You should check your local building code for the required thickness of non-combustible materials on the hearth. On a fireplace hearth the requirement is typically 1 inch. It sounds like 2 1/2" may be more than is required. Assuming that is what you want, I'm curious as to why you would do it with two thicknesses of the stone. Probably the best way is to do a bed of mortar (to whatever thickness you want) directly on top of the plywood and stick your stones into that (or let the mortar harden and use thinset to attach the stone). When you mix the mortar, use fire clay which makes the mortar more heat resistant and less likely to develop cracks from the heat. Also, why are you using pressure treated lumber? That's for when the wood is in contact with the ground. I'd be concerned about using it indoors, especially when it mught be exposed to some heat - it's got some pretty noxious chemicals in it.

Thanks Ventupete. I wasn't sure of the thickness required, that's why the two layers of stone. The cottage is on a slab right on the riverbank, so it is always damp inside. I thought it best to use pressure treated in that application. I made a hearth for the pellet stove at home in the same fashion, but tiled it. There's no smell from the wood. The reason I didn't want to use the mortar was that the cottage occassionaly gets flooded (every ten years or so). I wanted something that i could take apart easily if it got wet or ruined.
I'm not familiar with wood stoves. Do you think the durarock and 1 layer of store would be enough?

Got it. To be on the safe side (and give yourself some peace of mind), I'd put down two layers of the 1/2" cement board under the stone. I'd also mix the mortar you use to set and grout the stone with fire clay as I mentioned above. It will keep the mortar from crumbling due to the extreme temperature swings. Another option would be to do away with the wood entirely, form it out and just pour a concrete pad. That way there would be nothing to ruin if it got flooded. If you are concerned about eventually removing it, you could put a rubber membrane under the pad so that it would be easy to break up and remove at a later date. Just some thoughts. Good luck.

Thanks Ventupete. I like the fire clay idea.

FYI, some manufacturers call it "mortar clay" instead of "fire clay." The bag will have recommended proportions for the mortar mix. I've used it to install fire brick inside several fireplace boxes I've built, and have never had a problem with it failing.

I'll have to pick some up. Which raises another point for me.
I have a lot of that thinset left from the first heath I built. Could I mix that mortar clay in with the thinset to set the stone?
Thanks again Ventupete. I'm learning quite a bit from you.

Not sure how the clay might affect the thinset. Thinset is basically a type of mortar that has glue mixed in. I wouldn't put the clay into it as it might weaken it, especially since it's under the stone and won't be as directly affected by the heat. If the thinset you have isn't fortified, I'd get some liquid fortifier and use it when you mix up the thinset - it gives it a lot more strength. Use the clay for the mortar grout. Happy Holidays.

Thanks again Ventupete. same to you.