http://en.m.wikipedia.org/wiki/Topping_out
In building construction, topping out (sometimes referred to as topping off) is a builders' rite traditionally held when the last beam (or its equivalent) is placed atop a structure during its erection.
Nowadays the ceremony is often parlayed into a media event for public relations purposes.[1]
After topping out, numerous elements of construction remain, including interior finish and most mechanical, electrical, and plumbing systems.
mean that structurally, there should be no more structural beams needed