A Photo Guide to Modular Home Construction Identification & Inspection InspectAPedia® -
Definition and description of modular construction - how can we identify a factory-built home?
Definition and description of mobile homes, double wides, trailers, and campers
What is the difference between a mobile home, a manufactured home, a modular home, and panelized construction?
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Modular Construction, mobile homes, trailers, campers, doublewides, panelized construction -
what are the differences and how are these structures recognized? This series of articles describes the history and
characteristics of these different types of factory-built structures. Our page top photo shows a four-section modular home after the set-crew has finished placing the four individual sections of the building and the roof has been lifted and enclosed.
Characteristics of Modular Homes or Modular Housing
Modular homes, too, enjoyed a less than stellar reputation several decades ago, having the reputation of
flimsy construction. That is certainly no longer the case.
A modular home is constructed in a factory of one or more sections which are carried to the building site on
a trailer (photo above left) and lifted by a crane to be set upon a foundation which has been prepared ahead of time (photo above right).
Modular homes
can be quite large, involving four or quite a few more individual sections which are lifted and "set" into place
at the site (photo at left)
Some manufacturers provide custom architectural services and can deliver unique, but factory-built
homes in sections.
Contemporary modular construction of homes have these attributes:
The home is built indoors in a factory under controlled conditions, usually resulting in straight and square
construction of walls, ceilings, roofs, and floors.
Exterior wall sheathing and roof sheathing are glued as well as nailed to the surfaces of their
respective studs or rafters. Interior sub flooring and drywall on walls and ceilings are also
glued as well as nailed or screwed to their joists or studs. Some models by some manufacturers also install
a double layer of interior drywall. These methods result in a very stiff and strong construction.
The home is probably built to meet the building code requirements of all U.S. states, or at least all of the
states within a manufacturer's shipping area. You'll find an identifying label for the home with this information,
often on a kitchen sink cabinet wall.
The home is built strong enough to be lifted at the factory by crane for setting atop a steel trailer for
transport, then driven at 65 MPH down a highway, pushed or dragged on its trailer over an often hilly and rough
construction site, and lifted again by crane for final "set" atop its foundation.
Without falling apart. (Try this with a
stick-built house.) At the Contempri factory in Pennsylvania on a modular home construction tour about a decade
ago, suddenly all of the workers and managers dropped their tools, ran to their cars, and drove madly out of
the factory parking lot. We followed to see what was happening.
One of their modular sections was being towed
on U.S. interstate I84 when the trailer disconnected from the towing tractor. The tractor drove ahead. The trailer
ran off the highway at 65 MPH. The trailer ran head on into the end of a steel guard rail. The trailer stopped.
The modular home section kept going, and impaled itself centered atop the guard rail. When we got to the accident,
there was no debris anywhere except where the guardrail had punched out of the rear wall of the modular home section.
You could look in windows into the kitchen where the cabinets and appliances were perfectly in place.
When the modular home is transported to its site, it is moved by being lifted and set onto an independent
steel frame which has its own independent wheels. At the destination the modular home or home section is
lifted by crane and set onto an independent foundation, and the steel frame/wheel set returns to the factory
for re-use.
A modular home is normally set on a foundation which has already been placed at the building site.
If the modular home is placed over a basement or if there is to be an attached or detached garage, often that construction
is performed by a local building contractor rather than by the modular home manufacturer (sometimes resulting
in different quality of workmanship).
Keys to Identifying a Modular or factory built home after construction has been completed
A modular home can be difficult to recognize once its construction has been completed. However these clues will
work every time:
In the basement, if the ceiling is not fully enclosed, look at the main girder (photos above). Since most modular homes use at least
two long sections that have to be built and transported to the site, there will be at least two completely independent
floor framing systems, and at their mating point over the basement center, you'll see an unusually wide built-up
girder with (if properly installed) through bolts connecting the two building sections.
The modular home basement girder members should be touching. IF you see a gap at the center of this structure the building sections may not have been properly set on the foundation. Conversely, if there is to be any gap it should be at the top of the modular sections (visible in the attic, for example) - which assures that the bottom mating members are tight.
In the living area, if the building is a two-story unit, as you walk up the stairs from first to second
level, notice that there are a few more steps than usual between floors?
Since each of the four stacked sections in
a four-section two-story modular home has been framed with a complete floor, wall, and ceiling structure, the "ceiling"
between the first and second floors will be double the normal depth since it is comprised of both the first floor
section ceiling framing and the second floor section floor framing. So if 2x10 joists were used, there will be
about 20" of ceiling thickness between floor (a great place to run wires and ducts).
In the kitchen look under the kitchen sink base cabinet for the modular home manufacturers' labels (these labels may also be placed in a basement or by an electrical panel or at other locations) (Photos above).
In the attic is the fail-safe way to always identify a modular-built home unless there is simply no attic access
or all surfaces are covered. You'll find one, possibly two or even three features unique to modular home construction:
There may be hinged roof rafters (Photo at above left and sketch, below). Many modular homes have roof slopes which would be much too high
for the upper roof-bearing sections to travel up the highway. They travel with the roof laid flat atop
the upper floor module. The roof rafters are hinged, roughly 18-24" from the eaves of the home, and are lifted
up at the site, then supported by an attic knee wall. So you'll see two knee walls, one supporting the front
and one the back roof section. You may see the hinges on the rafters down near the eaves as well.
As the dimensions of a sloped roof will cover more area than the flat top section of the building, the roof of a modular or factory built home will often include an additional section that must be set in place, usually near the ridge (photo at above right) - you will see this separately framed structure in the attic of these homes.
There will
be a mating joint of the front and rear sections of the home visible as two girders in the attic floor, running along
the long dimension of the building, usually with a small gap between them, hopefully with insulation or other fire blocking
stuffed into the gap. The reason for the gap is that properly placed, the sections are set with their bottom girders
touching tightly, which may leave the top sections slightly separated at their highest point.
There will also be a mating joint at the ridge where instead of a single ridge board you may see up to four horizontal "ridge boards" - the roof sections need to be framed as stable sections that can be lifted into place. (Photo at left).
Just how Strong are Factory Built Homes? Stronger than Stick Built?
One modular home
I inspected had fallen off of its trailer while being lifted by the crane. It rolled over on its face. Like the
unit which had impaled itself on the guard rail, there was little damage other than broken windows.
But there was
a slight crease in all of the roof shingles about 24" up from the eaves.
The rafter hinges had all been slightly
bent when the section toppled. Outside, even on a modular section which has not fallen, you may see this telltale
line of slight shingle anomaly, parallel to the eaves.
Factory built homes are constructed so that each section can withstand being lifted onto a trailer, driven up the highway at 65 mph, pushed or pulled over an uneven, often sloped site, lifted into the air by the site crane, and set into place on the foundation.
Often factory built homes combine glue as well as conventional framing fasteners, extensive use of truss joists, girders, and additional framing of individual building sections so that each can be manipulated into place.
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Additional technical contributors & reference sources for this article are listed below.
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"Modular Home Construction, special defects and inspection methods" Dan Friedman, NY Metro ASHI Seminar, Holiday Inn, Crowne Plaza, White Plains NY, October 4, 1996
"Modular Home Construction, special defects and inspection methods" Dan Friedman, NY Metro ASHI Seminar, Holiday Inn, Crowne Plaza, White Plains NY, October 4, 199
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
Design of Wood Structures - ASD, Donald E. Breyer, Kenneth Fridley, Kelly Cobeen, David Pollock, McGraw Hill, 2003, ISBN-10: 0071379320, ISBN-13: 978-0071379328
This book is an update of a long-established text dating from at least 1988 (DJF); Quoting: This book is gives a good grasp of seismic design for wood structures. Many of the examples especially near the end are good practice for the California PE Special Seismic Exam design questions. It gives a good grasp of how seismic forces move through a building and how to calculate those forces at various locations.THE CLASSIC TEXT ON WOOD DESIGN UPDATED TO INCLUDE THE LATEST CODES AND DATA. Reflects the most recent provisions of the 2003 International Building Code and 2001 National Design Specification for Wood Construction. Continuing the sterling standard set by earlier editions, this indispensable reference clearly explains the best wood design techniques for the safe handling of gravity and lateral loads. Carefully revised and updated to include the new 2003 International Building Code, ASCE 7-02 Minimum Design Loads for Buildings and Other Structures, the 2001 National Design Specification for Wood Construction, and the most recent Allowable Stress Design.
Diagnosing & Repairing House Structure Problems, Edgar O. Seaquist, McGraw Hill, 1980 ISBN 0-07-056013-7 (obsolete, incomplete, missing most diagnosis steps, but very good reading; out of print but used copies are available at Amazon.com, and reprints are available from some inspection tool suppliers). Ed Seaquist was among the first speakers invited to a series of educational conferences organized by D Friedman for ASHI, the American Society of Home Inspectors, where the topic of inspecting the in-service condition of building structures was first addressed.
Defects and Deterioration in Buildings: A Practical Guide to the Science and Technology of Material Failure, Barry Richardson, Spon Press; 2d Ed (2001), ISBN-10: 041925210X, ISBN-13: 978-0419252108. Quoting: A professional reference designed to assist surveyors, engineers, architects and contractors in diagnosing existing problems and avoiding them in new buildings. Fully revised and updated, this edition, in new clearer format, covers developments in building defects, and problems such as sick building syndrome. Well liked for its mixture of theory and practice the new edition will complement Hinks and Cook's student textbook on defects at the practitioner level.
"Avoiding Foundation Failures," Robert Marshall, Journal of Light Construction, July, 1996 (Highly recommend this article-DF)
"A Foundation for Unstable Soils," Harris Hyman, P.E., Journal of Light Construction, May 1995
"Backfilling Basics," Buck Bartley, Journal of Light Construction, October 1994
"Inspecting Block Foundations," Donald V. Cohen, P.E., ASHI Reporter, December 1998. This article in turn cites the Fine Homebuilding article noted below.
"When Block Foundations go Bad," Fine Homebuilding, June/July 1998
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