Prefabrication, modular construction and DfMA

The most direct way to escape the productivity problems of the construction site is to do less of the work there. Prefabrication moves parts of the work into a controlled environment, a shop or factory, where workers stand at stations, materials arrive in sequence, tools are at hand, weather does not interfere and quality can be checked as work proceeds. Modular construction takes this furthest, building whole volumetric units off site and shipping them complete. Design for manufacture and assembly (DfMA) is the discipline of designing buildings so that they can be made this way. CON-260 lesson 10 introduced prefabrication and modular construction among emerging technologies; this lesson looks at them through a lean lens, as ways of improving flow, reducing waste and shortening schedules, and at the economics and limits of moving work off site.

A spectrum of off-site construction

Off-site construction is not one thing. It ranges from simple kits of cut materials to complete buildings assembled from factory-made modules. Each step along the spectrum moves more value-adding work into the factory and leaves less on site, but also requires earlier design decisions, more transport and more coordination between factory and site.

Why off-site work is more productive

Factories address almost every one of the eight wastes from lesson 2. Workers stay at stations rather than travelling to the work (motion and transportation). Materials are delivered to the station in the order needed (inventory and waiting). Jigs and fixtures make assemblies the same every time (defects). Work is protected from weather. Quality is checked at each station, and problems are fixed before the product leaves. Work can run in shifts and in parallel with site work, so a bathroom pod can be built in the factory while the structure that will receive it is being poured. Studies and contractor reports of prefabricated mechanical, electrical and plumbing assemblies commonly cite large reductions in site labour hours for the prefabricated scope, though results depend heavily on the product and the factory's maturity.

The economics of moving work off site

Prefabrication is not automatically cheaper. Factory labour is usually more productive and sometimes less expensive per hour, but the product must be designed, engineered, made, protected, transported, lifted and connected on site, and the factory's overheads and profit must be paid. The main savings often come not from lower direct cost but from a shorter schedule, which reduces general conditions, financing and escalation, and from fewer workers on a congested site. A simple model compares the cost of doing a scope on site with the cost of prefabricating it.

# Bathroom pods versus site-built bathrooms on Willowmere (84 bathrooms, illustrative)
n = 84
site_hours_each, site_rate = 96, 88              # multi-trade site hours and all-in $/h per bathroom
site_material = 7_400                            # $ per bathroom
pod_price = 15_200                               # factory price per pod, delivered
site_connect_hours, connect_rate = 10, 88        # set, connect and finish around each pod on site
crane_and_rigging = 38_000                       # extra lifting for the pods
design_engineering = 45_000                      # pod design, coordination, mock-up
weeks_saved, gc_per_week = 5, 23_000             # schedule saving and general conditions per week

site_total = n * (site_hours_each * site_rate + site_material)
pod_total = n * (pod_price + site_connect_hours * connect_rate) + crane_and_rigging + design_engineering
schedule_saving = weeks_saved * gc_per_week
print(f"Site-built bathrooms:  ${site_total:,.0f}")
print(f"Pods, before schedule: ${pod_total:,.0f}")
print(f"Schedule saving:       ${schedule_saving:,.0f}")
print(f"Net difference (pods minus site-built, after schedule saving): ${pod_total - schedule_saving - site_total:,.0f}")
print(f"Site trade hours avoided: {n * (site_hours_each - site_connect_hours):,} h")

Site-built bathrooms:  $1,331,232
Pods, before schedule: $1,433,720
Schedule saving:       $115,000
Net difference (pods minus site-built, after schedule saving): $-12,512
Site trade hours avoided: 7,224 h

In this illustrative case, pods cost about $100,000 more than site-built bathrooms before the schedule saving and slightly less after it, while removing about 7,200 hours of multi-trade labour from a congested site. Whether that is worthwhile depends on the owner's priorities: a rental building that earns income sooner may value the schedule saving more highly, and a site with skilled-labour shortages may value the labour reduction more than its direct cost suggests. A real decision would also weigh quality, warranty, the factory's track record and the risk of a supplier failing mid-project.

Design for manufacture and assembly

DfMA applies principles developed in manufacturing to building design: design products so that they are easy to make and easy to assemble. For a building, this means standardizing components and dimensions, repeating suite and bathroom types, designing connections that are quick and forgiving, and keeping module sizes within transport and lifting limits. DfMA decisions must be made early, during schematic design, because they shape the structure, the grid and the layouts. A building designed conventionally and then given to a modular supplier usually needs redesign and loses much of the benefit.

Transport limits are a hard constraint on volumetric modules. Road permits typically allow loads somewhat wider than a standard truck only with escorts, routes and time restrictions, and the limits differ by province and state. A module that is too wide or tall for the route cannot be shipped at a reasonable cost, so module dimensions are set as much by highways and bridges as by architecture. Lifting is the second constraint: the crane must reach the farthest module position at the required capacity, which can drive the choice of a larger and more expensive crane.

Off-site construction and lean flow

Moving work off site changes the flow of the project. The factory becomes a supplier whose output must arrive just as the site is ready, neither so early that modules must be stored and protected nor so late that crews wait. This makes the pull principle essential. Many modular projects use a delivery sequence that matches the erection sequence exactly, with each module or pod delivered on the day it will be set. The site, in turn, must be ready for each delivery: foundations, structure, connection points and services must be complete and accurate. A delay at either end ripples through both, so the Last Planner System and look-ahead planning extend upstream into the factory's schedule.

The policy context

Prefabrication and modular construction are receiving strong policy support in Canada because of the housing shortage. The federal agency Build Canada Homes, launched on 14 September 2025, has made modern methods of construction such as modular, panelized and prefabricated systems central to its approach. In February 2026 it issued a request for information to firms using these methods, closing in March 2026, as a step toward a prequalification process and a vetted list of suppliers, and it says projects it supports will favour Canadian materials and factory-built housing. Provinces have also published standardized housing designs intended to be built off site. The National Building Code applies to factory-built buildings as to site-built ones, and the CSA A277 standard sets out the certification procedure for factory-built buildings, so factory and site inspection must be coordinated.

Off-site construction also has a history of failures. Several well-funded modular start-ups in North America and Europe closed in the early 2020s after expanding too fast, underestimating the difficulty of keeping a factory busy through the construction cycle or taking on projects whose designs did not suit their systems. A factory has high fixed costs and needs a steady order book; construction demand is cyclical. Owners and contractors evaluating a modular supplier should check its financial strength, its order book and its record on similar projects, and should plan for what happens if it fails mid-project.

Choosing what to prefabricate

The best candidates for prefabrication share several features: they are repeated many times, they involve several trades in a congested space, their quality is hard to achieve in the field, they are on the critical path, or the site has limited access or labour. On Willowmere, Northgate chose bathroom pods for the 84 suites, multi-trade corridor racks for the residential floors and panelized exterior walls for the residential tower, while keeping the podium, lobby and amenity spaces conventional.