Library › Typical Residential Floor Image 1 of 1 Typical Residential Floor $4.00 Cold form steel joists with double gypsum layer ceiling and plywood subfloor. UL System No. L 527. STC 50 - 54 Add To Cart Added! Cold form steel joists with double gypsum layer ceiling and plywood subfloor. UL System No. L 527. STC 50 - 54