Steel vs wood modular: how to choose

Most modular buildings in North America are framed with wood or with steel, either light-gauge steel studs or heavier structural steel frames. The choice affects height limits, cost, which plants can build your project, and how the building behaves in transport. Here is how to think it through.

Start with the building code

The International Building Code sorts buildings into five construction types, from Type I to Type V, based mostly on whether the structure is combustible and how fire-resistant it is. ICC's training material on the 2021 IBC explains that Types I and II require noncombustible materials for the structural frame, walls, floors and roofs, while Type V allows any materials permitted by the code, including wood. The details are in IBC Chapter 6.

The type you need depends on the building's use, height and floor area. Wood-framed buildings generally fall under Type V (or Type III with noncombustible exterior walls) and face lower height and area limits than noncombustible types. Taller buildings usually push you toward steel or concrete.

So the first question isn't "steel or wood?" It is "what construction type does this building need?" Your architect or code consultant should answer that before you talk to manufacturers. Mass timber and other newer options have their own rules, so ask your code consultant whether they apply.

Wood modular

Strengths:

• Many plants build in wood, so you have more bidders.
• Wood framing is familiar to designers, inspectors and site crews.
• It is often the lower-cost option for low-rise buildings.
• Wood modules are lighter than comparable steel modules, which can help with crane size and trucking.

Watch for:

• Height and area limits under the code type.
• Moisture during transport and setting. Wet framing and drywall cause problems later, so weather protection matters.
• Fire separations and fire-stopping at module joints, which need careful detailing.

Steel modular

Steel modules come in two broad forms: light-gauge steel stud framing, often made with roll-forming machines that cut and label every member, and heavier structural steel frames with welded or bolted corner posts and beams.

Strengths:

• Noncombustible, which opens up taller buildings and construction types that wood can't meet.
• Stiff frames that handle lifting and transport well.
• Dimensional consistency. Steel doesn't shrink or warp with moisture.

Watch for:

• Fewer plants, so fewer bidders and potentially longer lead times.
• Heavier modules, which can mean bigger cranes and tighter trucking limits.
• Thermal bridging. Steel conducts heat, so insulation and detailing matter for energy code compliance.
• Corrosion protection at connections and anywhere exposed during transport.

The tallest modular projects tend to use steel. The AC Hotel New York NoMad, a 26-story tower, used prefabricated guestroom modules. The contractor, Skystone, said the rooms were steel modules built in a factory in Poland.

Hybrids

Many projects mix materials: a concrete or steel podium with wood modules above, steel modules with wood-framed infill, or light-gauge steel walls with other floor systems. The right mix depends on code type, cost and available plants.

How to decide

• Get the construction type first. Height, use and area set the options.
• Check which plants can build that type within reasonable shipping distance, and whether they are approved in your state. MBI's state program directory is a good starting point.
• Price both where both are allowed. For low-rise buildings, compare wood and steel bids on the same scope, including crane and transport.
• Ask about fire testing and listings for the assemblies at module joints, not just for the walls and floors.
• Consider moisture risk. If your site or season is wet, ask how each plant protects framing and drywall through transport and set.

Questions for a manufacturer

• Which construction types and heights have you built under the IBC?
• Are your fire-rated assemblies tested or listed, and can I see the listings?
• What does a typical module weigh, and how does that affect crane and trucking?
• How do you protect modules from moisture or corrosion in transit?

Bottom line

For low-rise projects, wood is usually the default and often the lower-cost choice. For taller or noncombustible buildings, steel is often required. Let the code decide what's allowed, then let bids and logistics decide the rest.

Acoustics and comfort

Steel and wood frames behave differently for sound and vibration. Both can meet code requirements for sound transmission between units when the assemblies are designed and built correctly. Ask any manufacturer for the tested ratings of the wall and floor assemblies they propose, and for how they detail the joints between modules, since flanking sound through joints often matters as much as the walls themselves.

Whichever frame you choose, ask to see a finished building from that plant in the same material. The way joints, fire-stopping and finishes look after a few years of use tells you more about a manufacturer than any spec sheet.