I lived in my gg great grandfathers house his slaves built in 1856. It had both sleepers and sills under the 4'' white poplar flooring . The 'sleepers' were 20 ft. hewed logs [2 sides of these logs was shaved down flat]. One flat side of the log rested on large sand rocks.[ 4 sand rocks under each log]. The sills were the same size logs , hewed flat on all four sides, the sills rested on the sleepers, and the floor was nailed to the top side of the sills.