NW conc. topping over plywood diaphragm/floor

[AaronMcD]

Hi, the internet doesn't seem to have info on this so I thought I'd ask here.

Should concrete be poured directly over plywood or should there be something between for protection? This is single family interior, and the plywood and engineered wood joists are structural. The concrete encases heating tubes and will be the permanent floor finish. I see online a lot of very thin concrete toppings, but since this is going to be the finish and we don't want (noticeable) cracking we are using 4" thick with reinf. The mech eng wants normal weight which is fine with me. We don't currently show any tar paper or other membrane or special treated plywood.

 

[XR250]

I put down 1" of foam insulation under mine. Keeps the heat going up. I would at least put some sort of bond breaker beneath.

 

[jayrod12]

Agree with XR. if there's room for it, 1" of rigid wouldn't be a terrible idea and would act as both a bond breaker and will help with heat losses.