The Tesseract was immovable. When Tony struck it with his attack, it didnt budget at all. This was done to show that that shit was well protected, but it also tells us that that shit ain't moving from that spot. It didnt transfer Tony attack into the roof. Its locked in place.
It transferred the attack back at Tony. There's no rule that the forcefield has to exert force in the direction that it was sent, into the rooftop. Just a bubbley energy field doing bubbley energy field stuff.
Also, Tesseract was floating in the center of a machine, and the machine was either powering the Tesseract or the Tesseract powering the machine, either way it was turned off by putting the Scepter to block the energy beam thingy (the Scepter wasn't a proper key or anything, Mr. Science Guy just said it "might" be used, because it could penetrate the bubble because deus ex). The forcefield itself was centered around the Tesseract itself (the machine had uncovered parts).
The machine itself is resting on the roof. While it
might have resulted in it just floating there
and helping the machine float somehow because of it being anchored into the cube (the whole setup was "self-powered", no longer needing energy from the ARC, but the Tesseract wasn't doing the portaling by itself) it isn't a foregone conclusion. Plus all the Frame of Reference stuff that Starver said, but those are usually handwaved in comic physics. (there is even a superhero that travels by standing "still" relative to Earth's rotation, but somehow never actually leaves Earth even tho Earth is moving. So, somehow anchored to the center of mass or something?)