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Modular Multiple Dwellings

Apartment buildings built up from cube-like modules assembled in a factory

Photo of red and white alternating modular buildings lined up like townhouses.

Although some might think that factory-built housing applies only to single-family homes, a number of builders are realizing time, and sometimes cost, savings with modular apartments. Modular multiple dwellings are available as customized buildings that look like site-built structures, assembled in a factory while earthwork, foundations, and utilities are prepared onsite. The scale and repetition that characterizes many multifamily buildings lends itself to an automated solution, especially in areas located near the factories. The controlled indoor environment and stable, experienced labor are conducive to assemblies with consistent quality, often at reasonable cost. Workers familiar with their product can more easily integrate unfamiliar materials and techniques with other trades working side-by-side. Also, the rapid building cycle of modular construction can reduce or eliminate jobsite and theft.

Modular apartment buildings first burst on the scene in the '60s with the celebrated "Habitat" in Montreal, designed by architect Moshe Safdie. The costly assembly of concrete cubes and terraces looks like an industrial-age Mediterranean hilltown. Today's modular multiple dwellings are less complex and almost always wood framed, except for those by such companies as Deluxe Homes, Inc., which also frames with steel, and Advanced Modular Concepts, which uses steel exclusively.

Modular multiple dwellings consist of boxlike sections stacked vertically and horizontally with a crane. Each box is approximately 14' wide by 50' long (this varies considerably), one-story high, and includes all partitions between a fully-framed floor and ceiling. Although companies accommodate various configurations, most are orders for side-by-side duplexes in two-story attached townhouses. Other arrangements include townhouse flats, duplexes over flats, and "quads" with two flats over two flats. Less common are apartment buildings with shared entrance halls. Signature Custom Homes is finishing a full city block of modular rowhouses with brownstone fronts. Signature estimates 80% of their multiple dwellings are installed over basements.


Affordability

Because of their manufacture at a factory, quality is improved and labor time is saved. Labor on-site is also reduced because of the quick assembly of the units.


Easy

Most jurisdictions allow residential wood-frame buildings up to four stories, some only three. Buildings higher than two or three stories may require stud spacing at 12" o.c., switching to engineered wood such as microlams, or changing wood species. Avis America, Simplex Industries, and Signature are among the manufacturers setting studs 24" o.c. as an option in interior partitions and exterior walls of lower floors, an optimum value engineering technique that improves energy efficiency while saving the builder $100 or more per dwelling unit.

Beyond three or four stories, concrete and/or steel framing is necessary, not to mention elevators. Avis has built modular apartment buildings with elevators, and reports that for them it is much the same as for conventional construction-the shaft is built before the modules are delivered. The hydraulic shaftway, however, must be drilled with precision; a main point of factory assembly is to minimize field alteration and adjustment.

If codes mandate sprinklers, most manufacturers will install the system in the factory, leaving connections to be made between sections in the field. Usually, each apartment has an independent HVAC system to simplify zoning, metering, and fire-rating issues-shared lines require fire dampers. However, manufacturers install a variety of mechanical systems in a variety of ways. For example, Avis often uses an individual through-wall heat pump for each room, but also installs central systems using self-contained heat pumps. Avis also uses water heaters with space heating capability to take advantage of the full capacity of the water heater, eliminating a separate boiler or furnace in some climates.

Most modular manufacturers experienced with multi-family housing are in the Northeast states. Signature delivers up to Maine and down to North Carolina, Simplex down to Virginia. Most manufacturers can deliver units within eight to ten weeks following an order, which is either receipt of architectural drawings (for a custom job) or choice of manufacturer's product/options.

Some states are increasing the width limits of delivering housing units to 16', and manufacturers are responding with wider boxes. These often require permits and flag cars to accompany the units down the road, especially for long-distance trips. These flag cars (independent contractors charging about $27/hr) have been known to not show up, and manufacturers pay for wasted time. So, according to Avis, shipping distances greater than approximately 350 miles become less cost-effective, especially as a truckload costs $2 to $5 per mile. As the housing industry continues to adopt modular building techniques, more western manufacturers will offer multifamily options.


Cost of delivered modules varies, depending upon materials used, complexity of the plans and, to a lesser degree, the manufacturer. For sections 80% to 90% finished, installed on foundations built by others, Simplex can place essentially custom units for a cost per square foot in the $20s, Signature in the $30s (using better materials). Avis starts at around $40/sf, with a higher level of finish. However, most manufacturers are willing to accommodate varying levels of finish; comparing apples to apples, bids will come in very close.


Modular building must be maintained like any other conventionally built building.


Like site-built housing, modular housing must comply with building codes of the area they are delivered to. Most codes do not allow non-fireproof construction beyond three or four stories in height. The Fair Housing Act requires barrier-free accessibility in elevator buildings and in single-story ground-floor units in non-elevator multiple dwellings.


Not Applicable


To stiffen the structure for trucking and craning, modular boxes are framed on all six sides, so there are double-thick walls where boxes join. These "mating walls" are either partitions within an apartment or fire-rated walls separating dwellings. Within an apartment, each mating wall is typically framed with 2 x 3s instead of the 2 x 6s used for exterior walls, resulting in some interior partitions being slightly thicker than others. Mating walls between dwellings are, depending on the state, 2 x 4s with a layer of firecode gypsum board on each side and an air space between each wall, creating a wall with four layers of gypsum board on two 2 x 4 frames.

In addition to protection from road stresses, multiple stories require stronger support for increased static (and possible seismic) loading, so corners are often strapped or lagbolted to unify the frame, while upper and lower stories are lagscrewed together to stiffen the structure vertically. Because of the many boxes that comprise a modular multiple dwelling, tolerances are important. Units must fit together both horizontally and vertically. Some manufacturers, including Signature, mate sections together in the factory to assure alignment, including roof raising, and reassemble the building onsite. To accomplish this, Signature builds its units on swiveling casters, instead of a rail-type production line, to more easily manipulate the sections on the factory floor.

Signature builds their roofs with an attic storage truss that provides additional space and hinges down to clear overpasses during delivery. Multistory buildings, however, require less roof area per square foot of floor area than do lower structures, so tilt-up roof designs are less likely to benefit from economies of scale in multistory buildings. Other methods include factory assembly of roof panels later positioned by crane, or site building the roof.

The tradeoff for going vertical is the need for stairs and landings, often in the form of runs to exterior balconies. These landings are often framed into the boxes themselves, which incorporate longer floor joists projecting out to be decked either in the factory or field. Simplex chooses to site-build balconies and railings to limit the delivered width of their units. Delicate components such as guard rails are often panelized and inserted into rail pockets onsite. If an elevator is required, most manufacturers will build a 2-hour fire-rated shaft right into the module, to which other trades install equipment.

Due to the many marriage lines (joints between sections) of multi-unit buildings, exterior finishes are either wholly or partially installed on site, though manufacturers will often deliver packaged finish materials with the modules and stack it on the lot or roof. Although vinyl siding is the default exterior wall finish, Avis often provides for more low-maintenance materials such as brick or fiber-cement siding.


Many manufacturers give a one-year warranty on materials and labor, similar to conventional site-built structures, with a longer structural warranty of perhaps 10 years.


Avis has installed a 42-unit apartment building, "sill plate to shingles," in 12 days, and says that a six-person crew can set nine modular sections per day. Of course, this doesn't include fabricating the sections, but that happens while the site is being prepared, providing ample opportunity to examine details, dimensions, and materials in the factory. This can mean savings in time, money, and potential aggravation.

Modular construction is tighter than average site-built construction, owing to the practice of glueing the components together, in addition to conventional fastening, for road-worthiness. This results in energy efficiency through decreased air infiltration, in addition to less squeaking and structural movement. Avis claims that, at 0.28 air changes/hour as independently blower door tested, its homes are so tight there is no need for a paper air barrier. Avis also has experience with passive solar features and integrating photovoltaics with its buildings, as featured in Solar Townhouses. Simplex accepts customer requests to meet Energy Star Homes standards.

Modular building does not lend itself to large clear spans, though for most residential designs this is not a problem. Avis says the limit is about 20' due to the need to provide supporting walls for transportation. According to Penn Lyon Homes, some modular manufacturers use up to 30% more lumber than site-built structures to ensure strength and to complete six-sided boxes. This can negatively impact energy efficiency and affordability.

Although trailers can carry sections up to 64' long and 16' wide, extra-large sections can be awkward in the factory and may require re-engineering framing such as changing to 2 x 12s. Long sections leave a portion hanging off the back, which can bounce and crack. Some of the big modules have even caused trailer tires to blow out. Since the manufacturer is responsible for repair, pricing for large modules may reflect their limitations.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.