First-person view (FPV) drones carrying repurposed shaped-charge warheads have inverted the cost-of-attrition logic of mechanised warfare, and crews have responded with improvised roof grids, the so-called 'cope cages', whose engineering value remains contested. This article asks a narrow question: under what geometric conditions does a roof-mounted, modular cage measurably reduce the perforation probability of a top-attack FPV-delivered shaped charge, and when does it fail to do so? The study integrates the grid-interaction probability model developed for slat armour against the RPG-7 threat with the empirically anchored standoff–penetration response of a representative warhead and with established jet-formation and penetration mechanics. Its original contribution is the Top-Attack Cage Effectiveness (TACE) index, a dimensionless composite that multiplies the warhead-disruption probability of a grid by the residual-penetration margin of any jet that survives initiation at the cage standoff, evaluated against the roof protection threshold. The index makes explicit a counter-intuitive result: because a shaped charge reaches peak penetration at a finite optimal standoff, about 317 mm against 278 mm at contact for the modelled warhead, a cage that merely adds standoff without disrupting the warhead can raise penetration rather than lower it. Illustrative computation across grid pitches of 30–60 mm and roof gaps of 250–450 mm shows TACE values ranging from below 0.30 for poorly placed coarse grids to above 0.80 for fine, disruption-dominated configurations. The analysis concludes that cope cages are defensible as the outermost layer of a graduated counter-UAS survivability architecture rather than as standalone protection, and that their design must be driven by warheaddisruption geometry, not standoff alone. All numerical TACE values are model-derived and require experimental validation before fielding.