Sheet Metal Specifications

Sheet metal specifications establish the fundamental material characteristics that determine duct structural performance, fabrication methods, and long-term durability. Gauge selection, material composition, and reinforcement design directly impact system pressure capacity, air leakage potential, and installation costs.

Galvanized Steel Sheet Metal

Galvanized steel represents the predominant duct construction material, offering corrosion resistance, structural strength, and economical cost. The zinc coating protects the steel substrate through both barrier protection and sacrificial anode action.

G90 coating designation indicates a minimum zinc coating weight of 0.90 ounces per square foot of sheet surface (both sides combined). This coating thickness provides adequate corrosion protection for interior ductwork in typical HVAC applications. G60 coating (0.60 oz/ft²) offers reduced corrosion protection, suitable for low-humidity environments or short service life applications.

Hot-dip galvanizing after fabrication (HDG) provides superior corrosion resistance compared to pre-galvanized sheet, particularly at cut edges and formed corners where the coating may be compromised during fabrication. HDG applications include outdoor installations, coastal environments, and corrosive atmospheres.

Gauge Standards and Thickness

The gauge numbering system for ferrous metals operates inversely, where larger gauge numbers indicate thinner material. This counterintuitive convention requires careful attention during specification and fabrication.

Standard gauge progressions for ductwork include:

• 26 gauge: 0.0187 inches (0.47 mm)
• 24 gauge: 0.0239 inches (0.61 mm)
• 22 gauge: 0.0299 inches (0.76 mm)
• 20 gauge: 0.0359 inches (0.91 mm)
• 18 gauge: 0.0478 inches (1.21 mm)
• 16 gauge: 0.0598 inches (1.52 mm)

Manufacturers’ gauge tolerances typically permit ±0.0015 to ±0.0030 inch variation from nominal thickness. These tolerances affect structural calculations and require conservative design approaches for critical applications.

Minimum Thickness Requirements

SMACNA tables correlate minimum gauge with duct dimensions and operating pressure, establishing baseline structural adequacy. Rectangular duct side dimensions determine base gauge selection, with smaller ducts permitting lighter gauge material.

Duct dimensions under 12 inches typically utilize 26 gauge minimum, while dimensions from 12 to 30 inches commonly require 24 gauge. Dimensions exceeding 30 inches generally mandate 22 gauge or heavier, with specific requirements varying by pressure classification.

Round duct diameter similarly governs gauge selection, with diameters under 12 inches often utilizing 26 gauge and larger diameters requiring progressively heavier material. The circular geometry provides structural advantages that permit lighter gauge compared to equivalent rectangular sections.

Stiffness and Deflection Criteria

Sheet metal stiffness determines resistance to deflection under operating pressure and external loads. Inadequate stiffness produces objectionable noise from panel flexure, accelerated joint failure from cyclic stress, and potential collapse under negative pressure.

Maximum permissible deflection typically limits to 1/240 of the unstiffened span for rectangular ductwork. This criterion prevents visible deformation and excessive stress concentration at joints and reinforcement attachments.

Aspect ratio (ratio of long dimension to short dimension) significantly affects rectangular duct stiffness. Ratios exceeding 4:1 require special consideration, often necessitating intermediate stiffeners or increased gauge to maintain structural adequacy.

Reinforcement Requirements

Reinforcement members stiffen duct panels to prevent excessive deflection and distribute stress around penetrations and joints. Reinforcement spacing correlates inversely with operating pressure, requiring closer spacing as static pressure increases.

Transverse Joint Reinforcement

Transverse reinforcement at joints typically utilizes formed angles, channels, or proprietary reinforcement systems. The reinforcement profile depth ranges from 0.75 to 2 inches, selected based on duct dimensions and pressure classification.

Corner reinforcement requires particular attention in rectangular ductwork, where stress concentration peaks at the intersection of perpendicular panels. TDC corners and integral corner angles distribute these stresses while facilitating gasket compression.

Longitudinal Reinforcement

Longitudinal stiffeners prevent panel deflection between transverse joints, positioned at intervals determined by SMACNA tables based on pressure and panel dimensions. External angles represent the most common longitudinal reinforcement method, though internal angles or beaded stiffeners serve specialized applications.

Intermediate Stiffeners

Intermediate stiffeners subdivide large duct panels to reduce unsupported span and increase natural frequency above potential resonance ranges. Stiffener spacing typically ranges from 24 to 60 inches, with closer spacing required for higher pressures and thinner gauge materials.

Standing seam stiffeners, formed directly in the duct panel, provide economical reinforcement for low to medium-pressure applications. Rolled beads offer alternative integral stiffening, though their effectiveness diminishes with increasing panel size.

Weight Classification

Duct weight classification correlates material gauge with application requirements, balancing structural performance against material and installation costs. Light weight construction utilizes minimum gauge material with reinforcement spacing at maximum allowable intervals.

Medium weight construction employs gauge selections one or two sizes heavier than minimum requirements, providing enhanced durability and reduced deflection. This approach suits applications demanding long service life or exposure to potential mechanical damage.

Heavy weight construction specifies substantial gauge increases and close reinforcement spacing for industrial applications, high-pressure systems, or corrosive environments requiring extended service life.

Material Selection Factors

Operating temperature influences material selection when air temperatures exceed 150°F, potentially requiring stainless steel or aluminum construction. Galvanized steel zinc coating volatilizes at temperatures above 750°F, limiting application in high-temperature exhaust systems.

Chemical exposure necessitates corrosion-resistant materials such as 304 or 316 stainless steel, aluminum alloys, or coated steel products. Laboratory exhaust systems, chemical processing facilities, and marine environments commonly warrant these upgraded materials.

Appearance requirements may dictate stainless steel or painted finishes for exposed ductwork in architectural applications. These specifications significantly increase material costs while providing aesthetic and durability benefits.