Charge Rolls and Vertical Distance

Bounced a rules question to the local discord, and though after much deliberating a tenuous conclusion was reached, no one could actually point to a written rule that confirmed one answer or the other, so I'm posting it here.

If infantry unit A is at ground level and wishes to charge unit B, which is 8.1" away and on the top floor of a 3" ruin, is the charge roll necessary 9< or 12