I have concrete slab basement floor with 3" cast iron sewer pipe sitting about 3" below the surface of concrete. To access this pipe I have a hole about 6" wide around this pipe (See picture). So if I try to screw flange over it the screws would end up in this hole or two close to it to be reliable.
I installed 3/4" Dricore panel subfloor and now plan to install some kind of flooring around 5/16" on top of it. Assuming that I put few tapcons to hold subfloor panel in place, would it be sufficient to attach flange with short 1" screws that would only embed in the subfloor panel?