Flooring Tile - sub floor requirements for tile

I've seen various recommendations for subfloor, but what I've seen never fits my situation. I want to tile my bathroom floor and my situation is this:

2x 10 joists, 16 inches apart
1.5 inch plank tongue and groove subfloor.
.5 inch plywood that I put down

I plan on putting .25 inch hardibacker down over the plywood and i want to put the thinset down, but i'm concerned about the plywood absobing moisture. I was thinking of putting a moisture barrier over the plywood before the thinset, then the backer board and then screw that down. Apparently the thinset is just to fill the gaps, and it really doesn't adhere to the wood anyway (true or false?), so why not the barrier? I've seen some people say to polyurethane the plywood first. the plywood is not pressure treated, so it will absorb moisture. what's the best thing to do to hopefully keep what I have?

2x 10 joists, 16 inches apart
1.5 inch plank tongue and groove subfloor.

Good so far.

.5 inch plywood that I put down

Are you sure? Half inch plywood for a subfloor doesn't meet any building code I'm aware of. The minimum would be 5/8".

I plan on putting .25 inch hardibacker down over the plywood

A minimum floor thickness of 1-1/8" is required for ceramic tile, more for stone tile.

and i want to put the thinset down, but i'm concerned about the plywood absobing moisture.

Not to worry. The plywood should be exterior grade however.

I was thinking of putting a moisture barrier over the plywood before the thinset, then the backer board and then screw that down.

Normally wouldn't be necessary.

Apparently the thinset is just to fill the gaps, and it really doesn't adhere to the wood anyway (true or false?),

True.

so why not the barrier? I've seen some people say to polyurethane the plywood first.

Nope!

the plywood is not pressure treated,

And it shouldn't be. Never want treated plywood or any treated wood in a tile installation.

so it will absorb moisture.

Pressure treated plywood probably wouldn't absord moisture, you wouldn't want it to anyway.

what's the best thing to do to hopefully keep what I have?

Ignore all the rules and throw fate to the wind.

i've seen different recommendations for sub floor, which I'm defining as the flooring above the joist, and i'm sure the 1.5 inch tongue and groove plank that i have is up to code. I thought the 1 1/8 was the thickness that's required on top of the joist excluding the plywood and backerboard. Including the hardiboard, i would have 2.25 inches over the joist. are you saying I should have 1.125 more than the 1.5 inch plank, for a total of 2 5/8 over the joist? if my .5 inch plywood is subpar, I can put more down. please clarify.

Hi Fletcher,

First of all your CBU does not count when measuring required subfloor thickness. So I guess you have 1.5x3/4" subfloor planks, with 1/2" plywood underlayment on top? Is this right?

If that is correct and in good condition, you have plenty of subfloor thickness. James Hardie says that 5/8" plywood is good enough. Most tile pros think a single 5/8" plywood sub is scary to say the least. It probably works most of the time, but there are a few *'s with that recommendation. Anyway, it's the bare minimum and we don't like bare minimums.:wall:

Personally I have done hundreds of floors over single 3/4" and that is plenty at 16" o.c. It is more than required. The other requirement is to make sure your joists are stiff enough. Please get back with this info; type, size, spacing and if possible species and grade of the joists. ALSO need to know the unsupported span of those 2x10's.

Jaz

First, thanks for the time and help. Actually the original subfloor is 1.5 inch thick by 6 inch wide tongue and groove plank over the joists. I don't know the length because the edges go under the walls. it's very solid and in great shape and there was tile in the bathroom prior which I removed (it was on a morter base). I don't know the span of the joists (at least 10 ft), but the room under is very large, 20 x 20 or more. I'm doing a bathroom and the floor is maybe 45 sq feet tops and I'm using 1x1 tile for the floor. I put down .5 plywood over the planks, which gives me 2 inches of wood on top of the joist. If I need more plywood, I can do it if I have to, but I'd like to spare myself the time and expense if what I have is satisfactory. This is where I'm confused: the 5/8 requirement for hardi; is that the total wood above the joist, or the wood that should be added to the original 1.5 inch plank subfloor? Thanks again.

My mistake, when I saw the 1/2" ply comment I overlooked the t&g planks. Add the Hardi for "bite" and you'll be fine. I doubt the overall span will have anything to do with that small of an area.:)