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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.)
Reference: SMACNA HVAC Duct Construction Standards (overview/PDF excerpts) and Ductmate quick guide referencing SMACNA.
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
ASTM A653 (galvanized sheet) overview & conversions: U. S. Steel note and GalvInfoNote 1.1.
Process background (continuous vs batch hot-dip): American Galvanizers Association explainer.
UL 181 (mastics & tapes): Carlisle/Hardcast UL 181 note.
Amazon affiliate picks
As an Amazon Associate, K & E may earn from qualifying purchases.
Cold-galv touch-up (zinc-rich) — helps protect field cuts/screws
UL 181 duct mastic (water-based) — for sealing joints: (UL 181 reference above.)
UL 181B-FX foil tape — for code-compliant sealing:
Sheet-metal screws (hex-washer, self-drilling) — for field assembly:
5 Tools We Use Every Day in the Shop (And Why They Matter)
Tools of The Trade
Here are a few simple tools to get you started on the road to becoming a tin knocker extraordinaire.
In sheet metal, precision and efficiency aren’t just buzzwords—they’re survival. Over time, we’ve developed a short list of tools that never leave our side in the shop.
If you’re just getting started—or want to tighten up your daily workflow—here are five tools that earn their keep every single day.
🔧 1. Wiss M1 Aviation Snips
Nothing beats a good pair of snips. The Wiss M1s cut clean, hold an edge, and don’t fight your wrist.
We keep a few extra pairs around because they always walk off.
📏 2. 16-ft Magnetic Tape Measure
Why 16 feet? It’s compact, fits in a pouch, and you’ll rarely need more in the shop. The magnetic end speeds up solo measuring on duct runs.
Bonus points if it has fractional markings and a standout over 10 feet.
📐 3. Milwaukee 12V Drill/Driver Kit
Light, fast, and just enough torque for fab work and install screws. The 12V M12 kit is our go-to—it’s nimble and charges fast.
Perfect balance of power and portability for duct shops.
✏️ 4. FastCap Flatback Tape + Mechanical Pencil
When accuracy matters, a flat-back tape plus a mechanical pencil makes layout tight and repeatable.
🛒 FastCap Flatback Tape
🛒 Mechanical Pencil
For layout on metal or when making a series of repeat cuts, this combo beats soapstone or Sharpie.
🛠 5. Malco Hand Seamer
For quick bends, adjustments, and cleanup—nothing beats a quality hand seamer. Malco's 3” seamer grips well and gives you leverage when you need it.
Ours gets used daily—on site and at the bench.
““Every tradesperson should have a go-to set of tools in a single pouch. It saves time, avoids shop chaos, and lets you work like a pro.””
From the Shop
We’re a small fabrication team working on tight deadlines, custom projects, and smarter workflows. No fluff—just field-tested tools and methods.
Disclaimer:
Some of the links above are affiliate links. If you click through and make a purchase, we may earn a small commission—at no extra cost to you. It helps us keep the shop running and the blog alive.
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.
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!
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
Understanding The Role of an HVAC Ductwork Fabricator
Have you ever heard of the term 'tin knocker'? There's a good chance you have come across the term somewhere along the line. Although, there's also a really good chance you have no idea what I'm talking about.
So what is a tin knocker anyway?
Have you ever heard of the term 'tin knocker'? There's a good chance you have come across the term somewhere along the line. Although, there's also a really good chance you have no idea what I'm talking about.
So what is a tin knocker anyway?
Although there is no real dictionary definition of the term, the accepted definition of a tin knocker is some one works with HVAC ductwork, a sheet metal worker. Meaning someone who fabricates duct and duct fittings for residential or commercial dwellings. Tin knocker can also be assigned to someone who installs ductwork on a regular basis. However, the term 'duct installer' is more common for the men and women working that job.
My daily job, along with co-owner duties, here at K & E Sheet Metal is HVAC ductwork fabricator or tin knocker. I fabricate the components used for the duct running through your home so heat and air conditioning can get to each of the rooms.
Our main business
Our main business is fabricating the plenums, duct, elbows and transitions needed to run the trunk line (ductwork) in the basement or attic. Many times the installer will need to run the duct under existing plumbing pipes or around a corner. Whether the piece needed is a standard fitting or a custom section, contractors or customers will call with measurements for what they need to finish the job. I, as an HVAC ductwork fabricator, build the duct to keep the job moving along.
Method to the madness
Well, in our shop it's a much simpler process than that of the larger, unionized shops knocking out ductwork for major projects around the world. Our facility takes an order from the customer--no job being too small--where I will either enter the dimensions in our computer system running the CNC plasma machine or start the cutting process by hand. Either with had tools or with our 52" wide shear, which is used to cut larger sections of duct.
After the pieces are cut, I will use our 8' sheet metal brake and other metal bending tools to form the duct fittings and custom sections one flat side at a time. Our ductwork is formed either in half sections or four different sides to create the square or box that will be the duct. This also gives me the chance to run each side through whichever roll forming machine is needed to create the joints necessary for assembling each component.
The assembly process is performed either with a hand held or air hammer, with the air hammer being the quickest form of assembly at our shop. Depending on size or type of fabrication, each piece can take anywhere from 10 minutes to 45 minutes.
Obviously. this is a very quick overview of what I do as an HVAC ductwork fabricator, but you can get a sense of the basic steps needed and where our services are very important to the installation process. In order to keep the air flow and efficiency at peak you need to have the proper sized duct and fittings to move the air. Therefore a good fabricator is a must for anyone who runs an HVAC installation company.
Keep the line moving sir
You can equate what I do to a short order cook, who needs to have something done quickly and accurately to keep the line moving. When an order is called in, the time window for me to finish is tight, roughly within 24 hours. With many orders coming through the door, this leaves hours to minutes for getting the fabrication complete, depending on the size of the order. All lead times aren't always a rush, some are weeks or even months down the line. Nevertheless, the majority of what we do in the residential market is a very quick turn around because time frames for many of the homes going up these days is fast.
Other factors that will determine what an HVAC ductwork fabricator produces on a daily basis is the size of his shop, amount of equipment, technological advancement of that equipment and number of employees fabricating ductwork.
The role of the duct fabricator hasn't changed much over the years, but the size of the shops have. There aren't many job shops like ours around these days giving homeowners and smaller companies the ability to get small orders fabricated on demand for a good price.
If anything, understand that an HVAC ductwork fabricator is an integral part of the construction of your home or commercial building.
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.
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. For some reason my immediate thought was urn or something like it. However, I was completely wrong, the customer was only looking for a box to rake old ash and embers from an outdoor grilling station.
I had a tiny idea of what he might be talking about, growing up in a house with a wood stove and many family members with the same. Nevertheless, asking for any documentation or drawings is always worth asking. I emailed a request for a picture or something he could draw to give me a starting point. Surprising to me, he had a picture of exactly what he needed.
Receiving a quality photo of what the customer needs is rare, with me usually coming up with something on the fly or deciphering a crude drawing on a scrap sheet of paper. This is a rare and extremely beneficial occasion.
The customer requested I do my best to keep it as waterproof as possible because the box would reside outside most of the time. My goal was to keep the simplicity of the box he had sent me thereby keeping the cost down. Needless to say this was not the most difficult of fabrications, but that doesn't mean I don't keep all customer requests in mind and do my best with each order.
I got to work and basically made the same box with a few minor adjustments to the original design with the biggest one being to the sliding cover. The extended handle was unnecessary, so I simply rolled a hem flange up with a 1/2" extension down to help keep any water from running down inside the box. You can see this depicted in one of the photos below.
The finished box is not completely waterproof mind you, but any chance to keep water out is a good thing and something I kept in mind. Hence the cross break on the cover, helping runoff of any water, keeping it from pooling on the top. There is also a 1/4" extension at the opposite end of the cover to help with water drainage.
My goal with any customer is to make sure they get what they're paying for no matter how simple the request may be. It's always fun to fabricate the more intricate items, but the simple pieces are the ones people remember because they usually solve a simple problem. Every customer is important.
Plus, they're the first ones to promote you when things come up in conversation.
Where Have All The Salespeople Gone?
Business is getting better. The fabrication shop has been very busy for the majority of the year. Yes, some of the production can be attributed to our new CNC plasma table, but not all. The business environment seems to be on the steady.
Where does my optimism come from?
Business is getting better. The fabrication shop has been very busy for the majority of the year. Yes, some of the production can be attributed to our new CNC plasma table, but not all. The business environment seems to be on the steady.
Where does my optimism come from?
Lack of salesman.
When the economy was tanking I noticed a big increase in the number of salespeople darkening my doorstep. Men and women from sheet metal, HVAC, you name it companies were lurking around on what seemed like a daily basis. Many of the companies I had never heard of because they were based outside our region.
So why would they be talking up the likes of me?
They were making the trip and extra effort because they needed new business in a big way. Any business would suffice, even the likes of our tiny company, which probably ordered a tenth of what their major customers were buying. They needed cash.
Fast forward to today and none of those people have come within 10 miles of our tiny little shop to hock their wares. Their business is doing fine (or maybe imploded :/) so tiny orders like ours are not worth their breath or gas miles. All my new best friends have suddenly disappeared.
One thing I have learned about our business since the economic crisis. Watching the flow of salespeople through the door can be a forward look at what may be to come. A canary in the coal mine if you will.
There are the regulars and the normal flows of new blood, but an abnormal increase can spell a slowing in business. The workload trickles down. If those bigger companies above us aren't doing well, then our drought will not be far behind.
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 :)
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 :)
Taking measurements
Today we received an order for a plenum with transitions needing to get from one measurement, to another and then back to yet another. Below is a crude drawing of the sizes needing to be transitioned.
In order to make the transitions and fit the AC coil, I will need to fabricate two transitions and a straight plenum, placed in the center, that will house the coil and have a door for servicing. The transitions can be entered into the computer system and cut with our new CNC plasma machine, while the straight box I will cut with tin snips.
Looking down inside the AC coil plenum at the shelf that will hold the coil
The fabrication
The bottom transition, 13 1/4 x 18 up to 17 1/4 x 19 3/4 will have a 1/2" flange on the new furnace side and S-lock on the top side that will accept the straight plenum box. Inside the plenum box will be a 24 gauge metal shelf (Picture to the right) to hold the AC coil inside the 17 1/4 x 19 3/4 box.
From the top of the AC coil box I will transition to the existing duct inside the customer's home. Basically, the old duct work leftover after tearing out the old furnace.
All being said the fabrication for this order sounds much harder than it actually is. Many times the hardest thing is translating what the customer is thinking into how my brain processes things. Once this hurdle is completed the process goes rather quick and easy. This particular order foe example ended up being two simple custom transitions with a straight AC coil box sandwiched in between.
Below is a photo of the finished plenum ready to be picked up and installed
