What is the difference between composite floor deck and roof deck?

Composite floor deck and roof deck serve fundamentally different structural functions. Composite floor deck (1.5VL, 2.0N, 3.0N profiles from manufacturers like Vulcraft and Verco) has embossments (indentations) rolled into the webs and ribs that mechanically interlock with the concrete fill, creating a composite slab that acts together with the steel beams. The embossments transfer horizontal shear between the concrete and the deck, making the deck part of the structural system. Composite deck ribs are typically 1.5 to 3.0 inches deep, with 6- or 12-inch rib spacing. 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 inches. Roof deck, on the other hand, carries roofing loads only (membrane, insulation, ponding) and does not receive concrete fill. Common roof deck types include Type B (WR, 1.5 in deep), Type F (1.5 NR), and Type A (3.0 NR). Roof deck has no embossments since composite action is not required. Roof deck spans are typically shorter than composite deck for the same gage because there is no concrete to stiffen the deck. Form deck (non-composite) is a third type that acts only as permanent formwork for concrete — it has no embossments, contributes no structural capacity to the slab after concrete curing, and is typically shallow (9/16 to 1 inch deep). Cellular deck adds enclosed raceways for MEP distribution, with or without acoustic perforations.

How are unshored span limits determined for steel deck?

Unshored span limits define the maximum distance steel deck can span during construction, before the concrete fill cures. The deck alone must support the wet concrete weight (typically 40-50 psf for normal weight concrete at 3-4 inches above the deck ribs), the deck self-weight, and a uniform construction live load of 20 psf per SDI and OSHA requirements. SDI publishes span tables for each deck profile and gage combination for single-span (1-Span), double-span (2-Span), and triple-span (3-Span) conditions. Multi-span conditions significantly increase the allowable unshored span because of moment redistribution across supports — a 3-span condition typically offers 25-35% longer spans than a simple span. For example, a 22 gage 1.5VL composite deck has max unshored spans of 5 ft 6 in (1-span), 6 ft 6 in (2-span), and 7 ft 0 in (3-span). Going to 20 gage increases these to 6 ft 6 in, 7 ft 8 in, and 8 ft 6 in respectively. For a deeper 3.0N profile at 18 gage, max unshored spans reach approximately 13 ft (1-span), 15 ft (2-span), and 16 ft (3-span). Beyond the unshored span limit, temporary shoring is required during concrete placement. Designers typically select deck gage to avoid shoring, since shoring adds significant cost and schedule delay. The construction load governs 90% of deck selections — composite strength after curing usually has ample capacity because the hardened concrete provides compression resistance.

What gage steel deck should I specify for typical office floor framing?

For typical office building floor framing with beams at 8 to 10 ft spacing, the most common specification is 20 gage 2.0VL or 2.0N composite deck (2.0 inch rib depth) with 3.25 to 3.5 inches of normal weight concrete above the top of the deck rib (total slab depth 5.25 to 5.5 inches). At 8 ft beam spacing, 20 gage 2.0N deck handles the wet concrete plus 20 psf construction live load in a 3-span condition with margin to spare. At 10 ft spacing, 18 gage 2.0N provides adequate unshored capacity. For 12 ft spacing, a 3.0N profile at 18 or 16 gage is required to avoid shoring. The 3.0N profile at 12 ft spacing with 3.5 inches of cover weighs approximately 56 psf for the concrete plus 3 psf for the deck. Gage selection also affects diaphragm shear capacity: 20 gage 1.5VL with 3.25 inch LW concrete over 4.5 inch total depth and 36/4 puddle weld pattern provides approximately 350-450 plf diaphragm shear. If higher diaphragm demand exists (e.g., long narrow buildings, high seismic), move to 18 gage or closer weld spacing. Attachment to beams is typically by 3/4 inch puddle (arc spot) welds through the deck to the beam flange, or by powder-actuated fasteners. SDI requires a minimum of 4 welds per sheet width at end supports and 2 welds per sheet at intermediate supports. For composite deck, the minimum end bearing on steel is 1.5 inches. For roof deck, specify Type B 22 gage for spans up to 6 ft (3-span), Type F 22 gage for 7-8 ft spans, and Type A 22 gage for 9-12 ft spans. Industrial roofs with heavy equipment should use deep deck profiles (4.5-7.5 in) at 16-18 gage.

How do cellular deck and composite deck differ for MEP coordination?

Cellular deck integrates enclosed metal raceways (cells) within the deck profile for electrical, data, and HVAC distribution, while composite deck is a solid profile that requires separate ceiling plenums, cable trays, or raised access flooring for MEP services. Cellular deck cells run parallel to the deck span and are typically 3 to 6 inches wide (narrow cell) for power and data, or 6 to 12 inches wide (wide cell) for larger cable bundles and air distribution. The cells are formed by a flat bottom plate welded to the underside of the deck ribs, creating an enclosed rectangular or trapezoidal raceway. Access to cells is via preset inserts (floor monuments) at 6 to 12 foot spacing that provide power, data, and communication connections at workstation level — this eliminates the need for core-drilling and conduit runs from the ceiling. In a typical office building, cellular deck can eliminate 15-20% of MEP rough-in cost by removing the need for separate conduit systems. Acoustical cellular deck adds perforations in the bottom plate with sound-absorbing insulation inside the cells, providing both MEP distribution and noise reduction (NRC 0.70-0.85 with glass fiber fill). The structural capacity of cellular deck is similar to non-cellular deck of the same depth and gage for gravity loads — the bottom plate adds some dead weight (1-2 psf) but does not significantly affect flexural capacity. However, cellular deck diaphragm shear values are typically 10-15% lower than non-cellular deck of the same profile because the bottom plate detail affects weld access and shear transfer. Cellular deck requires closer coordination between the structural engineer and MEP designer to plan cell runs, insert locations, and service entry/exit points.

What is steel deck diaphragm shear capacity and how is it calculated?

Steel deck acts as a structural diaphragm, transferring lateral loads (wind and seismic) from the roof or floor to the vertical lateral force-resisting system (braced frames or shear walls). The diaphragm shear capacity depends on the deck profile, gage, concrete fill (if composite), attachment pattern, and support spacing. SDI publishes diaphragm shear strength tables for each deck type and attachment configuration. The diaphragm shear capacity for bare (non-composite) roof deck is typically 200-500 plf depending on gage, support spacing, and weld/pin spacing. For composite floor deck with concrete fill, diaphragm shear capacity increases dramatically — typically 800-3,000 plf — because the concrete slab provides significant in-plane shear resistance through diagonal compression struts. The AISI S310 (North American Standard for the Design of Profiled Steel Diaphragm Panels) governs the design. Key parameters affecting diaphragm shear capacity include: (1) Deck-to-support attachment: 3/4 in puddle welds at 12-36 in o.c. or powder-actuated fasteners. The closer the spacing, the higher the diaphragm capacity. (2) Side-lap connections: screws or button punches between adjacent deck sheets at 12-36 in o.c. Side-lap fasteners are critical because deck sheets are only 24-36 in wide and the diaphragm must transfer shear across sheet edges. (3) Concrete fill: composite slabs use a combination of deck-to-support connections and concrete bearing/shear-friction for shear transfer — the concrete adds 2-5x more capacity over bare deck. (4) Support stiffness: a stiff support pattern (beams at 8 ft o.c.) provides higher diaphragm capacity than flexible supports (open-web joists at 6 ft o.c.) because the deck is stiffer in-plane when supports are closer. For preliminary design, a composite 20 gage 2.0N deck with 36/4 weld pattern and 3.25 in LW concrete provides approximately 1,200-1,500 plf diaphragm shear — adequate for most mid-rise buildings. High-seismic applications may require 18 gage or closer attachment spacing.

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.

Cellular deck (for MEP distribution)

Type	Depth	Cell Width	Typical Gage	MEP Integration
Cellular floor	1.5-3.0 in.	3-6 in.	22-18 ga	Electrical, data conduit
Cellular acoustical	1.5-3.0 in.	3-6 in.	22-18 ga	Acoustic perforations + insulation
Deep cellular	3.0-4.5 in.	6-12 in.	18-16 ga	Large cable trays, HVAC

Cellular deck provides enclosed raceways for electrical and data cabling, eliminating the need for separate conduit systems. Acoustical versions have perforations in the bottom flange with acoustic insulation to reduce sound transmission.

Unshored span limits by deck profile and gage

1.5 in. composite deck

Gage	Thickness (in.)	Weight (psf)	Max Unshored Span 1-Span	Max Unshored Span 2-Span	Max Unshored Span 3-Span
22	0.0295	1.6	5 ft 6 in.	6 ft 6 in.	7 ft 0 in.
21	0.0329	1.8	5 ft 10 in.	7 ft 0 in.	7 ft 6 in.
20	0.0358	2.0	6 ft 4 in.	7 ft 6 in.	8 ft 0 in.
19	0.0418	2.3	7 ft 0 in.	8 ft 6 in.	9 ft 0 in.
18	0.0474	2.6	7 ft 8 in.	9 ft 6 in.	10 ft 0 in.

3.0 in. composite deck

Gage	Thickness (in.)	Weight (psf)	Max Unshored Span 1-Span	Max Unshored Span 2-Span	Max Unshored Span 3-Span
22	0.0295	1.9	9 ft 0 in.	10 ft 0 in.	11 ft 0 in.
20	0.0358	2.3	10 ft 0 in.	12 ft 0 in.	13 ft 0 in.
18	0.0474	3.0	12 ft 0 in.	14 ft 6 in.	15 ft 6 in.
16	0.0598	3.8	14 ft 0 in.	16 ft 6 in.	17 ft 6 in.

Span limits assume normal-weight concrete at 150 pcf, total slab weight including deck. 3-span condition is most common in practice.

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.

Diaphragm shear capacity (1.5 in. composite deck, puddle welds)

Gage	Weld Pattern	Side-Lap	Nominal Sn (plf)	phi*Sn (LRFD, plf)
22	36/36	Button punch	470	306
22	36/24	Button punch	560	364
20	36/36	Button punch	620	403
20	36/24	Button punch	720	468
20	36/36	Screw (#10)	680	442
18	36/36	Button punch	780	507
18	36/24	Screw (#10)	950	618

phi = 0.65 for LRFD diaphragm design per SDI DDM04. Weld pattern notation: 36/36 = 36 in. along-beam spacing / 36 in. side-lap spacing.

Typical diaphragm shear demands

Building Type	Bay Size (ft)	Wind Demand (plf)	Seismic Demand (plf)	Typical Deck/Gage
Low-rise office (3 st)	30 x 30	150-250	200-350	1.5 in., 22 ga
Mid-rise office (10 st)	30 x 30	250-400	300-500	1.5 in., 20 ga
High-rise (20+ st)	30 x 30	400-600	400-700	1.5 in., 20 ga + screws
Warehouse (1 st)	40 x 50	100-200	150-300	1.5 in., 22 ga
Parking garage	30 x 60	100-200	250-400	3.0 in., 20 ga

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:

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.

Shear stud placement in deck ribs

Per AISC 360-22 Section I8.2d, shear studs in steel deck must comply with:

Requirement	AISC 360 Value	Notes
Max. stud diameter	3/4 in. (or 7/8 in. if rib width permits)	Larger studs need wider ribs
Min. rib width at top	2 in. for one stud per rib	4 in. for two studs per rib
Stud height above deck	>= 1.5 in. above top of deck rib	Must embed in concrete
Stud reduction factors	Rg = 0.85 (2 studs), Rp = 0.75 (weak pos.)	Strong position Rp = 1.0
Number of studs per rib	1 preferred, 2 max per rib	3+ requires Rg = 0.70

Rg and Rp factor combinations

Condition	Rg	Rp (strong pos.)	Rp (weak pos.)	Combined (strong)	Combined (weak)
1 stud per rib	1.0	1.0	0.75	1.00	0.75
2 studs per rib	0.85	1.0	0.75	0.85	0.64
3+ studs per rib	0.70	1.0	0.75	0.70	0.53

Weak-position studs lose 25% capacity. Two-stud configurations lose an additional 15%. Specifying stud position on drawings is important for design-to-construction accuracy.

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
3.0 in. composite deck + normal weight concrete	3 hour	4.5 in. above top of rib	UL D982
Roof deck + SFRM spray	1 hour	N/A (spray thickness per UL)	Various UL assemblies
Roof deck + SFRM spray	2 hour	N/A (spray thickness per UL)	Various UL assemblies

Lightweight concrete reduces the required slab thickness for the same fire rating compared to normal-weight concrete.

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
Deck gage designation	MSG ( Manufacturers Standard Gage)	BMT (Base Metal Thickness)	Nominal thickness (mm)	MSG (same as US)

Common mistakes to avoid

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.
Not specifying stud position (strong vs. weak). The Rg x Rp reduction for studs in the weak position can reduce shear stud capacity by 25-47%. Always specify stud position on structural drawings.
Using normal-weight concrete when lightweight was assumed. Lightweight concrete (110-120 pcf) reduces slab weight by 20-25%, but some contractors substitute normal-weight (150 pcf) for availability. This increases dead load and may overstress the framing.

Frequently asked questions

What is the most common composite deck profile? 1.5 in. deep, 22 or 20 gage, with ribs at 6 in. spacing. It is the default for office building construction with beam spacings of 8-10 ft. Use 3.0 in. deck for beam spacings over 12 ft.

How thick should the concrete be above the deck? Minimum 2 in. above the top of the deck rib per AISC 360 Section I3.2c for composite action. Typical total slab depth is 4.5-5.5 in. (including the deck). For fire ratings of 2 hours or more, 3.5-4.5 in. above the top of rib may be required.

Do I need shoring during construction? Only if the deck span exceeds the unshored span limit from the manufacturer's load table. For 1.5 in. deck at 20 gage, the limit is approximately 8 ft. For 3.0 in. deck at 20 gage, it is approximately 13 ft. Most office construction with 10 ft spacing does not need shoring.

What is the difference between Type B and Type F roof deck? Type B has narrower ribs (wider flats), providing higher diaphragm shear capacity. Type F has wider ribs (narrower flats), allowing longer spans but lower diaphragm capacity. Use Type B when diaphragm action is critical.

Can steel deck be used as a structural diaphragm? Yes. Steel deck with welded or mechanically fastened connections to the supporting frame acts as a horizontal diaphragm. Diaphragm shear capacity ranges from 300-600 plf depending on gage, profile, and connection pattern. See SDI DDM04 for design tables.

How do I detail deck at the perimeter and openings? At the building perimeter, provide closure angles or pour stops to contain the wet concrete. At openings larger than 12 in. x 12 in., provide reinforcing around the opening per SDI recommendations. Small openings (< 12 in.) can be cut in the field without reinforcement.

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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 this information.