Galvanized Ductwork 101: Gauges, G-Ratings, and What We Use at K & E

When you’re choosing duct, two specs matter most: steel gauge (thickness) and galvanized coating weight (zinc layer, e.g., G60 or G90). Thicker steel resists dents and spans farther; more zinc slows rust at seams and scratches. (Intro to coating weights and ASTM overview: GalvInfoNote, U. S. Steel quick conversions.)

Gauge (Thickness): What it Means

  • 30 ga (thin, light): common on small-diameter round pipe for low-pressure runs.
    Pros: light, inexpensive. Cons: dents easily; not ideal for long spans or rough handling.

  • 28 ga: step up in stiffness for round and small fittings.
    Pros: better durability. Cons: slightly higher cost/weight.

  • 26 ga: standard for many residential trunks and larger fittings.
    Pros: sturdier panels, cleaner seams. Cons: heavier, pricier than 28–30.

  • 24 ga: for larger trunks, long spans, light commercial, or tougher handling.
    Pros: strong, holds shape, reduces oil-canning. Cons: more cost/weight; may need heavier hangers.

Rule of thumb: As duct gets larger, longer, or exposed to abuse, go thicker (lower gauge number). (SMACNA context: see the HVAC Duct Construction Standards.)

G-Ratings (Zinc Coating): Why G60 vs. G90 Matters

The G-number is total zinc weight on both sides (ASTM A653). More zinc = better corrosion resistance.

  • G60: balanced protection for interior duct in conditioned spaces.

  • G90: extra margin for damp areas, basements, crawlspaces, garages, rooftop curbs, or where condensation risk is higher.

Important: G60 vs. G90 does not change steel thickness—only the protective zinc layer. (Definitions and conversions: GalvInfoNote, G60 vs G90 explainer.)

K & E Shop Standards

  • We stock and fabricate 26 ga and 24 ga as our core thicknesses.

  • For both 26 ga and 24 ga we use G60 and G90 zinc coatings depending on environment.

  • Heavier gauges (22 ga, 20 ga) are used as the job and specifications require (size, pressure class, span, and reinforcement).

  • For sealing, we follow UL 181 listed mastics/tapes for HVAC duct systems. (UL 181 context.)

Quick Picks (Save & Share)

  • Small round runs (dry interior): 28–30 ga, G60

  • Typical residential trunks/returns: 26 ga, G60

  • Basement/crawl/garage or any condensation risk: 26 ga, G90 (or 24 ga if large spans)

  • Large trunks/long spans/light commercial: 24 ga, G60/G90 per environment

  • Outdoor/rooftop/curb: G90 minimum; consider 24 ga or heavier

Advantages & Tradeoffs at a Glance

  • Thicker gauge (24 vs. 26): more rigidity, fewer oil-cans, longer spans → higher cost/weight.

  • Higher zinc (G90 vs. G60): longer rust resistance → slightly higher material cost.

  • Smart combo: match gauge to size/span and G-rating to moisture risk.

Helpful Definitions & Deep Dives

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SaraSpa Rod & Gun Club Duct Fabrication

After installing the filter boxes and elbows shown in my previous post, it is now time to fabricate the next set of duct and transitions that will run up the wall. There are three total units and each will need a 90" drop. Below are the in-progress fittings lined up in a pretty little row.

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Our next task is to cut and install the Ductmate Flange System on each section of duct for easy installation when we get to the jobsite.

Wolla!

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We finished up a few 90 elbows with turning vanes for the job as well. Probably could have managed a better photo but I had to assist my helper uncle with that tub of screws :)...JK

 

Custom Galvanized Sheet Metal Ash Box

The benefit of our fabrication shop is the ability to fabricate small numbers of custom projects for a range of products. Our main focus being HVAC custom ductwork and fittings, but we fabricate a lot of specialty items for many of our customers.

Last week I received a request through our website for a custom 15" x 15" x 15" ash box with a sliding top cover.

 

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Custom Duct Transitions For AC Coil

Installing a new AC coil

There are times when installing a new AC coil to existing duct work can be simple and times when the task can be difficult. Usually you can get the proper cased AC coil for the furnace you are installing and either easily transition the duct to the existing duct line or you can have a custom plenum made fitting the coil to the furnace.  

Then there are times when you need to transition from your furnace to the size of the uncased AC coil and then back again to the custom size of your existing duct work. When this happens your first call should be to us, K & E Sheet Metal, because this is exactly why we exist. Custom plenums and transitions are our specialty :)

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