Steel Deck Types — Composite Floor Deck, Roof Deck, and Form Deck

Steel deck is the most common floor and roof substrate in structural steel buildings. It serves as formwork for concrete placement, contributes to composite action with beams, and acts as a structural diaphragm for lateral load transfer. The Steel Deck Institute (SDI) and individual manufacturers publish load tables, while AISC 360-22 Chapter I governs composite beam design with steel deck.

Deck categories and common designations

Composite floor deck

Composite floor deck has embossments (indentations) on the webs and/or ribs that mechanically interlock with the concrete, creating a composite slab. Common profiles:

Designation	Rib Depth	Rib Spacing	Typical Gage	Max Unshored Span	Typical Slab Depth
1.5 VL (Vulcraft) / 1.5 B (Verco)	1.5 in.	6 in. or 12 in.	22-18 ga	8-10 ft	4.5 - 6.5 in.
2.0 VL / 2.0 N	2.0 in.	6 in. or 12 in.	22-18 ga	9-12 ft	5.0 - 7.5 in.
3.0 N / 3.0 NI	3.0 in.	6 in. or 12 in.	22-16 ga	11-15 ft	6.5 - 8.5 in.

The 3 in. deep profile is used for longer spans (12-15 ft unshored) but requires more concrete fill. For composite beam design per AISC 360 Section I3.2c, the effective slab thickness above the top of the deck rib must be at least 2 in.

Roof deck

Roof deck carries roofing loads (membrane, insulation, ponding) and does not receive concrete fill (except in some industrial applications). Common types:

Type	Depth	Typical Gage	Max Span (3-span)	Application
Type B (WR)	1.5 in.	22-20 ga	6-8 ft	Light-duty, short span
Type F / 1.5 NR	1.5 in.	22-18 ga	7-9 ft	Standard roof deck
Type A / 3.0 NR	3.0 in.	22-18 ga	12-16 ft	Wide rib, long span
Deep deck (4.5, 6, 7.5 in.)	4.5-7.5 in.	18-14 ga	16-30+ ft	Industrial, heavy roofing

Form deck (non-composite)

Form deck acts only as permanent formwork for concrete — no composite action. It has no embossments and is typically 9/16 in. to 1 in. deep, used where thin concrete fill-over is placed on top of supporting beams. It contributes no structural capacity to the composite slab.

Diaphragm design with steel deck

Steel deck with proper attachment to the structural frame acts as a horizontal diaphragm that transfers lateral loads (wind, seismic) to the vertical bracing or moment frames. Diaphragm shear capacity depends on:

Deck profile and gage: Heavier gage and deeper ribs generally provide higher shear strength.
Attachment to frame: Puddle welds (arc spot welds) at 36/36 or 36/24 pattern, mechanical fasteners (Hilti, Pneutek), or powder-actuated pins. SDI DDM04 provides capacities.
Side-lap connections: Button punches, screws, or welds at deck-to-deck overlaps.

Worked example — diaphragm shear capacity

Given: 1.5 in., 20 gage composite deck. Puddle welds at 36/36 pattern (weld every 36 in. along support beams, 36 in. side-lap spacing). Deck span = 8 ft. A653 SS Gr. 33 deck material.

From SDI DDM04 Table (or manufacturer tables):

Nominal diaphragm shear strength: S_n = 620 plf
phi = 0.65 (SDI ASD factor = 2.35 for welded connections)
phi _ S_n = 0.65 _ 620 = 403 plf (LRFD)

For a building bay of 30 ft x 30 ft with 8 kips seismic diaphragm shear at the bay edge:

Required shear capacity: v_u = 8,000 / 30 = 267 plf
403 plf > 267 plf (OK, utilization = 66%)

If the demand exceeds capacity, upgrade to 18 gage deck, tighter weld spacing (36/24), or add side-lap screws.

Fire rating considerations

Assembly	Rating	Concrete Thickness Above Deck	Reference
1.5 in. composite deck + normal weight concrete	1 hour	3.5 in. above top of rib	UL D916
1.5 in. composite deck + normal weight concrete	2 hour	4.5 in. above top of rib	UL D916
2.0 in. composite deck + lightweight concrete	2 hour	3.25 in. above top of rib	UL D925
Roof deck + SFRM spray	1 hour	N/A (spray thickness per UL)	Various UL assemblies

Code comparison

Aspect	AISC / SDI (US)	AS 2327 (Australia)	EN 1994-1-1 (Eurocode 4)	CSA S16 (Canada)
Composite slab standard	AISC 360 Ch. I + SDI C-2017	AS 2327 composite structures	EN 1994-1-1 + EN 1993-1-3	CSA S16 Cl. 17
Shear stud placement	One stud per rib minimum (AISC I8.2d)	Per AS 2327 Sect. 8	Per EN 1994-1-1 Sect. 6.6.5	Per CSA S16 Cl. 17.7
Diaphragm design	SDI DDM04	No specific standard	EN 1993-1-3 Annex	CSA S136
Deck material	ASTM A653 SS Gr. 33/40/50	AS 1397 G300/G550	EN 10346 S320GD+	CSA G40.21 230W

Key clause references

AISC 360-22 Section I3.2c — Composite beam with steel deck, concrete above rib thickness
AISC 360-22 Section I8.2d — Shear stud placement in deck ribs
SDI C-2017 — Composite Steel Floor Deck design standard
SDI DDM04 — Diaphragm Design Manual (shear capacities)
SDI RD — Roof Deck design standard
ASCE 7-22 Section 12.10 — Diaphragm design forces

Topic-specific pitfalls

Placing shear studs in every rib vs. every other rib without checking AISC limits — AISC 360 Section I8.2d limits the stud diameter to 3/4 in. for deck with rib width less than the stud diameter. Also, the stud reduction factor Q_n applies when the e_mid-ht / h_r ratio is unfavorable (narrow rib with off-center stud).
Ignoring deck orientation for diaphragm action — deck ribs running perpendicular to the supporting beam create strong-direction diaphragm action parallel to the ribs. Rotating deck orientation 90 degrees significantly changes the diaphragm capacity.
Specifying unshored span beyond manufacturer limits — unshored construction requires the bare deck (no concrete yet) to support its own weight plus wet concrete weight. Exceeding the unshored span limit causes visible sag that becomes locked into the slab.
Neglecting ponding on roof deck — flat or low-slope roof deck without adequate stiffness can accumulate rainwater in deflection-created ponds. AISC 360 Appendix 2 requires a ponding check when the roof slope is less than 1/4 in. per foot.

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Related references

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.