Effect of Insulation Boards on the Flexural Stiffness of Cold-Formed Steel Deck Systems Under Downward Pressure

Cold-formed steel (CFS) roof decks are widely used in modern building systems, often as part of multi-layer assemblies where mechanically fastened insulation boards alter the flexural stiffness. This study experimentally investigates the influence of a nailbase insulation board (50.8-mm (2-in.) rigid insulation with a 12.7-mm (0.5-in.) OSB top layer) on the vertical deflection and flexural stiffness of a three-equal-span, 0.76-mm (22-gauge) B-deck assembly. Full-scale vacuum box tests measuring 3.66 m × 7.32 m (12 ft × 24 ft) were conducted under uniform downward vertical pressure across three setups: a bare-deck baseline, a minimum screw pattern (16 screws per panel), and a maximum screw pattern (32 screws per panel). Vertical deflections and pressures were monitored using an OptiTrack motion-capture system and pressure transducers. Experimental results showed that adding OSB insulation boards significantly increases system flexural stiffness. Relative to the bare deck, the 16-screw pattern reduced vertical deflections by 5–18%, while doubling the fastener density (32 screws) only decreased deflections by an additional 0.2–7%, confirming that the minimum pattern captures the majority of the stiffness contribution. To evaluate these experimental findings analytically and verify the physical data limits, a traditional 2D continuous finite element line model was developed in MASTAN2. Numerical analysis sets were created by varying cross-sectional properties to establish bounds representing non-composite and fully composite behavior. The analytical 2D line-element model successfully serves as a robust engineering check, demonstrating that the full-scale experimental vertical serviceability response aligns reasonably within expected structural bounds.