Need help with my hearthpad :plywood and durock leveling

So I am building a hearths for a woodstove .

I have the location and clearances all figured out.

Under the stove I need .5 r. I chose two layers of durock on a plywood pad. That gives me .78 r so I am good.

The problem is that the plywood underlayment is not exactly level. There is about 1/4 inch of "play" between the edge of the plywood and the exsisting wood floor.

This means when you walk on the edge of the plywood it moves. I am afraid that once the durock and tiles are on, the thing will either flex and crack tiles when walked on or be wobbly.

Any suggestions?

Bumped because I changed the title and question. Hopefully I can get some help.

The last time I installed a stove this place was awesome.

I think that the plywood is slightly bowed.

Screwing into the floor is Not an option. I do not want to tear up the wood floor. I went to home depot and all the plywood is a little bowed. Looks like I will be building a frame. Oh Well.

I wanted a raised hearth anyway. Wife wanted a lower profile.

Tried her way now I will do it my way.

Any advice on the proper thinset for the Durock?

Went ahead and built a frame.

Here is the frame.





And the Durock mortared down and taped.



This is my first experience with with mortar.

Yes. 3/4 inch plywood base. It was the same piece that was slightly bowed. The frame totally fixed that issue. I will put one more piece of Durock down over the current one to give a r = .78 value. It will have the seems all offset by a good bit.

Ok so a bit of a snag. For my top layer of Durock, does it need to be screwed into the plywood or can it just be screwed into the base layer of Durock. I ask because it seems the screws going all the way into the plywood would allow heat to transfer into the plywood. I am planning to use thinset under the top layer.

Final layer of Duroc down.