We entered the first floor of a residential structure and found floor to be intact, smoke conditions were light, and visibility was about 15 feet in front. While performing a search, I noticed the floor felt hot through my PPE. I immediately notified the other firefighter I was with, and he agreed with me. We advised command and exited the structure. Approximately 15 minutes into the incident it was determined that the fire originated in the crawl space, and the floor on the first floor began to sag from weight of furniture.