Since an HE grenade's damage drops off completely within 8m, since I see your main purpose is to blow up the roof, i'll just apply the damage directly to the roof and say you blue it up right near the roof. I'll say it was an explosive in contact with the roof, so it's the DV x 2 vs. the Armor 6+ Structure 4. Every square meter has a condition monitor of 4. Since you guys are about 5m from the blast, 6P is left over, and the roof soaked 4 of that, so that's 2P left over for each of you, which i'll just ignore, so don't even roll to soak that.