Yeah, I'm not aerospace engineer, but couldn't you just put the roof like 10 cm above the rotors so it has enough air flow?

To generate lift. It's most certainly not to shield the helicopter from attacks from above.

Exactly. How do you imagine rotors generating lift when enclosed by a roof?

Something as basic as a wire mesh would fit the bill.

It probably wouldn't, when the rotor has incredible suction and the tangle wire is very thin. I would certainly expect an attacker using monofilament and a defender using several-mm mesh to go the way of the attacker. Too much protection and you lose the airflow.

I must immediately tape speaker mesh to my AR drone! FOR SCIENCE!

Not that.