They can be installed at reasonable intervals, high enough to permit maximum coverage by their illumination. Cover vulnerable bulbs with metal screens. Patrol officers should note units that are not working. Such units should be repaired or replaced the following day. Roofs are too often ripe to be cut through, bashed through, or otherwise penetrated.
Skylights are particularly vulnerable.
The rooftop outlets of HVAC systems are other possible vulnerable points, especially if they can be removed and laid aside. Keep the parapet around the rim of your roof low. The less there is to hide behind, the better. Consider using a surveillance camera to check the roof, or use a light alarm system in which rooftop floodlights go on if someone walks on the roof. Alarm your interior ceiling so that if anyone breaks through, your guards are notified.