What a volumetric module actually is, and why the road decides its shape.

Start with a definition, because most confusion about modular construction begins with the word. In the taxonomy used by the UK's MMC framework and adopted in Canada by the University of New Brunswick's Off-site Construction Research Centre, there are four broad kinds of off-site construction: volumetric, in which three-dimensional room-sized modules leave the factory structurally complete and largely finished; panelised, in which flat wall, floor and roof panels are shipped and assembled into volumes on site; hybrid, which mixes the two, typically volumetric wet rooms inside a panelised or conventional frame; and component or sub-assembly, meaning factory-made parts such as roof trusses, bathroom pods or façade cassettes.^1,2

Four kinds of off-site

These are different businesses. Panelised construction moves framing indoors and leaves most finishing on site; it ships densely and tolerates irregular buildings. Volumetric construction moves nearly everything indoors and ships air — a module is mostly empty space — which is why its economics are so sensitive to distance and why its buildings want to be regular. Hybrid schemes try to capture volumetric's advantage where it is strongest, in the labour-dense, tolerance-critical kitchen and bathroom, while leaving the open-plan spaces to cheaper methods.

This site's focus is the first category, and when we say "modular" without qualification we mean volumetric. When a market report or a politician says "modular," it is worth asking which of the four they mean, because the answer changes the factory, the truck, the crane and the cost.

The road sets the dimensions

A volumetric module's dimensions are not an architectural decision. They are a transport decision, made by whichever highway authority the truck must cross. Z Modular, the Zekelman Industries steel-module business, puts the practical width ceiling around 15 feet 9 inches, describes 14 feet 6 inches as the optimal width for efficiency, and notes that modules narrower than about 12 feet 6 inches become inefficient — you are paying to ship a box that does not hold much room.³ Length can reach 72 feet, though around 68 feet is preferred to leave room for vertical mechanical chases at the module ends.³ Beyond roughly 30 feet of depth, interior daylighting suffers, which is why modular floor plates are long and thin.³

Width above about 14 feet generally means oversize permits; above 16 feet, escorts, route surveys and, in some jurisdictions, night moves.⁶ Height is bounded by bridge clearances. Weight is bounded by axle limits. Every one of these is a cost that a site-built building never incurs, and every one is locked in the moment the design team picks a module width. The good modular architects design the building from the truck backward.

How the line is sequenced

A volumetric factory is organised as a line of stations, and the sequence is remarkably consistent across manufacturers even though no single published source enumerates it identically. Synthesising process descriptions from Factory_OS, Z Modular and MBI, the module typically proceeds as follows: a floor cassette is framed and decked on a jig; wall panels, built on separate tables, are stood and fixed to it; a ceiling or roof cassette closes the box; mechanical, electrical and plumbing rough-in runs through the open framing while it is still accessible from both sides; insulation, drywall, finishes, cabinetry and fixtures follow; and finally the module is wrapped for weather and transport.^5,9

Two things about this sequence matter for anyone evaluating a factory. First, the MEP station is where most of the labour value and most of the quality risk sits, and it is where factory conditions — working at bench height on a wall that is lying flat, with the drawings on a screen — outperform a site most dramatically. Second, the line only earns its keep if it moves. A station with nothing to do is the most expensive square footage in construction. This is why factory utilisation, not factory technology, decides whether a modular business survives.

The 60-to-90-percent question

MBI's standard description is that 60 to 90 percent of construction is completed in the factory before modules ship, which is what makes possible the roughly halved schedules the industry advertises.⁴ That range is wide enough to hide a great deal. A module at 60 percent arrives as a structural shell with rough-ins; a module at 90 percent arrives with tile, appliances and light fixtures, needing only mate-line finishing, utility connection and corridor work.

The difference is not academic. Every percentage point left for site completion is a percentage point that returns to the productivity, weather and coordination problems the method was meant to escape. When a factory quotes you, the completion percentage is the number to press on, and the answer should be specific to your project rather than a range from a brochure.

Steel, timber, and height

The structural system decides how high a modular building can go. Light-gauge steel modules stack furthest: 101 George Street in Croydon, south London, comprises two towers of 44 and 38 storeys built from 1,526 steel modules manufactured by Vision Modular Systems for developer Tide Construction, delivering 546 build-to-rent apartments in roughly half the time of conventional construction.⁷ It remains the tallest volumetric building in the world. Marriott's AC Hotel NoMad in Manhattan was designed as a 26-storey, 168-module steel tower — architect Danny Forster said stacking would reach a floor a day — but the project stalled in 2020 and in September 2024 was cancelled outright and replaced with a conventional condominium.⁸

Wood-framed modules dominate the North American mid-rise market — the four-to-six-storey multifamily and supportive-housing projects that make up most volume — and are constrained by fire-code height and area limits for combustible construction rather than by the method itself. We have not found a source stating a universal height ceiling for wood modular, and treat the "mid-rise wood, high-rise steel" split as a description of practice rather than a rule. Concrete modules, common in Hong Kong and Singapore, trade weight for fire and acoustic performance and depend on cranes and roads that North American sites rarely have.

Two claims that need better data

Two widely repeated factory-side claims did not survive our attempt to source them, and we flag them rather than repeat them. The first is the acoustic benefit of the "double wall": because adjacent modules each carry their own wall and ceiling, party walls become air-gapped double-leaf assemblies that should outperform a single stud wall. The physics is sound and the double-leaf literature supports it, but we could not find a modular-specific test report with an STC or IIC number. A manufacturer that publishes one will have a real differentiator.

The second is throughput. Modules-per-day figures for named North American factories — Boxabl, Fading West, the former Factory_OS plant in Vallejo now operating as Harbinger Homes — circulate in press coverage, but we could not tie them to dated primary disclosures.⁹ Public companies' filings are the exception. Until factories publish audited output, "capacity" should be read as aspiration.

What to do with this

Design from the truck backward, and get a transport engineer's sign-off on module dimensions before schematic design closes. Ask for the completion percentage, not the range. Match the structural system to the height you actually need rather than the height that makes a press release. And treat factory tours as sales events: the numbers that matter — utilisation, completion at shipping, permit costs on real routes — are on spreadsheets, not the shop floor.