19.1 Fabricating Rectangular & Round Ducts
Key Takeaways
- Trade Area B (sheet-metal ducts) is 9 percent of both the Class A and Class B trade exams; the outline splits rectangular under 2 feet, rectangular over 2 feet, and round.
- SMACNA HVAC Duct Construction Standards, Metal and Flexible, 4th Edition (2020) is the construction book: lookup pressure class (½, 1, 2, 3, 4, 6, or 10 in. w.g., positive or negative), largest rectangular side or round diameter, and transverse joint spacing, then read gauge and reinforcement.
- FBC Mechanical 2023 Section 603.4 sends metallic ducts to SMACNA; the dwelling-unit exception (Table 603.4) only covers ½-inch and 1-inch water gauge and dimensions through about 20 inches, so a 24-inch (2-foot) side is already a SMACNA-table problem.
- Round metallic joints need not less than three equally spaced screws or rivets (FBC Mechanical 603.4.1); tape or mastic alone is not the mechanical fastener.
- Negative-pressure return and exhaust try to collapse the duct; SMACNA negative rows often add reinforcement sooner than the matching positive-pressure row of the same class.
19.1 Fabricating Rectangular & Round Ducts
Trade Area B (Sheet Metal Ducts) is 9 percent of both the Class A and Class B trade exams. The CILB outline splits the shop work into rectangular ducts under 2 feet, rectangular ducts over 2 feet, and round ducts. Chapter 17 sized the duct with the Ductulator. This chapter builds it. The 2026 Air Conditioning CBT references that matter here are SMACNA HVAC Duct Construction Standards Metal and Flexible, 4th Edition (2020), SMACNA Fibrous Glass Duct Construction Standards, 8th Edition (2021), and Florida Building Code — Mechanical, 2023 Chapter 6. Kitchen grease duct is still NFPA 96 (2021) and FBC Mechanical Chapter 5 — that metal is not a 26-gauge snap-lock (Section 19.4).
Quick Answer: Pressure class is the static the shop builds to (½ through 10 in. w.g., positive or negative). Gauge and reinforcement come from the SMACNA tables using that class, the largest rectangular side (or round diameter), and joint spacing. Under 2 feet (24 inches) is typically light-gauge, S-slip and drive, Pittsburgh or snap-lock, little or no intermediate stiffener. Over 2 feet is standing seams, TDC/TDF, angles, and tie rods. Round uses hoop strength and lighter metal. FBC Mechanical 603.4 sends you to SMACNA except a dwelling-unit table that stops at about 20 inches and 1 in. w.g.
Why gauge is not a coil-line habit
Pearson VUE will hand you a width, a pressure class, and a joint spacing and ask which SMACNA row you are on. “We always run 26 gauge” is not an answer. Pressure class is a construction rating: ½, 1, 2, 3, 4, 6, or 10 in. w.g., positive or negative. It is not the blower’s nameplate external static. A 2-inch-class supply trunk on a 16-ton rooftop is built to 2 in. w.g. even if the fan is scheduled for 1.4 in. external static. Specs that say “SMACNA 2 inch” mean the 2 in. w.g. table, not 2-inch-thick metal.
Negative pressure (return and exhaust) tries to collapse the duct. SMACNA prints separate negative-pressure rows that often add reinforcement sooner than the matching positive row. Building a 36-inch return like a 36-inch supply because “it is the same size” is how you oil-can a ceiling.
FBC Mechanical 2023 Section 603.4 states that metallic ducts shall be constructed as specified in SMACNA HVAC Duct Construction Standards — Metal and Flexible. Exception: ducts in single dwelling units may use Table 603.4, which only goes to ½-inch and 1-inch water gauge and to about 20 inches of dimension. A 24-inch (2-foot) side, or any 2-inch class duct, is already off Table 603.4 and on the SMACNA book. That is why the CILB outline splits under 2 feet from over 2 feet.
SMACNA rectangular lookup, every time:
- Pressure class (½ through 10 in. w.g.), and whether the duct is positive or negative.
- Largest side of the rectangle — not the short side, not equivalent round diameter, not cfm.
- Transverse joint spacing (commonly 4, 5, 6, 8, or 10 feet of duct between joints).
- Read minimum galvanized gauge and the reinforcement code (joint type plus any intermediate stiffeners or tie rods).
The tables assume G-60 lock-forming galvanized steel (ASTM A653 / A924). Stainless or aluminum is a corrosion choice; you still have to meet the stiffness of that SMACNA row. Cross-breaking or beading flat panels adds stiffness but does not let you skip a required angle or tie rod.
Rectangular under 2 feet
A 20×12 trunk has a largest side of 20 inches — under 2 feet. At ½-inch or 1-inch class, the light-pressure SMACNA rows commonly land on 26 gauge, Pittsburgh or button-punch snap-lock longitudinal seams (Section 19.2), and S-slip and drive transverse joints (SMACNA T-1 / T-5 family) at 4- or 5-foot lengths, with no intermediate reinforcement. That is the residential and light-commercial coil-line product. Aspect ratio is still the Chapter 17 design limit (keep near 4:1); fabrication still has to close the lock on that 20-inch side. A 20×5 pancake (AR = 4) is under 2 feet on the long side and still a miserable friction and metal-area choice.
Worked example. A Class B qualifier in Polk County builds a 16×12 supply (largest side 16 in.) at 1 in. w.g. positive, 5-foot joints. Open SMACNA 4th 2020 to the 1 in. w.g. rectangular table. Find 16 inches, the column for 5-foot spacing, and read the gauge and whether a reinforcement letter is required. You do not jump to 18 gauge because “commercial always uses 18.” You do not use FBC Table 603.4 if this is a tenant-fit strip center — that table is the dwelling-unit exception.
Rectangular over 2 feet
A 36×12 trunk has a largest side of 36 inches — over 2 feet. At 2 in. w.g., the same 5-foot joint spacing now demands heavier gauge and/or standing joints and intermediate reinforcement. Typical hardware once you cross 24 inches at commercial pressure:
- Standing S-slips, bar slips, or TDC/TDF (SMACNA T-25a / T-25b) four-bolt companion flanges with gasket.
- External angles on the wide sides, spaced per the table.
- Tie rods (internal) on very wide ducts, with SMACNA washers so the rod does not become a dirt dam or a liner snag.
Failure mode on a 36-inch unreinforced 26-gauge panel at 2 in. w.g. is oil-canning, seam pop, and collapse on the negative side. The 2-foot line on the exam is this stiffness jump, not a license-class line. Class A and Class B both fabricate over-2-foot duct; Class B is still capped at 25 tons / 500,000 Btu per system (F.S. 489.105(3)(g)), but a 16-ton 32-inch main is Class B work that still uses the over-2-foot table.
Negative 2 in. w.g. on a 40-inch return is often one table heavier than the matching supply. Exhaust that is not grease duct still follows SMACNA HVAC tables. Type I grease duct is FBC 506.3 / NFPA 96: 16-gauge steel or 18-gauge stainless, continuous liquid-tight weld — not a 26-gauge snap-lock (Section 19.4).
| Lookup | Under 2 ft (largest side ≤ 24 in.) | Over 2 ft (largest side > 24 in.) |
|---|---|---|
| Typical ½–1 in. w.g. positive | 26 ga, Pittsburgh or snap-lock, S-slip and drive, often no intermediate stiffener | Standing S or TDC, possible angles; do not keep the residential recipe |
| Typical 2 in. w.g. | May still be light if the side is small — read the table | Heavier gauge and/or standing joints, angles, tie rods |
| Negative of the same class | Check the negative row; collapse is the failure | Negative row almost always stiffens sooner |
| FBC Table 603.4 | Dwelling-unit exception only, and it stops near 20 in. / 1 in. w.g. | Off the dwelling table; SMACNA 4th 2020 |
Round ducts
Round duct has the least perimeter per area, so the least metal and the least leakage for a given cfm. Hoop strength lets SMACNA round tables use lighter gauge than a rectangular duct of the same capacity and pressure class. A 14-inch spiral supply at 2 in. w.g. is a different row from a 14×14 rectangle.
| Round product | How it is made | Exam use |
|---|---|---|
| Spiral lockseam | Helical lockseam from coil | Default commercial round; low leakage; listed pressure class |
| Longitudinal snap-lock / grooved | Pipe machine, snap or grooved long seam | Light commercial and residential; pressure class limited compared with spiral |
| Welded longitudinal | Shop-welded pipe | High pressure; kitchen grease uses the 16/18-gauge welded rule, not HVAC spiral |
Joints: crimp-and-bead (male crimp into the female bead) or couplings. FBC Mechanical 603.4.1: round metallic ducts shall be mechanically fastened with not less than three sheet-metal screws or rivets spaced equally around the joint. If the joint is partly inaccessible, three fasteners go on the exposed portion so the pipe cannot hinge. Tape or mastic alone is not the mechanical fastener. Seal the joint after it is screwed (Section 19.2).
Worked example. 1,200 cfm on 12-inch round at about 1,500 fpm. The shop builds spiral, 2 in. w.g. class to match the air handler, bead-crimp joints, three screws, then Seal Class A mastic. Substituting 12-inch flex for that trunk is not SMACNA metal construction. Flex is a listed UL 181 connector, fully extended, for short runouts — Chapter 24 hangs it; this chapter does not fabricate it.
Sheet metal vs fibrous glass vs flex
Sheet metal (galvanized, aluminum, stainless) is the SMACNA 4th 2020 metal book. Fibrous glass duct board is the 8th Edition 2021 fibrous-glass standard: rigid board, UL 181 Class 1, closed with 181A-P / 181A-M / 181A-H, limited pressure (commonly 2 in. w.g. class) and velocity. Flexible duct is listed UL 181B. The shop does not “fabricate” flex. Duct board is not interior-lined metal (Section 19.2). FBC Energy Conservation 2023 still wants attic metal R-8 (3 inches diameter and up) and a seal; construction class and insulation class are different rows on the takeoff.
Florida HVAC scenario
Gulf Coast Mechanical, a certified Class B shop in Lee County, takes a 16-ton (under 25-ton) strip-center job. Drawings say 2 in. w.g. supply and 1 in. w.g. return. The estimator prices every rectangle as 26 gauge, 5-foot S-slip and drive because that is what the residential coil line makes. The 18×12 runouts (under 2 feet) at 1 in. w.g. return may actually be 26-gauge unreinforced. The 32×16 supply main (largest side 32 in. — over 2 feet) at 2 in. w.g. is not. SMACNA 4th 2020 wants a heavier row: standing joint or TDC, possibly tie rods. Round 18-inch spiral for the same cfm would drop metal weight and leakage. Class B may fabricate all of this — Trade B is 9 percent on both outlines, and 16 tons is inside 489.105(3)(g). The failure is using the under-2-foot recipe on an over-2-foot pressure-class table.
Traps: (1) Reading FBC Table 603.4 on a commercial 2-inch-class trunk. (2) Using the short side of a 36×10 pancake as the lookup dimension. (3) Treating 24 inches as “still residential.” (4) Fastening round pipe with tape only. (5) Building negative-pressure return like positive-pressure supply of the same size.
A 32×16 supply main is specified SMACNA 2 in. w.g. positive with 5-foot joints on a 16-ton Florida strip-center job. Which construction statement is correct?
Which statement about round metallic duct joints on the Florida trade exam is accurate?
FBC Mechanical 2023 Section 603.4 tells a Florida HVAC contractor to construct metallic ducts to which standard, and when may the dwelling-unit thickness table be used instead?