She's got the zombies *and* the bad guys trapped in a building. The heroine has had a change of heart and doesn't want the bad guys to become zombies, so she wants to instead destroy the building by blowing up some cars in the basement carpark, or alternately just siphoning the petrol onto the carpark floor and exploding that. How many litres/Gallons of petrol detonated 2 floors below might give her the destruction of at least 50 square meters of real estate, as in a small block with shops on the ground floor and offices above. The bad guys and zombies are trapped on the upper floor.
I've read of a couple of instances - This one in Prahran, Melbourne in 1968 had "several gallons" of petrol, and would have destroyed the office and set the building alight. This one in Sydney last year was only 10 litres of petrol, which doesn't seem like very much. Whereas this idea of the Fuel Air bomb didn't need much more than a fuel can's worth - or so he said. Yet I feel that wouldn't be enough.
Searches done on "blowing up buildings with petrol", "blowing up buildings with gasoline" and the old favourite "why am I doing this for my friend?"
Ideas? Scientific studies? Anonymous confessions of similar instances?