Help with Hardibacker installation in bathroom

[Hue]

i'm about to use some for an upstairs bathroom project and i could use some input. here is the scenario:

the current subfloor is 2 inch thick toungue and groove car decking sitiing on 4x8 inch fir beam stringers. while this car decking is the floor on the second level of the house, it's the ceiling on the first level of the house. i have concerns about applying thinset directly to the car decking, even though it is the subfloor. should i put down plywood over the car decking, apply thinset to that, then apply the hardibacker? fyi, we are putting down 11 inch porcelain tile on the hardibacker. also, hardibacker suggest 5/8 inch plywood, but if i put plywood over the car decking could i use plywood slightly thinner?

thanks in advance for any help you can provide.

hue

[Randyj]

The instructions say to use the 5/8" plywood directly over the joists. If the t&g is sturdy enough and there is no deflection you should be okay. Read the directions thoroughly to choose what fits your situation best... I'm not sure what "car decking" is..new term to me. I would use screws that do not penetrate the decking in your situation.
Here's a link to installation instructions: http://www.holdenhumphrey.com/jh/hardibackerinstall.htm

[Hue]

I suppose my concern is mostly to do with applying thinset directly to the surface of the car decking (2x6 t&g boards... perhaps it's a northwest term). I know hardibacker says to install to subfloor. But when I took up the old floor there was one inch particle board down over the car decking. ON top of the particle board was vinyl. If you apply thinset directly to the car decking, won't this make any future attempts at changing out the flooring very difficult?

I can't use 5/8 plywood directly over the joists as the floor on the second story is the ceiling on the first story.

Thanks for the input,
Hue

[Randyj]

I doubt many people install ceramic tile with the intention of replacing it later. Funny how people are tho'.... they want it to stay down forever but fuss if it won't come up fun and easy when they want to remove it.

[Hue]

i won't be replacing it again, that's for sure guess i'm just thinking of how easy it was for me to take up the old flooring and hoping someone might thank me 50 years down the road if i make it easy for them.

hue

[DIY]

A few things i would like to suggest not particularly an answer here:
What shape is your car decking/t&g in? I pressume it is pine? Reason i ask Is you might consider renting or having a proffesional come in and sand and finish the t&g in a varnish,laquer etc. I have seen on several remodeling jobs nice floors of wood get covered up with tile or paint or sometimes carpet. Anyway to each there own,and if the look fits with rest of house etc. I just really like the look of raw finished wood especially in open type spaces like floors. It sounds to me to if you go with a thinner plywood over the t&g it would work just fine. ( 1/2,1/4 or 3/8). I don't really see the need for it other than providing a nice level flat surface to run floor tile on. Put a level on the t&g and look for high and low spots across the floor. Any unacceptable low spots are filled in with a floor leveling material (i have seen them add a little extra thin set in low spots that were not real bad) I do not know your experience with hardibacker,but my experience has been is it does not cut and snap like drywall,as there demonstration video shows and says even with the carbide scoring knife hardibacker has...lol However, hardibacker does seem a bit easier to work with than Dur-rock. Hopefully, the room you may need the backerboard in requires little to no cutting at all. Happy flooring

[Hue]

the T&G is fir. it has a finished side (ceiling below) and and unfinished, unbeveled side (subfloor above). no chance of keeping it as is. tile will be the thing. i've used hardibacker before. works well... but i do use a saw to cut it rather than trying to score and break. thanks for the input!

hue

[DIY]

Yep, i reached for my saw to after the scoring bit was useless and time consuming.

[jadnashua]

If you haven't already done anything yet, for your tile to have the best chance of survival, you need a layer of plywood (min of 3/8") over the planks. This is to aid the isolation - dimensional wood moves too much to survive, even with the cbu in between. Then, mortar under the cbu with it screwed down (or galvanized roofing nails) in the proper pattern.

[Hue]

i used half inch plywood things are moving along fine at this point. my thanks to all who contributed their input. it helps

hue