If there is a unit in the building you are targetting the unit (if multiple units in the building, you must choose one). If the building is empty, you can target the building.
Direct fire you roll to hit as normal, ignore cover – all other normal to hit modifiers apply.
Indirect fire you follow the usual to hit ranging in process – if you score a hit, you roll to see which floor it goes off, starting with the uppermost floor – usually that will be the roof (4,5, or 6 it goes off on the current floor – 1,2, or 3 try again for the next floor down). If it reaches the ground floor, it explodes there.
Assuming you score a hit, roll for the number of hits scored using the “Hits vs targets in buildings” column of the HE chart, p70. This number is the hits that can bring down a building – you need 10 or more to bring the building down, which is why you need at least 2D6. Target unit being down halves the hits they suffer but doesn’t halve the hits when working out if the building is destroyed.
Assuming the building hasn’t been destroyed (which kills units inside) roll to damage for all hits suffered – no extra protection vs HE, normal pen for the HE type applies.
If I’ve missed something, I’m sure someone will fill the gaps.
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