Sheet Metal Fabrication

In-house. The shop cuts, forms, seals and labels rectangular and round duct, fittings, plenums, transitions and curb adapters for our own projects. The practical difference shows up mid-job: when a field condition does not match the drawing, we remake the piece on our own schedule instead of joining an outside shop’s queue behind three other contractors.

It is set by the largest dimension of the duct and the pressure class it has to hold, not by preference. A 12 in. trunk at low pressure is comfortable in 26 gauge; a 60 in. duct at 2 in. water gauge wants 18 gauge with angle reinforcement and tie rods or it will drum and flex every time the fan starts. The table above covers the common combinations. Where a specification calls out a class we build to it and note it on the shop drawing.

A Pittsburgh lock is a folded pocket seam: one edge is formed into a pocket, the other is inserted and the pocket is closed over it. It holds mechanically, so it does not rely on sealant to stay tight, and it is the normal longitudinal seam for rectangular duct. A button-punch snap lock is quicker and fine on lighter gauge at lower pressure. On round and spiral, the seam is rolled continuously along the pipe. Seam choice matters most on long runs and higher pressure, where a marginal seam leaks in a hundred places at once.

Three classes are in common use. Class C seals transverse joints. Class B seals transverse joints and longitudinal seams. Class A seals both plus every penetration of the duct wall — hanger points, taps, damper shafts, access doors and test ports. Higher pressure and longer runs need class A, because leakage on a long duct compounds and the terminal at the end of the run never sees its design airflow. We seal to the class called for and record it on the shop drawing.

Yes. Stainless for kitchen, dishwasher, laboratory and process exhaust, aluminium for pool rooms and humid runs, and welded black iron for grease duct behind commercial kitchen hoods — welded liquid-tight, pitched back toward the hood, with access at the required points.

Routinely — it is one of the most common single items we build. A technician measures the existing curb opening and the new unit’s supply and return connections, and the adapter is formed to bridge them cleanly with insulated internal divisions so supply and return do not short-circuit inside it. A sloppy adapter is a permanent air leak sitting directly under the unit, which is exactly where it does the most damage.

Talk to us. The shop is sized for our own project work, so our jobs come first, but we do take fabrication packages for general contractors and mechanical firms we work with, particularly on fittings, transitions, plenums and adapter work that a supply house will not carry. Send the drawings or the field dimensions and we will tell you honestly whether the schedule works.

As complaints that never resolve. The zone at the end of the run stays warm no matter what the thermostat says, the fan runs harder than the design called for, and the tenant nearest the air handler is always cold. On return duct in a ceiling plenum it also pulls unconditioned air out of the ceiling cavity, which is how a building gets a musty smell in July that no amount of filter changing fixes. Sealing and static pressure testing find it; guessing does not.