only use hardi though.

10-27-2009, 02:41 PM
Assuming joists meet the deflection criteria I would add 1/2" ply, properly laid and fastened, followed by Ditra then tile.