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Guide to modular home construction, inspection, troubleshooting, diagnosis, repair: how are modular homes recognized?
How is a modular home built, brought to a homesite, and assembled? What portions of a modular home were not made in a factory? What is the quality of modular homes? What are their features, common defects, problems, solutions.
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.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Characteristics of Modular Homes or Modular Housing
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) and lifted by a crane to be set upon a foundation which has been prepared ahead of time (photo below). 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.
Modular homes, earlier in their conception, enjoyed a less than stellar reputation several decades ago, having the reputation of
flimsy construction. That is certainly no longer the case.
Contemporary modular construction of homes have these attributes:
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 or other modular building is usually 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, sometimes in or at the electrical panel.
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
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).
How to Identify 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.
Modular home uppermost sections that will include a roof 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.
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 Wood Framed Houses?
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.
Modular Office Building Labeling & Code Compliance Certification
[Click to enlarge any image]
Reader question: how do I establish that my modular office meets my local state building code?
I bought a 12'x56' modular office building from a government auction in sc and got it hauled to my business in NC. But to my surprise I was told by the city code inspector, since the modular was built in the state of SC and has codes and approval of that state I won't be allowed to get the permits until it is certified to the NC building codes. Please suggest what are my options and how can I take care of this issue.
This modular office building was built in 1998 by AAA modular based in SC or GA at the time, unfortunately seems like they are not in business anymore, it has sc stamps etc, let me take some photographs and send them over. - Anonymous by private email 2016/07/23
Reply: how to find and check the labels affixed to the structure, check with the labeling agency
Virtually every modular building construction company in North America builds their products to meet all of the state codes where they can imagine their building will end-up. Take a look at the data tags on your building. Send me some sharp photos of the data tag and a couple of the whole building and I can comment further. [Done, shown here - Ed. ]
Check for one or more industry / agency / manufacturing certifying labels that for any modular home or office will identify at least:
The manufacturer of the modular office or home
The modular home or office model, manufacturing date, and serial number
A building code compliance certification agency
Depending on the age and type of modular structure, the identifying labels are usually found in or at the main electrical panel, under the main or kitchen sink affixed to a cabinet door, and/or on a weatherproof data tag riveted to one end of the structure near ground level and near a corner.
Then give a call to the manufacturer of your building if they are still in business. Give them the same data tag information: model number, serial number, etc. and ask them what codes their product meets.
When the manufacturer of your modular structure has disappeared, don't give up: you may still be able to obtain documentation of the building codes with which your building complies by contacting the original certifying agency.
Radco who certified your modular building is still in business even though the original manufacturer, Triple A Modular Buildings, Inc., has disappeared: see this ASTM citation for Radco
Reader Question: is it OK to modify or remove modular home attic ceiling framing?
Can the protruding mating joints on the attic floor of modular home be safely trimmed down? Would like to level flooring for finishing of attic. - Anonymous, 22 July 2015
I cannot say for sure if what you want to trim is safe or not - it depends on the amount. Trimming off 1/4" or even 1/2" of the upper surface of mating 2x lumber of the mating beam in the center of an attic is likely of no import.
For a greater difference or maybe even in all cases as it's easier and faster, I'd consider instead using some shims to level the surface before installing flooring. Use tapered wood shims (available at any building supplier) or left-over tapered wood siding if that's available.
Thank you for response. I'm not sure I described the issue properly. I have a setup similar to the image here:
[Photo shown at left]
[Click to enlarge any image]
The existing flooring is significantly lower than the bulky joint in the middle.
Your photo is a very different case from what I was describing. It appears to me that there is a raised header installed where that 2x lumber projects up into the attic.
If so it's probably spanning an open doorway or opening in a wall partition below.
If that is the case you definitely cannot remove or chop down that beam. Your options are to box it in and leave it alone or to hire a structural engineer to re-design a dropped beam in that location. I recommend the former.
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Double-wide mobile home with no insulation?
Do these come pre trimmed in the inside? Or do I have to pull them off and insulate underneath them? Im renting and its costing me 600 a month to heat. I don't think the renters insulated it at all. I pull a piece of trim from around the slider door and no insulation there. The pipes, no caulk or anything. I can throw a penny down to the ground from the bath pipes. It is a double wide. The house has a gap in the floor where it looks like the house is coming apart. I can stick a long needle rite threw it to the ground. - Kevin 8/13/12
Do Double wide Mobile come pre trimmed to the sellers in the inside? Or do the businesses that sell them have to have to pull them off and insulate underneath them? I'm renting a 2004 and its costing me $500-$600 a month to heat. I don't think the renters insulated it at all. I pulled a piece of trim from around the slider door and no insulation there. I put my hand underneath the cement foundation and the floor. No insulation there as well.
The heat comes out semi cold. It only has a 56000 BTU heater and is new. But.. This seems low for a 40X60 or 40X70 Im not sure which one this is. But it runs for house trying to get the house warm. The dishwasher water freezes in the inside of it during the winter. I tried pulling it out to see why. But its hooked in. Cold air rushes from the light sockets to where it blows a lighter out. I came from a 3 story victory. So I know little about pre-manufactured homes. The pipes, no caulk or anything. I can throw a penny down to the ground from the bath pipes. I have caulked around it now.
The house has now formed a gap in the floor where it looks like the house is coming apart. I can stick a long needle rite threw the carpet it to the ground. Is the house just settling? Also the electric is $300-to $400 a month. The light bulbs flash like a strobe light all the time. They had a 12,000 volt cow fence hooked and water for their 50 head of cows in when we got here. After paying for that (unknowingly) for 6-8 months I realized that it was hooked in.
When I turn off the electric to the house. The fence and water would turned off. So they put in a separate pole. But the lights still flicker and the fuse box blows fuses all the time (for no reason) like if I have the washer on. I have two small girls. The renters say this is all normal. - Computer Geekz - 8/13/12
In order to sort out the question of what are common construction practices and what is usually included or not with a manufactured home, we first need to get a couple of confusing terms straightened out.
You have posted a question about double-wide mobile home construction here in an article on modular construction
(MODULAR HOME CONSTRUCTION) .
There in the Mobile Homes FAQs section we include your original question along with detailed answers.
Question: What building codes regulate the construction of modular homes
Does a modular home or factory built home made in another state or province have to meet local state, provincial or other building codes?
A modular home is built using conventional 2x4 or 2x6 wood framing much like a stick built house, but it is constructed in several sections that are then trailered to a building site, set upon a conventional building foundation or slab, and fastened together there. The floor of each section is built strong enough to be placed on a temporary trailer for transport to the building site where the trailer chassis is removed prior to assembly of each section.
Building codes and standards for modulars are essentially the same as for a stick-built residential home. Even when the modular home is built in a different state from which it is to be assembled, the home must comply with state building codes.
In fact because modular home manufacturers want to be able to ship their models to multiple states or provinces, often the units are constructed to meet the most stringent code requirements of the various states/provinces served - a condition that means a modular home might exceed local building code requirements.
Question: modular home roof ventilation plus plumbing not vented outside SNAFU leads to mold contamination
9/12/2014 Dan said:
We bought a used 1995 modular home in 2005 that consists of one middle pod (10 ft ceilings) and two side pods (8 foot ceilings). The roofline is 4/12 pitch. When you enter the attic space up to the middle pod (which is overlaid with metal sheeting) you can see all the way to the other end. However, in order to get from the middle pod to the side pods, you have to crawl through a small opening that was cut out by one of the workers. Everywhere else there is a wall of plywood that separates the lower section (side pods) from the upper (middle pod). I did not think much of it until we found out that the lower sections now have mold growing on the underside of the plywood for the roof. A mold inspector told us that this plywood was preventing air from circulating. In short, he said that the eave vents have no way of circulating air with the middle pod's roof vents. My question: What is the purpose of the plywood? Should it have been removed?
During this discovery, we found out that a sewer vent, and two other air vents were never vented out through the roof. In other words, the vents were exhausting warm air into the attic. The mold inspector commented that the combination of these two failures brought on the mold which is now making my family sick. 2nd question: What can I do about this huge problem we now face?
What a mess. Under a 4/12 roof the accessible area under the lower slopes is very limited - too limited to think about adequate cleaning & insulation replacement from inside the attic space.
That leaves two approaches: tear off all of the roof, remove all insulation, clean and seal all surfaces, re-insulate, re-roof,
Tear down ceilings indoors and do the same job from inside.
The second approach is more disruptive to the occupied interior and increases the risk of cross contamination and thus the need for still more indoor HEPA vacuum cleaning and wiping.
Normally I oppose heroic, expensive roof tearoffs to address attic mold as not being justified. But I would not want to leave a large problematic mold reservoir in a building: indeed air movement can move "backwards" down into a structure in some conditions.
I would first do some careful sampling of the molds present: on plywood, in insulation (by vacuum testing) and on the attic side of ceiling drywall. If none of the molds found are highly mobile - highly harmful (e.g. if it's just some simple Cladosporium) I might do nothing except fix the venting. Otherwise, I'd consider the remediation I outlined above.
The plywood was doubtless in place to enclose and protect the structure during transport.
Leaving it in place indeed blocks the proper roof ventilation.
Failure to vent plumbing vents to outdoors is a site set-crew SNAFU which is another example of why modular companies often want their own crews to set the house. Modulars encourage construction by builders who don't know enough about building.
Tell me the state where you are located and the brand and model of your home and I can research this further.
Kudos to your home inspector.
Question: roof connections on modular homes
(Feb 10, 2015) L Stokes said:
Our modular was built in 1971. The crawl space looks to have a lot of metal vs wood. Was the roof solely attached by tabs wrapped around the metal and then stapled to the plywood? It seems to have weathered very well all these years!
Please use our email found at the CONTACT link at page bottom to send me some photos as I'm not clear what we're discussing. I don't know how in a crawl space (under a building) we are seeing roof construction details.
Some modulars have hinged roof trusses or rafters that are erected after each top floor section is set. And wood roof trusses may be assembled using steel plates. And trusses or rafters may be secured to the building wall top plate using steel hurricane ties. But I can only guess from just your note.
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 Section 184 Indian Home Loan Guarantee Program, U.S. Department of Housing & Urban Development, web search 1/5/2012, original source: portal.hud.gov/hudportal/HUD?src=/program_offices/public_indian_housing/ih/homeownership/184 - Quoting:
The Section 184 Indian Home Loan Guarantee Program is a home mortgage specifically designed for American Indian and Alaska Native families, Alaska Villages, Tribes, or Tribally Designated Housing Entities. Section 184 loans can be used, both on and off native lands, for new construction, rehabilitation, purchase of an existing home, or refinance.
Also see Freddie Mac & Fannie Mae
 Native American Housing Loan Guarantee Program HUD Section 184 Loans At A Glance, FannieMae, web search 1/5/12, original source: efanniemae.com/sf/mortgageproducts/pdf/section184aag.pdf
 "Modular Home Construction, special defects and inspection methods" Dan Friedman, NY Metro ASHI Seminar, Holiday Inn, Crowne Plaza, White Plains NY, October 4, 1996
 New York State: "Manufactured Homes: an installation guide for the code enforcement official," undated. [Div. of Code Enforcement & Admin. - 518-474-4073, George E. Clark, Jr., Director] - this is a guide tool, not an enforcement code or standard.
 HUD State Administrative Agency (for 36 states) (NY: 518-474-4073) - for complaints
 manufactured Housing Institute, 2101 Wilson Blvd. Ste. 610, Arlington VA 22201 703-558-0400 www.mfghome.org
 NYMHA, 35 Commerce Ave., Albany NY 12206-2015 518-435-9859 800-721-HOME (they want the Star Program to provide for separate assessment of manufactured homes)
 Consumer Reports: www.consumerreports.org - special report 2/98
 Thanks to home inspector Peter Bennett for eagle-eye editing assistance regarding spelling at this web article series. Little Silver, NJ 07739 Office 732-758-9887 Fax 732-758-8993 Cell 732-245-9817 email@example.com
 Wikipedia provided background information about some topics discussed at this website provided this citation is also found in the same article along with a " retrieved on" date. NOTE: because Wikipedia entries are fluid and can be amended in real time, we cite the retrieval date of Wikipedia citations and we do not assert that the information found there is necessarily authoritative. - Entry on Mobile Homes, original source: http://en.wikipedia.org/wiki/Mobile_home#Regulation, retrieved 8/14/2012
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones