How to Install a Rectangular Trunk Line

This guide walks through a rectangular trunk line installation step by step, what the International Mechanical Code and SMACNA require at each stage, and the mistakes that cause rework. If you already have dimensions in hand, go ahead and submit your custom ductwork quote request.

Key Takeaways

• A rectangular trunk line is the main supply or return duct running from the air handler, with branches taken off it to serve individual rooms or zones. Installing one is a sequence, and the order matters: size, pressure class, route, assemble, seal, support.
• The International Mechanical Code requires duct joints, seams, and connections to be constructed to the SMACNA HVAC Duct Construction Standards – Metal and Flexible, and to be both securely fastened and sealed.
• Sealing is not optional on most systems. The code’s only relief is for ducts with a static pressure classification below 2 inches water column using continuously welded or locking-type joints — and that relief does not extend to snaplock or button-lock joints outside conditioned spaces.
• Hanger spacing comes from the SMACNA tables and varies with duct size and hanger type. There is no single correct spacing number, which is why supports should be selected from the table for your duct rather than from habit.
• Rectangular trunk sections are joined at transverse joints using a flat S cleat on the long sides and a drive cleat on the short sides.

What a Rectangular Trunk Line Is and Where It Sits in the System

A rectangular trunk line is the main duct that carries conditioned air away from the air handler, furnace, or rooftop unit, with branch ducts taken off it to feed individual rooms and zones. It is the backbone of most forced-air systems.

Rectangular is the standard choice wherever headroom or structure limits what a round duct can do — tight ceiling cavities, spans between floor joists, and wall chases. Round duct moves air with less friction, but it needs vertical clearance that a joist bay often does not have.

A complete trunk installation involves more than straight sections. Plan for plenum boxes at the equipment, offset transitions around obstructions, takeoffs at every branch, and an end cap where the run terminates. Duct2Go fabricates all of these in single-piece and multi-piece configurations from our rectangular duct collection, dimensioned to SMACNA standards so they mate with standard fittings from any manufacturer.

The Install Sequence at a Glance

Working out of order is the most common cause of rework on a rectangular trunk line. This is the sequence, with the governing reference for each stage.

Step 1: Confirm Duct Sizing Before Anything Is Cut

Never cut metal against an assumed size. The code names the sizing method: ducts within a single dwelling unit are sized in accordance with ACCA Manual D, the appliance manufacturer’s installation instructions, or another approved method, and ducts in all other buildings are sized per the ASHRAE Handbook of Fundamentals or an equivalent computation procedure.

An undersized trunk cannot be fixed downstream. Balancing dampers redistribute air; they do not create static pressure headroom that the duct geometry never had.

This is also the point to confirm the trunk reduces correctly along its length. A trunk that carries full airflow at the plenum and only a fraction at its far end should step down in size, which means transitions need to be on the material list before the first section is hung.

Step 2: Establish the Pressure Class and Seal Class

Set the pressure class before ordering, because it drives gauge, reinforcement, and how much sealing the run needs. A duct’s pressure classification must equal or exceed the design pressure of the system it serves, so you round up to the next class rather than down.

Where the designer has specified no pressure class, SMACNA treats 1-inch water gauge as the basis of compliance regardless of the velocity in the duct. On a residential or light commercial trunk that default is often correct, but it should be a decision rather than an accident.

Seal class follows from pressure class and defines which parts of the assembly get sealed — transverse joints only, transverse joints plus longitudinal seams, or those plus duct wall penetrations. Confirm both numbers before your trunk sections are fabricated, because gauge cannot be changed on site.

Step 3: Plan the Route, Elevation, and Obstructions

Walk the trunk line route and mark the elevation before hanging anything. Joists, beams, sprinkler mains, plumbing, and electrical all occupy the same cavity, and the trunk is usually the least flexible of them.

Mark branch takeoff locations on the route at this stage too. Cutting a takeoff into an installed trunk is possible, but it is far easier to position it while the section is on the ground, and the cut is cleaner.

Where the route has to shift laterally around an obstruction, that is an offset rather than an elbow, and it needs to be on the order. If the geometry does not match a standard part, send the dimensions to our custom fabrication team rather than field-modifying a stock fitting into place.

Step 4: Dry-Fit the Run Before You Fasten Anything

Lay the full trunk line out and dry-fit it before a single screw goes in. This is fifteen minutes that regularly saves half a day.

What you are checking is cumulative: whether the section lengths actually add up to the run, whether fittings land where the drawing says, and whether the last section reaches the end cap without a gap. Small errors compound across a long trunk.

Check orientation while you are there. Transitions and offsets have a direction, and a reducer installed backwards is an increaser. Standard 5-foot galvanized rectangular trunk sections make the arithmetic straightforward on typical runs.

Step 5: Assemble Transverse Joints with S Cleats and Drive Cleats

Rectangular duct sections join end to end with a flat S cleat on the long sides and a drive cleat on the short sides. This is the slip-and-drive joint, and it is the standard transverse connection for rectangular ductwork.

The S cleat slides over the mating edges of both sections and holds them aligned in the same plane. The drive cleat is then driven over the short sides, which pulls the joint tight and locks the S cleats in position. Both are supplied as straight stock and cut to length on site.

Step 6: Fasten and Seal Every Joint, Seam, and Connection

Mechanical fastening and sealing are two separate requirements, and the code asks for both. Joints, longitudinal and transverse seams, and connections must be securely fastened and sealed with welds, gaskets, mastics, mastic-plus-embedded-fabric systems, liquid sealants, or tapes.

Closure products carry listings that matter at inspection. Tapes and mastics for metallic and flexible duct are listed to UL 181B and marked 181B-FX for pressure-sensitive tape or 181B-M for mastic. Products for fibrous glass duct are listed to UL 181A and marked 181A-P, 181A-M, or 181A-H. Closure systems must be installed in accordance with the manufacturer’s instructions, which for most mastics means a specified wet film thickness rather than a thin smear.

There is one narrow exception worth knowing precisely, because it is widely misread.

Step 7: Hang and Support the Trunk Line

Ducts must be supported in accordance with the SMACNA HVAC Duct Construction Standards – Metal and Flexible. Hanger type, hanger material, and spacing all come from those tables, selected by duct size.

There is no single correct spacing figure, and this is worth saying plainly because published numbers vary widely. SMACNA sets spacing intervals against duct half-perimeter and hanger material, so a small trunk on light strap and a large trunk on trapeze hangers are governed by different rows of the same table. Work from the table for the duct in front of you.

Three principles hold regardless of spacing. Additional hangers belong near fittings, where load concentrates and geometry changes. Hangers attach to building structure, not to other ductwork, piping, or conduit. And the code’s broader requirement is that duct be braced and reinforced to provide structural strength and durability, which supports alone do not satisfy on larger runs.

Step 8: Cut In Takeoffs and Branch Connections

Cut takeoffs into the trunk line at the locations marked during routing, working on the ground wherever the sequence allows. A straight tap is the standard branch fitting on rectangular duct; where a round branch leaves a rectangular trunk, a takeoff collar or spin-in tap does the job.

Cut accurately. An oversized hole cannot be sealed reliably and becomes a permanent leak at exactly the point where the system is trying to divert air, which is also where turbulence is highest.

Round metallic duct connections are mechanically fastened with not less than three sheet metal screws or rivets spaced equally around the joint. Where the connection is partially inaccessible, three fasteners go on the exposed portion so the joint cannot form a hinge. Our HVAC duct fittings collection covers takeoffs, taps, elbows, and transitions built to match the duct they connect to.

Step 9: Connect the Trunk to the Plenum and Equipment

Duct connections to the flanges of air distribution equipment must be sealed and mechanically fastened. Both, not either — this is the joint that leaks most often, and it sits where the static pressure is highest.

The plenum is where the trunk meets the air handler, and it is the part most often built custom because its dimensions have to match the equipment outlet exactly. We fabricate supply and return plenum boxes to air handler outlet dimensions.

Clearance to combustibles at a furnace connection follows the furnace manufacturer’s installation instructions rather than a general rule. Check the specific appliance documentation before finalising the connection detail, because clearances differ between models.

Step 10: Install Balancing Dampers and Provide Access

Volume dampers or another means of supply air adjustment must be provided in the branch ducts or at each individual register, grille, or diffuser. Each damper used in balancing must be provided with access.

That access requirement is the one that gets missed. A damper sealed above a hard ceiling with no access panel cannot be adjusted at balancing, which means it may as well not be there.

Fit dampers at the branch rather than at the outlet wherever the layout allows. Throttling at the takeoff keeps the noise upstream of the room instead of generating it at the grille. Many takeoff collars ship with an integral damper for exactly this reason.

Step 11: Insulate and Control Condensation

Provisions must be made to prevent condensation forming on the exterior of any duct. On a cooling system running below the surrounding dew point, that means insulation with a continuous vapour barrier.

The vapour barrier is the part that does the work, and it fails at the details rather than in the middle of a run. Seams, hanger penetrations, and terminations are where moisture reaches the metal, and where the insulation later sags away from it.

Material choice interacts with this. Galvanized steel handles normal interior humidity well, but sustained condensation is a corrosion problem regardless of the metal — our galvanized, aluminum, and stainless steel guide covers where each material belongs.

Special Locations: Garages, Exterior Runs, and Ducts Near Earth

Several code rules apply to where the trunk line runs rather than how it is built, and each one changes the installation.

Duct passing through a wall or ceiling separating a dwelling from a private garage must be continuous sheet steel of at least 0.0187 inch, which the code identifies as No. 26 gauge, with no openings into the garage. Ducts must not be installed in or within 4 inches of the earth unless they comply with the underground duct provisions.

Ducts exposed to mechanical damage from vehicles or other causes must be protected by approved barriers, and any duct installed on the exterior of a building — including its linings, coverings, and vibration isolation connectors — must be protected against the elements.

Flexible Connectors: The 14-Foot Rule and Where They Cannot Go

Flexible air connectors are limited in length to 14 feet, and they must not pass through any wall, floor, or ceiling. Both limits are absolute, and both are commonly breached on retrofit work.

Flexible air ducts are a different product with different rules and are not limited in length. Connectors and ducts are frequently confused on site because they look similar on the reel, so check the listing on the product rather than assuming.

Common Rectangular Trunk Line Installation Mistakes

• Sealing only the joints you can reach. The code requires transverse joints, longitudinal seams, and connections to be sealed, and the seams above a trunk are exactly the ones that get skipped when the run is already hung.
• Supporting from the wrong thing. Hangers belong on building structure, not on piping, conduit, or another duct, and a hanger that shares a load path with a sprinkler line is a problem for both trades.
• Field-modifying stock parts to cover a measurement error. A cut-and-taped transition leaks, adds turbulence, and is invisible once insulated.

Ordering the right part from rectangular duct and fittings, or having it made to dimension, costs less than the airflow it saves. One practical note before you start: sheet metal edges and cleat ends are sharp enough to warrant cut-resistant gloves, and dry-fitting overhead is a two-person job on anything but a short run.

Conclusion

Installing a rectangular trunk line well comes down to sequence and to two requirements the code states plainly: every joint gets both mechanically fastened and sealed, and every duct gets supported to the SMACNA tables. Size it first, set the pressure class deliberately, and dry-fit before you fasten.

Duct2Go fabricates rectangular trunk sections, plenums, transitions, takeoffs, and end caps to order in Chandler, Arizona — configure standard sizes in the ductwork configurator or send us your dimensions and we will build to them.