We frame our ADUs with light-gauge cold-formed steel — engineered, precision-made, and built for earthquake country. Here is how it works, why it builds faster and stands stronger, and the honest truth about what “Type II” does and doesn’t mean.
Most backyard cottages and garage conversions going up in California right now are wood boxes, framed the same way houses were framed a hundred years ago. At Mars Construction Industries, we frame our ADUs with steel — light-gauge cold-formed steel that is engineered and precision-made before it ever reaches your backyard. This page walks through exactly how that process works, why it builds faster, stands stronger, resists fire, and sips energy compared to a conventional wood frame — including the honest parts most builders would rather not explain.
Watch: our 8-minute walkthrough of the steel process — and what “Type II” really means.
How a Steel ADU Frame Is Made
Every wall, floor, and roof member starts as galvanized sheet steel that is roll-formed at room temperature into studs, tracks, and joists, cut and punched to the exact dimensions your plans call for. Engineers design every member to the AISI standard — AISI S100, the national design standard for cold-formed steel — with light-frame construction per California Building Code Section 2211 and AISI S200/S220. The connections are engineered too: screws, steel-sheet shear walls, and strap bracing form a complete lateral system that California recognizes for earthquake country, designed to AISI S400 in higher seismic design categories.
The components arrive on site ready to assemble, and our crews put the frame together the way it was drawn, piece by piece and panel by panel, until the structure standing in your backyard matches the engineered plans line for line. Nothing about it is improvised, and nothing depends on the luck of the lumber pile.
Why Steel Builds Faster
That precision is exactly why steel builds faster. A roll-formed steel stud comes off the line straight and true, and it stays that way, so our crews never burn hours sorting through a stack of warped, twisted, and crowned lumber looking for the few good pieces. Steel does not swell when it gets wet, and it does not shrink as it dries, so a wall framed on Monday is the same wall on Friday, in the rain or in the heat. The members are light enough to handle easily, and they go together with engineered screw connections, quickly and repeatably. Because every piece arrives the size the plans promised, the framing flows instead of fighting us at every step — straighter material means fewer surprises, and a frame that goes up the way it was drawn is a frame that finishes sooner. That is where the speed comes from.
Stronger: Earthquakes, and Everything That Eats Wood
In California, strength starts with earthquakes. A steel frame is light, and lighter buildings feel less force when the ground moves. Steel itself is ductile, which means it can flex and bend without snapping. Our frames are engineered as a complete system, with steel-sheet shear walls or strap bracing designed to the AISI seismic standard, so the building has a real, code-recognized path for earthquake forces to travel safely into the ground.
Then there is everything that quietly destroys a wood frame over the years. Steel does not rot. Termites cannot eat it, because there is nothing in it for them to eat, and mold finds nothing to feed on either. The frame that goes up straight stays straight, year after year, without the slow decay that wood invites from day one.
Fire: What “Type II” Actually Means
This is where steel really separates itself — and where you will hear the words Type II. Type II is not a product, and it is not a brand. It is a fire classification in the California Building Code, Section 602.2, and it describes a whole building whose structural frame, bearing walls, floors, and roof are all built from noncombustible materials — the elements listed in Table 601. Type II-A means those elements are also protected to a one-hour fire-resistance rating. Type II-B means the same noncombustible construction without the hourly rating. Steel passes the noncombustible test perfectly, because steel does not ignite, it does not burn, and it adds zero fuel to a fire.
Now the honest part, the part we will always tell you straight: noncombustible does not mean fireproof. Steel loses strength as it gets hot, giving up roughly half of it around 1,000 degrees. That is exactly why the protected rating in Type II-A has to be earned with real protection, like spray-applied fire-resistive material, intumescent coatings, or gypsum and concrete encasement. We would rather explain that honestly than sell you a slogan.
The Energy Envelope: Continuous Insulation and Title 24
Steel also wins on energy, as long as it is detailed right — and here again the details matter. A steel stud conducts heat, so a steel wall insulated only between the studs will leak energy straight through the framing, a problem called thermal bridging. We build the envelope the way California’s energy code, Title 24, Part 6, rewards: with continuous insulation wrapped over the outside of the steel frame, so the thermal bridge is broken and the whole wall performs as one tight, efficient envelope. The result is an ADU that holds its heating and cooling, feels comfortable in every season, and costs less to run month after month.
The Foam-Core Panel Warning
One warning that could save you from a costly mistake. Some systems on the market wrap thin steel skins around a core of foam plastic and market the result as Type II steel construction. It is not, and here is why: the foam core is a combustible material, and steel’s fire performance does not transfer to it. Foam is allowed in this kind of construction only as insulation, under its own chapter of the code (Chapter 26) — never as the structural core of the building.
If a system truly earned a Type II classification, there would be an evaluation report saying so, issued in the manufacturer’s own name. Ask for that report by name, and if it does not exist, the claim does not either. We tell you this because we would rather earn your trust with the truth than win a job with a label.
So Is a Steel ADU “Type II”? The Honest Answer
One more piece of honesty. A typical detached house or ADU in California is permitted under the California Residential Code, which covers steel framing under its own rules (Section R603) — so a steel ADU does not automatically carry a Type II label. What steel gives you is a frame made from a material that can meet that commercial-grade fire standard when the whole building is detailed for it, and a structure that contributes nothing to a fire in the meantime. That is the steel advantage as we build it: a frame that is engineered and precision-made, that goes up faster because every piece arrives straight and true, that stands stronger in an earthquake and shrugs off rot, termites, and mold, that adds no fuel to a fire, and that wraps your home in a tight, efficient envelope. More answers on our ADU FAQ page.
Tell us about your lot and what you want to build — we will check feasibility, walk you through the steel framing for your project, and map out the cost and timeline. Fill out the contact form or call or text us — free consultation, no obligation.
Call or Text 888-627-1131 Get a Free ConsultationGeneral information about the California Building Code (Sections 602.2 and 2211, Table 601), the California Residential Code (Section R603), and the California Energy Code (Title 24, Part 6), not legal or engineering advice. Whether a specific building carries a Type II classification is determined element by element by the local building department under the code edition in force; we confirm what applies to your project during feasibility.
