What are the different types of steel roof deck profiles and when is each used?

Steel roof deck profiles per SDI are designated by type letter and depth: Type B (Narrow Rib, 1.5 in) — narrow flute spacing, used for architectural roofs and steep slopes where the narrow rib pattern provides more attachment points. Span: 4-8 ft. Type F (Intermediate Rib, 1.5 in) — intermediate flute width, general-purpose roofing. Span: 5-9 ft. Type N (Wide Rib, 1.5 in) — the most common structural roof deck in North America. Wide ribs provide maximum structural efficiency and the largest bearing surface for insulation. Span: 6-12 ft. 1.5N20 (20 gauge) is the default spec for most commercial roofs. Type A (Deep Rib, 3.0 in) — deep profile for long spans 10-18 ft, used for canopies, long-span roofs, and where fewer support members are desired. Type J (Interlocking, 1.5 in) — side joints interlock mechanically, providing better diaphragm action and eliminating the need for sidelap fasteners in some applications. The designation system: e.g., 1.5N20 = 1.5 in depth, Type N profile, 20 gauge (0.0358 in design thickness). 20 gauge is the most common for structural roof decks. 22 gauge (0.0295 in, 1.7 psf) for light loads/short spans; 18 gauge (0.0474 in, 2.6 psf) for heavy loads/long spans; 16 gauge (0.0598 in, 3.3 psf) for floor deck or heavy roof loading.

How far can steel roof deck span and what load does it carry?

Steel roof deck span capability depends on deck type, gauge, and number of continuous spans: 1.5N22 (22 gauge wide rib): 1-span — 127 psf at 4 ft, 56 psf at 6 ft, 32 psf at 8 ft, 20 psf at 10 ft. 2-span — 160 psf at 4 ft, 71 psf at 6 ft, 40 psf at 8 ft, 25 psf at 10 ft. 3-span — 144 psf at 4 ft, 64 psf at 6 ft, 36 psf at 8 ft, 23 psf at 10 ft. 1.5N20 (20 gauge): 1-span — 102 psf at 5 ft, 71 psf at 6 ft, 40 psf at 8 ft, 25 psf at 10 ft. 2-span — 128 psf at 5 ft, 89 psf at 6 ft, 50 psf at 8 ft, 32 psf at 10 ft. 3-span condition gives intermediate values. 3A18 (3 in deep rib, 18 gauge): 1-span — 52 psf at 10 ft, 36 psf at 12 ft, 26 psf at 14 ft, 20 psf at 16 ft. Multi-span (2 or 3 span) conditions provide 25-35% more capacity than single-span because continuity reduces the maximum bending moment. Continuous deck over 3+ supports is standard practice — single-span deck is rare. Typical roof design loads: dead load 3.5-8 psf (deck + insulation + membrane), snow/live load 20-50 psf (varies by location), wind uplift 15-40 psf (varies by location). For a 6 ft joist spacing with 30 psf snow load + 5 psf dead load = 35 psf total: 1.5N22 is adequate (56 psf 1-span, 71 psf 2-span). For 8 ft spacing with same load: 1.5N20 required (40 psf 1-span, 50 psf 2-span). Deflection limits: total load L/240, live load L/360, ponding L/240 + 1/4 in max additional deflection. Always verify with manufacturer-specific load tables.

How does steel roof deck function as a structural diaphragm?

Steel roof deck acts as a horizontal diaphragm transferring lateral wind and seismic loads from walls to braced frames or shear walls. This is essential for building stability — without diaphragm action, each frame would need to resist lateral loads independently. Diaphragm capacity depends on four factors: (1) Deck gauge — thicker gauge = higher shear capacity. 20 gauge provides approximately 25% more diaphragm shear than 22 gauge for the same connection pattern. (2) Support connection pattern — the weld pattern notation '36/7' means 7 arc-spot (puddle) welds per 36 in wide sheet. Common patterns: 36/7 (7 welds per sheet = every other flute), 36/5 (5 welds per sheet), 36/4 (4 welds = minimum). More welds = higher shear capacity. (3) Sidelap connections — between adjacent sheets: welded (1/4 in fillet at 12-24 in), button punch (at 12-24 in, for lighter gauge), screwed (#10 TEK at 12-24 in). Sidelap connections transfer shear between adjacent deck panels. Without sidelap connections, each panel acts independently, dramatically reducing diaphragm capacity. (4) Deck orientation — deck ribs perpendicular to the lateral force direction provide the strongest diaphragm action. Deck ribs parallel to the force direction have approximately 50-70% of perpendicular capacity. Typical diaphragm shear capacities: 22 gauge with 36/7 welds + screwed sidelaps = 150-250 plf; 20 gauge with 36/7 welds + welded sidelaps = 250-400 plf; 18 gauge with 36/5 welds + welded sidelaps = 350-550 plf. For design: follow SDI Diaphragm Design Manual or IBC Section 2306. The deck manufacturer provides tested diaphragm shear values (not calculated from first principles).

What are puddle welds and how are roof deck connections detailed?

A puddle weld (arc-spot weld) is the standard connection method for welding steel deck to structural supports. The process: an electric arc burns through the deck into the supporting steel member (joist or beam), creating a circular weld pool ('puddle') that fuses the deck to the support. The welding electrode passes through a ceramic ferrule or is guided by the welder. Minimum weld diameter: 5/8 in for structural deck. The visible weld on top of the deck is the 'weld button.' Connection requirements: Support connections — each flute (rib) of the deck must be welded to the support at end spans and at every other flute at interior supports. The 36/7 pattern (7 welds per 36 in sheet width) is standard. For mechanical fasteners: #12 or #14 TEK screws, minimum 1 per flute at supports, with a minimum embedment of 3/8 in into the steel support. Sidelap connections — between adjacent deck sheets: welded (1/4 in fillet weld at 12-24 in spacing), button punch (crimped mechanical connection at 12-24 in, for 22 gauge and lighter), screwed (#10 TEK screws at 12-24 in spacing). Sidelap connections are critical for diaphragm action — without them, each panel moves independently. Perimeter connections — all deck edges must be connected to perimeter supports. Minimum bearing: 6 in at steel supports, 4 in at masonry/concrete walls. Deck must be attached to ALL supports — skipping intermediate supports is not permitted. Puddle weld quality: welds should be visually inspected for: full fusion to support (no 'cold' welds), adequate diameter (≥5/8 in), no burn-through or excessive spatter. AWS D1.3 governs structural welding of sheet steel.

How does roof deck selection interact with insulation and roofing systems?

Steel roof deck must be compatible with the insulation and roofing system above it. Key interactions: Insulation attachment — polyisocyanurate (PIR, R = 6.0/in, 2-4 in thick), XPS (R = 5.0/in), EPS (R = 3.8/in), mineral wool (R = 3.7/in), composite board. Insulation can be: mechanically fastened (screws + plates through insulation into deck ribs), adhered (low-rise foam adhesive applied to deck ribs), or loose-laid under ballasted membrane. Mechanical fastening into deck ribs requires the deck rib width to accommodate the fastener + plate diameter. Type N (wide rib) provides more flat surface for adhesive than Type B (narrow rib). Thermal bridging — steel deck ribs act as thermal bridges, conducting heat through the insulation at rib locations. The effective R-value of insulation over steel deck is approximately 20-35% lower than the nominal R-value due to thermal bridging through the ribs. Using cover boards or increasing insulation thickness compensates. Minimum insulation thickness: 1.5 in over steel deck to prevent condensation (thermal bridging through ribs drops surface temperature). For cold climates, 2-4 in minimum is typical. Vapor retarders: placed between deck and insulation (warm side in cold climates) to prevent interior moisture from reaching the cold deck surface. The vapor retarder must be compatible with the deck attachment method — mechanical fasteners penetrate the retarder at every fastener location. Steep-slope roofing (>2:12): deck provides substrate for standing seam metal roofing, tile, or shingles. The deck must resist the dead load of the roofing system plus wind uplift. Low-slope roofing (≤2:12): minimum 1/4 in/ft slope for drainage. Flat roofs use tapered insulation to create slope. Ponding (standing water) can cause progressive deflection — AISC Design Guide 3 addresses ponding stability. The deck must be checked for ponding where the roof slope is less than 1/4 in/ft and the deck span exceeds 20 ft.

Steel Roof Deck Design — Types, Spans & Load Tables

Steel roof deck is the most common roof substrate in commercial and industrial construction. It serves as a structural element (supporting roof loads and transferring them to joists or beams), a diaphragm (transferring lateral loads to braced frames), and a substrate for roofing materials (insulation, membrane, standing seam).

This page covers deck types, span tables, connection requirements, and design guidance per the Steel Deck Institute (SDI) standards.

Steel Roof Deck Types

By Profile

Type	Profile	Depth (in)	Common Spans	Application
B	Narrow rib	1.5	4-8 ft	Architectural roofs, steep slopes
F	Intermediate rib	1.5	5-9 ft	General purpose roofing
N	Wide rib	1.5	6-12 ft	Most common structural roof deck
A	Deep rib	3.0	10-18 ft	Long span roofs, canopies
J	Intermediate rib	1.5	5-8 ft	Interlocking deck
PLN	Cellular (acoustic)	Varies	Varies	Acoustical ceilings

The most common roof deck in North America is 1.5N (1.5 inch deep, Type N wide rib).

By Gauge (Thickness)

Gauge	Design Thickness (in)	Weight (psf)	Typical Use
22	0.0295	1.7	Structural roof deck
20	0.0358	2.0	Structural roof deck (most common)
18	0.0474	2.6	Heavy loads, long spans
16	0.0598	3.3	Floor deck, heavy roof
14	0.0747	4.1	Heavy industrial

Note: Galvanized steel weighs approximately 5% more due to zinc coating.

Common Designations

1.5B22: 1.5 inch depth, Type B (narrow rib), 22 gauge
1.5N20: 1.5 inch depth, Type N (wide rib), 20 gauge
3A18: 3 inch depth, Type A (deep rib), 18 gauge
1.5N22-36: 36 inch coverage width, 22 gauge

Span Tables — Allowable Uniform Loads (psf)

1.5N (Wide Rib) Roof Deck — 22 Gauge (0.0295 in)

Span (ft)	1 Span	2 Span	3 Span
4	127	160	144
5	81	102	92
6	56	71	64
7	42	52	47
8	32	40	36
9	25	31	28
10	20	25	23

Values are allowable uniform load in psf for Fy = 33 ksi steel. Actual capacity depends on manufacturer.

1.5N (Wide Rib) Roof Deck — 20 Gauge (0.0358 in)

Span (ft)	1 Span	2 Span	3 Span
4	159	200	180
5	102	128	115
6	71	89	80
7	52	65	59
8	40	50	45
9	31	39	35
10	25	32	28
11	21	26	23
12	18	22	20

3A (Deep Rib) Roof Deck — 18 Gauge (0.0474 in)

Span (ft)	1 Span	2 Span	3 Span
8	81	102	92
9	64	81	72
10	52	65	59
11	43	54	48
12	36	45	41
14	26	33	30
16	20	25	23
18	16	20	18

Note: All span table values are approximate. Always verify with the specific manufacturer's load tables.

Design Considerations

Load Combinations for Roof Deck

Typical roof deck design loads:

Load Component	Low Slope (≤ 2:12)	Steep Slope (> 2:12)
Deck self-weight	1.7-3.3 psf	1.7-3.3 psf
Insulation	1.5-3.0 psf	1.5-3.0 psf
Roofing membrane	0.5-1.5 psf	0.5-1.5 psf
Snow load	20-50 psf (varies)	20-50 psf (varies)
Wind uplift	15-40 psf (varies)	10-30 psf (varies)
Total dead load	3.7-7.8 psf	3.7-7.8 psf
Total live/snow	20-50 psf	20-50 psf

Deflection Limits

Condition	Limit
Roof deck under total load	L/240
Roof deck under live load	L/360
Ponding consideration	L/240 + 1/4 in max

Diaphragm Capacity

Steel roof deck acts as a diaphragm to transfer lateral wind and seismic loads to braced frames. Diaphragm capacity depends on:

Deck gauge: thicker gauge = higher capacity
Connection pattern: stitch, button punch, or welded sidelap connections
Support connections: weld pattern (e.g., 36/7 or 36/5) or mechanical fasteners
Deck orientation: deck parallel vs perpendicular to the lateral force direction

Typical diaphragm shear capacities range from 100-500 plf for 20 gauge deck depending on connection pattern.

Connection Requirements

Structural supports (joists, beams):

Welded: 5/8 inch diameter arc-spot welds ( puddle welds) or 3/8 inch fillet welds
Minimum connection: each flute at supports (36/7 pattern = 7 welds per 36" width)
Mechanical fasteners: #12 or #14 TEK screws, minimum 1 per flute at supports

Sidelap connections (between deck sheets):

Welded: 1/4 inch fillet welds at 12-24 inch spacing
Button punch: at 12-24 inch spacing (for 22 gauge and lighter)
Screwed: #10 TEK screws at 12-24 inch spacing

Perimeter connections:

All deck edges must be connected to supports
Minimum 6 inches of bearing at supports
Deck must be attached to ALL supports

Roof Deck and Insulation Compatibility

Insulation Type	Thickness (in)	R-Value	Attachment
Polyiso (PIR)	2.0-4.0	6.0/in	Adhesive or mechanical
XPS (Extruded Polystyrene)	1.5-4.0	5.0/in	Adhesive or mechanical
EPS (Expanded Polystyrene)	1.5-6.0	3.8/in	Adhesive or mechanical
Mineral Wool	2.0-4.0	3.7/in	Adhesive or mechanical
Composite board	1.0-2.5	Varies	Adhesive or mechanical

For low-slope roofs with insulation over steel deck:

Minimum insulation thickness: 1.5 inches (to prevent thermal bridging)
Mechanical attachment recommended for wind uplift resistance
Insulation boards should be staggered to avoid continuous joints

Fire Rating Considerations

Steel roof deck assemblies can achieve fire ratings with:

Rating	Construction
Non-rated	Steel deck + insulation + membrane
1-hour	Steel deck + 2" concrete fill, or fire-rated insulation
2-hour	Steel deck + 2.5" concrete fill + spray-applied fire resistive material

Frequently Asked Questions

What gauge steel roof deck should I use? 20 gauge (0.0358 in) is the most common for structural roof decks. Use 22 gauge for light loads and short spans. Use 18 gauge for heavy loads or spans over 8 ft.

What is the difference between Type B, F, and N deck? Type B has narrow ribs (narrow flute spacing), Type F has intermediate ribs, and Type N has wide ribs. Type N provides the best structural capacity and is the most common for roof applications.

How far can steel roof deck span? 1.5N20 (20 gauge wide rib) can span approximately 8-10 ft for typical roof loads. 3A18 (18 gauge deep rib) can span 12-16 ft. Always verify with manufacturer load tables.

Does steel roof deck need to be galvanized? Galvanized coating (G60 or G90) is recommended for corrosion protection, especially in humid environments. Painted deck (primer only) is used for dry, enclosed conditions.

What is a puddle weld? A puddle weld (arc-spot weld) is made by burning through the steel deck into the supporting member, creating a circular weld. It is the most common attachment method for steel deck to structural supports.

How do I design steel roof deck as a diaphragm? Follow SDI Diaphragm Design Manual or IBC provisions. Key parameters: deck gauge, span, connection pattern (welds per sheet width), and sidelap connection type. Diaphragm shear capacity is provided by the deck manufacturer.

Can steel roof deck be used for flooring? No. Roof deck profiles are not designed for the concentrated loads and vibration requirements of floors. Use composite floor deck (1.5W, 2W, or 3W profiles) for floor applications.

What is the minimum roof slope for steel deck? Minimum 1/4 inch per foot (1:48) for drainage. Flat roofs must have tapered insulation to achieve positive drainage. Standing water (ponding) can cause progressive collapse.

Steel Deck Calculator — Roof and floor deck analysis
Steel Beam Sizes — Open web joist and beam dimensions
Snow Load Calculator — ASCE 7 snow load determination
Wind Load Calculator — Uplift pressure on roof assemblies
Deflection Limits — L/240, L/360 criteria
Steel Grating Sizes — Bar grating for platforms and walkways

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.

Span (ft)	1 Span	2 Span	3 Span
4	159	200	180
5	102	128	115
6	71	89	80
7	52	65	59
8	40	50	45
9	31	39	35
10	25	32	28
11	21	26	23
12	18	22	20

Span (ft)	1 Span	2 Span	3 Span
8	81	102	92
9	64	81	72
10	52	65	59
11	43	54	48
12	36	45	41
14	26	33	30
16	20	25	23
18	16	20	18

Span (ft)	1 Span	2 Span	3 Span
4	159	200	180
5	102	128	115
6	71	89	80
7	52	65	59
8	40	50	45
9	31	39	35
10	25	32	28
11	21	26	23
12	18	22	20

Span (ft)	1 Span	2 Span	3 Span
8	81	102	92
9	64	81	72
10	52	65	59
11	43	54	48
12	36	45	41
14	26	33	30
16	20	25	23
18	16	20	18