Good thinking. You lock the door, even though you don't think the zombies could get in downstairs. You set up one of the guns on the edge of the roof and eliminate all of your hindering obstacles outside the building.
You rush down the stairs, keeping your guard up in case any got in. You are lucky, none did. You have your bag of guns and ammo in hand, and you haul your ass quickly to your car. You quickly take off.
You have survived so far, but how much longer do you think you'll stay lucky? You've got to go somewhere, where you can stay out of reach of the zombies and have some food to thrive on until this is over. So you head over to...