Another tiling sub-floor question

Hi all,
I'm new to the forum but not to the DIY lifestyle. I've been reading a lot before posting this and can't find a direct answer to my question. I've been able to piece together info from multiple postings but I'm not comfortable with what I'm putting together in my head.

My project is fairly large in scope. I'm adding a door between an existing bathroom and bedroom to make it like a master suite. Bedroom was carpet, bathroom was linoleum. This will be replaced with ceramic or porcelain tile in both areas with no transition. Tearing out the old tub and shower and making a curb-less walk-in shower. This is all being done for direct handicap access.

My existing floor consists 3/4" OSB only, over 9" joists. According to the load calculator I'm okay with my unsupported span but I am very close to the minimum.

I was planning to use 1/4" Hardibacker as the UL for the tile

Now finally for my direct question. Do I need to add a layer of plywood between the OSB and Hardi to make this right? If so what is the MIN thickness of plywood I can get away with?

 

What is your spacing, and span of your floor joists?

 

16" center spacing with 12.5' span. I don't know if this matters but the OSB is glued with construction adhesive to every joist

 

how big will the shower pan area be?. curbless showers need to have recesses joists, blocked between joists and reinforced

 

I shoot for a minimum 1 1/4" subfloor before the addition of hardie. However, in order to create your slope in your shower, I would add and additional 3/4" which will provide you with enough height for 1/4" drop over 3 feet if going with a trench style drain. Under the shower pan itself, just the single layer.

 

about 3x5. I'm definitely cutting out for some depth on this and adding support and blocking it all. Thanks