If there is a road or floor directly above the place you want to mine the mouse changes to a red x and you are not able to mine there. But mining first and building the road or floor afterwards works.
Steps to repuduce:
build a road or floor
mine one slice down
try to mine under the road
I expect that mining directly under a road or floor is allowed
you cant mine directly under a road or floor
Alpha 7 r 183