It was 04:45 on a somewhat quiet night when Squad 34 was alerted for a dwelling fire. First arriving units found heavy fire in the basement of a middle of the row townhouse. Upon our arrival, command instructed us to check for extension in the Delta exposure then set up lighting on side Charlie of the invloved structure. When these tasks had been completed, we reported back to command for an additional task. The three of us from SQ34 were sent to the second floor of the involved structure to check for any extension. After masking up, we entered the structure and headed up to complete our assignment. As the three of us neared the top of the steps, my partners foot fell through the weakened floor board. He yelled back to inform us then attempted to step over the hole to the next step. In doing so, the steps he was standing on gave way and he began to fall down through the hole. I immediately grabbed his SCBA straps in an attempt to hold him back. Within a few seconds, the steps I was standing on gave way and we both fell down through the first floor and into the basement. Interior attack crews in the basement, who had just knocked the last bit of fire, immediately radioed to command informing them of the collapse and that we were both ok. After being escorted out by the crew, command immediately called a PAR because the location of our third crew member was unknown. He eventually met us on side Alpha unscathed. All in all, neither of us sustained any serious injuries, and we remained on scene for the duration. It was later determined the steps sustained no direct flame impingement, and that rotted floor boards were the cause of the collapse.
More caution must be exercised when ascending a stairwell which may have been compromised by a heavy fire from below. If a partial collapse is evident or pending, command should be notified immediately and an alternative route to safely reach your destination may be needed.