The study explores an original idea that responds to the urgent need to reduce the detrimental environmental impacts of load-bearing floor construction in new buildings by reusing saw-cut reinforced concrete (RC) pieces salvaged from soon-to-be demolished structures. Cutting and reusing large RC pieces rather than crushing them to rubble is an untapped emerging circular construction method with a high potential for reducing waste generation, natural resource consumption, and upfront greenhouse gas emissions. Through an iterative design and analytical process, the study demonstrates how discarded cast-in-place RC floors can be cut and reused to build new low-carbon, little-extractive, load-bearing building floors. The study provides two new floor design solutions that valorise frequently discarded construction components (reinforced concrete slabs and steel profiles), combining construction technologies already used by the industry. The parametric design of 20′280 combinations of donor and receiver structures and their environmental analysis through Life-Cycle Assessment show that the new floor systems have shallow detrimental environmental impacts, with a reduction of upfront greenhouse gas emissions averaging 80 % compared to conventional practice. Floor-system solutions as low as 5 kgCO2e/m2 have been obtained. Structural assessments additionally show that flat slabs that are currently demolished meet the structural requirements at the preliminary design stage for reuse in new office or housing buildings. In particular, thanks to mandatory minimum reinforcement, 18-cm thick or thicker flat slabs built in Switzerland after 1956 and spanning up to 4 m are expected to be technically reusable as-is over their entire span. Overall, this study sets up a new benchmark for innovative floor systems with minimum environmental impacts and calls for considering soon-to-be demolished RC structures as mines of valuable construction components.