I'll do even on better here is some of the most played maps with the spawnkill limits marked out and described. If in doubt, check this page
And as for your exact case that happened, let's see: if your mortar shot landed indeed on the roof or inside, which would seem to be OK. However, the script takes position of damaged player, not landing spot of mortar round. So you might have aimed on roof but probably dealt dmg to someone in the protected area which is outside of the building (mortar has really big splash area). That's why you got warmed. It's better to err on side of caution, respect if you already have 2 warnings. Maybe next time consider very different spot away from spawn or different class altogether to avoid the 15 min kick I'm sure it was the script being triggered, it gives 15 min. Admins tend to show a bit of mercy if you're not serious offender ❤️