Identify these conditions before the incident with good and complete pre-fire planning. Or consider them as you do your own size-up. How much fire is in the building and what effects is it having? If you do not know for sure whether a roof is trussed or not use caution. Perform vertical ventilation from the ends at the highest point. Stay close to bearing walls while inside. Trusses have a tendency to fail in v-shaped collapses. Remember wooden trusses always, ALWAYS, have a gusset exactly in the middle of the building on the top and bottom chord. This configuration fails most often. Concrete can and will fail with enough heat underneath. If it is too hot to kneel on the floor there is a good possibility there is fire under you. Unless you have definite rescues to be made it may be wise to operate outside of this area. Think as you go in. Gravity is waiting to grab you at the first opportunity. The trick is don't be above or below a failure. Stay Safe.