When is custom sheet metal ductwork required instead of standard products in California?

Custom sheet metal fabrication is required or strongly preferred when: (1) the duct geometry doesn't fit standard sizes or offsets (irregular-shaped spaces, historic building retrofits, architectural exposed ductwork); (2) the duct pressure class requires heavier gauge than standard catalogue products (2-inch and 3-inch static pressure classes, high-velocity VAV systems); (3) seismic Zone D requirements necessitate heavier construction or custom bracing that catalogue products don't accommodate; (4) grease duct applications requiring 16-gauge welded carbon steel or 18-gauge welded stainless per NFPA 96; (5) high-temperature applications (kitchen exhaust above 400°F); or (6) the project timeline doesn't accommodate standard product lead times.

What gauge does SMACNA require for each pressure class?

SMACNA HVAC Duct Construction Standards (4th edition) gauge requirements for galvanized steel rectangular duct by pressure class: 0.5-inch static (±0.5" w.g.): 26-gauge minimum for sizes up to 12"; 24-gauge for 13"–30"; 22-gauge for 31"–60". 1-inch static class: 24-gauge up to 12"; 22-gauge for 13"–30"; 20-gauge for 31"–60". 2-inch static class: 22-gauge up to 12"; 20-gauge for 13"–30"; 18-gauge for 31"–60". 3-inch static class: 20-gauge up to 12"; 18-gauge for 13"–30"; 16-gauge for 31"–60". Reinforcement and tie-rod requirements are additive to gauge requirements at larger duct sizes.

What is the typical fabrication lead time for custom ductwork in California?

Fabrication lead times for custom commercial ductwork in California depend on shop workload and project complexity: Standard rectangular duct in common sizes and 0.5- to 1-inch pressure class - 1–2 weeks from approved shop drawings in a well-staffed shop. Complex geometry, large high-pressure fittings, or high-volume projects - 2–4 weeks. Welded grease duct (16-gauge carbon steel or 18-gauge stainless) - 2–3 weeks for standard runs; 3–5 weeks for complex systems with multiple changes of direction. For comparison, standard catalogue duct products (spiral pipe, flexi duct, standard rectangular from a regional distributor) typically ship 3–10 business days - but they require standard sizes and shapes that custom work does not.

When is spiral duct preferred over rectangular duct on a California commercial project?

Spiral duct is preferred over rectangular duct when: (1) the duct runs are long and straight (spiral duct has lower leakage rates, fewer joints, and lower installed cost per linear foot for straight runs); (2) the architectural design calls for exposed ductwork with an industrial or modern aesthetic; (3) the pressure class is 2-inch or higher (spiral duct maintains round section integrity better than rectangular under high pressure); (4) the installation team is primarily mechanical mechanics rather than sheet metal workers (spiral connects with slip-and-drive fittings that require less sheet metal expertise). Rectangular duct is preferred when: ceiling cavity space is tight in one dimension; fittings must be custom to accommodate irregular building geometry; or the duct serves as a structural plenum (rectangular better integrates with ceiling grid).

How does an in-house C-43 fabrication shop reduce GC schedule risk?

An in-house C-43 sheet metal shop under the same company as the C-20 mechanical contractor eliminates the coordination risk between a separate fab house and the installation team. With in-house fabrication: design changes during construction can be incorporated into the fab queue immediately without a change-order delay to a separate vendor; rush fabrication is possible when field conditions reveal scope changes; the fab shop foreman communicates directly with the field foreman on fitting geometry; and the contractor is not dependent on a third-party fab house's workload or delivery schedule. For GCs on fast-track TI projects, the ability to call for a custom fitting on Day 1 and have it on the job site on Day 3 is a significant schedule advantage.

Fabrication · Sheet Metal · Published 2026-06-03

Custom sheet metal ductwork fabrication for California commercial projects: when shop fabrication beats standard products

Standard catalogue ductwork - spiral pipe, standard rectangular sections, flexible duct - covers the majority of California commercial HVAC installations. But a significant category of projects requires custom shop fabrication: high-pressure VAV systems, restaurant grease duct, architecturally exposed ductwork, seismically braced high-bay systems, and any project where the building geometry doesn't accommodate standard sizes. This guide covers when custom fabrication is required, SMACNA gauge selection by pressure class, the rectangular vs. spiral vs. flat-oval decision, and how Sierra's in-house C-43 shop eliminates the schedule risk of a separate fab vendor.

Sheet metal fabrication shop welder fabricating custom commercial ductwork in California — Sierra Mechanical's in-house C-43 fabrication shop - custom ductwork fabricated on-site means design changes can be incorporated same-day, without a separate vendor's lead time.

When custom fabrication is required or strongly preferred

The decision to use custom shop fabrication rather than catalogue duct products is driven by one or more of the following conditions:

Pressure class requirements

Most standard catalogue rectangular ductwork is manufactured to 0.5-inch or 1-inch static pressure class. High-velocity VAV systems, pressure-independent terminal boxes, and complex multi-floor duct systems may require 2-inch or 3-inch pressure class, which demands heavier gauge than catalogue standard. At 2-inch static class and above, custom-fabricated ductwork is typically faster and more cost-effective than sourcing specialty high-pressure catalogue sections from limited-stock distributors.

Grease duct applications

NFPA 96 § 7.5 requires Type I grease exhaust duct to be 16-gauge carbon steel or 18-gauge stainless steel, continuously welded liquid-tight. This is custom fabrication by definition - no standard catalogue product satisfies this requirement. All grease duct for commercial kitchen exhaust is shop-fabricated and welded. For a complete guide to grease duct construction requirements, see our NFPA 96 grease duct guide.

Unusual building geometry

Historic building retrofits, buildings with non-standard ceiling heights, angled ceilings, sloped roofs penetrated by ductwork, and buildings with unusual structural bay dimensions all generate duct geometry that standard catalogue fittings cannot accommodate. Custom fabrication handles any angle, any offset, any transition between sizes - standard fittings do not.

Architectural exposed ductwork

Exposed ductwork in office TIs, restaurant dining rooms, and industrial-aesthetic retail spaces requires a level of finish quality that catalogue ductwork - designed for concealed installation - doesn't consistently provide. Custom-fabricated exposed ductwork uses higher-quality galvanized or stainless steel, tighter tolerances, and visual connections (flanged rather than slip-and-drive) that look intentional in an exposed ceiling environment.

Seismic bracing in California SDC D/E zones

California's Seismic Design Categories D1, D2, and E - which cover most of the densely developed urban California - require seismic bracing on duct systems above certain size and weight thresholds. SMACNA's Seismic Restraint Manual and ASHRAE Standard 171 set requirements for hanger spacing, lateral bracing intervals, and sway bracing on heavy duct sections. In some high-seismic applications, the duct gauge and connection must be upgraded from standard to provide the structural integrity the seismic bracing requires. Custom fabrication of the duct and the seismic bracing frame is often the cleanest solution.

SMACNA gauge selection table by pressure class

SMACNA HVAC Duct Construction Standards, 4th Edition, sets minimum gauge requirements for galvanized steel rectangular duct by pressure class and duct width. These are minimum requirements - heavier gauge is always permissible and sometimes specified for noise, structural, or longevity reasons:

Pressure Class	Up to 12" wide	13"–30" wide	31"–60" wide
0.5" static (±0.5" w.g.)	26 ga.	24 ga.	22 ga.
1" static class	24 ga.	22 ga.	20 ga.
2" static class	22 ga.	20 ga.	18 ga.
3" static class	20 ga.	18 ga.	16 ga.
4" static class	18 ga.	16 ga.	16 ga. + reinforcement

Note: Reinforcement requirements (tie-rods, standing seam ribs, intermediate frames) are additive to gauge requirements at larger widths. Full SMACNA tables also include joint type, hanger spacing, and bracing requirements. For the complete SMACNA duct pressure class guide for California, see our companion article.

Rectangular vs. spiral vs. flat oval: application guide

Rectangular duct

Best for: ceiling cavities with limited height clearance (the aspect ratio of rectangular duct allows it to fit in shallow cavities where round duct cannot); architectural exposed ductwork requiring clean lines and flush connections; ductwork that must integrate with structural steel or concrete deck (rectangular bolts against structure cleanly); and fittings with complex geometry that don't map to circular cross-sections.

Disadvantages: more joints and connections per linear foot than spiral, higher leakage potential, higher installed labor cost.

Spiral duct

Best for: long straight runs in open-ceiling warehouse, industrial, or retail applications; high-pressure systems where the circular section maintains structural integrity better than rectangular; architecturally exposed modern/industrial aesthetic; and projects prioritizing lower air leakage (spiral has fewer transverse joints). Spiral duct installation is typically faster per linear foot than rectangular on straight runs.

Disadvantages: requires more ceiling height; radius fittings require more space than rectangular elbows; spiral is not ideal for tight multi-directional changes in confined ceiling cavities.

Flat oval duct

Flat oval is a hybrid: the cross-section is an oval (flat on top and bottom, rounded sides), providing better aspect ratio than round for ceiling cavities while maintaining the structural integrity and leakage performance closer to spiral. Flat oval is increasingly specified for high-performance office and healthcare HVAC where acoustic performance and low leakage are priorities. It is more expensive than rectangular but has better acoustic performance and lower leakage.

Cost premium for custom fabrication

Custom shop-fabricated ductwork typically costs 20–40% more per pound of steel than catalogue standard ductwork at the same pressure class. However, the cost comparison must account for:

Installation labor savings: Custom-fabricated fittings that are dimensioned to the actual field conditions install faster than standard fittings that require field modification. The labor savings can offset 30–50% of the fabrication premium on complex fitting-heavy jobs.
Schedule value: On a fast-track project, the ability to have custom ductwork fabricated and delivered in 5 business days (vs. waiting for a standard catalogue order) has schedule value that typically exceeds the fabrication cost premium.
Rework avoidance: Catalogue fittings that don't quite fit the field geometry are modified in the field - usually with tin snips, Vise-Grips, and duct tape, none of which provide a code-compliant connection. Custom-fabricated fittings that fit are more reliable, have fewer leakage points, and don't require field modification.

How Sierra's in-house C-43 shop reduces GC schedule risk

Sierra Mechanical holds both a C-20 (HVAC and Refrigeration) and C-43 (Sheet Metal) license, with an in-house fabrication shop in Rancho Cordova. The practical benefits for GCs:

No third-party fab house coordination: Design changes that emerge during construction - field conflicts, structural obstructions, owner-requested modifications - go directly to the Sierra fab shop. Same-day or next-day response on standard fittings.
Integrated estimating: Because the same company estimates and fabricates, there is no disconnect between the bid scope and the shop drawings. Fabrication quantity takeoffs are done by the estimating team that understands the installation sequence.
Welded grease duct in-house: We fabricate 16-gauge carbon steel and 18-gauge stainless grease duct in our shop to NFPA 96 § 7.5 spec, with full TIG capability for stainless. Grease duct fabricated by a separate vendor and delivered to the field for installation by a different crew is a common quality-control gap; in-house fab eliminates it.
Schedule certainty: When a GC puts Sierra on a project, the fab shop capacity is reserved for that project. There is no risk of a separate fab house prioritizing another customer's order over yours.

For our services overview including sheet metal fabrication scope, and for project inquiries, contact our bid desk with your project address, scope description, and bid date.

Working with Sierra Mechanical on custom ductwork projects

Sierra Mechanical's C-43 fabrication shop has been producing custom commercial ductwork since 1996, including grease duct for QSR and full-service restaurant rollouts, high-pressure VAV duct for multi-story office TIs, architecturally exposed spiral systems for industrial-aesthetic retail, and seismically braced high-bay duct for warehouse and industrial applications. We self-perform design, fabrication, and installation under a single subcontract to GCs across California.

Bid desk: (916) 638-8605. Request a bid online with your project scope and schedule requirements.

References: SMACNA HVAC Duct Construction Standards (4th edition); NFPA 96 (2024 edition) § 7.5; SMACNA Seismic Restraint Manual for HVAC (2nd edition); California Mechanical Code (2025). Information current as of 2026-06-03.

This article is general guidance. Gauge requirements are from SMACNA tables and are minimums only; engineer of record may specify heavier gauge. Consult your licensed mechanical engineer for project-specific duct specifications.