The interior of a fast food restaurant is not well suited to establish a defensive position. One of the main problems with surviving in a fast food restaurant is the large windows. If a large number of zombies tries to break the windows, they eventually will, and barricading the windows won't do anything to slow that process. The roof is not a viable option, because there is only one way to get to the roof or get off of it, so if zombies find the entrance to the roof, your only choice is to jump off the roof and risk breaking a leg, or die of starvation/dehydration. Obviously, there may be food inside the restaurant, but whether or not it is safe to eat depends on how long it has been without proper refrigeration. As with all bases, procuring and storing supplies before hunkering down is crucial to survival.