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Home / Assemblies / UL D121
UL D1214-hr Fire RatingFloor/CeilingLoad-BearingSTC 58
Steel JoistMulti-Layer GypsumFiberglass BattHigh STCDesign

Steel Joist Floor/Ceiling — 4 layers 5/8" Type X gypsum board

UL-listed 4-hour fire-rated steel joist floor/ceiling assembly. Open-web steel joists at 24" o.c. with R-19 fiberglass batt, 4 layers of 5/8" Type X gypsum board ceiling.

Cross-Section Diagram

12345678FLOOR/CEILING ASSEMBLYSECTION · NOT TO SCALE8 LAYERSsee legend for ① ② ③
Legend
1Floor covering(varies)
23" concrete on steel deck(3")
3Open-web steel joists at 24" o.c.(14.0")
4R-19 fiberglass batt
55/8" Type X gypsum board(5/8")
65/8" Type X gypsum board(5/8")
75/8" Type X gypsum board(5/8")
85/8" Type X gypsum board(5/8")

Specifications

Fire Rating
4-Hour
STC Rating
STC 58
Load Bearing
Yes
Total Thickness
18"
Source / Publisher
UL (Underwriters Laboratories)
Design Number
D121
Assembly Type
Floor/Ceiling
Test Standard
UL 263
Construction Group
Non-Combustible
Number of Layers
8 layers

Applicable Codes

IBC 2024 Table 722.1IBC 2024 §722IBC 2021 §722

Construction Summary

Open-web steel joists at 24" o.c. with R-19 fiberglass batt, 4 layers of 5/8" Type X gypsum board ceiling

Material Layers

LayerMaterialThickness
1Floor coveringvaries
23" concrete on steel deck3"
3Open-web steel joists at 24" o.c.14.0"
4R-19 fiberglass batt—
55/8" Type X gypsum board5/8"
65/8" Type X gypsum board5/8"
75/8" Type X gypsum board5/8"
85/8" Type X gypsum board5/8"

Important Notice

This assembly listing is for reference only. Always verify against the current UL Product iQ database before specifying in construction documents. Material substitutions may void the fire rating. Consult with a qualified fire protection engineer for project-specific requirements.