The Modular Process In-depth
Introduction
The best way to appreciate the modular building process is
to see it firsthand. And the only way to do that is to visit a modular home
manufacturing facility. Fortunately, many companies encourage home buyers to
visit the factory during non-production time to walk through the entire process
from start to finish. Before you make a decision, call up your selected builder
or a modular home manufacturer to arrange for a guided tour. Here we outline the modular home construction process in-depth. The steps in building a modular home closely follow a traditional stick-built home. Regardless, before you spend money, its best to understand the process of building a modular home - to answer the question: Just how are modular homes built.
Comparison of Strengths and Weaknesses of Each System
Whether your new home is built using the modular building
process or some other building system, there are strengths and weaknesses
associated with each system. The chart below compares the three types of
building systems that utilize standard dimensional lumber: stick-built,
panelized and modular.
Modular
|
Panelized
|
Stick-built
|
|
Protection from exposure to
weather during construction
|
excellent
|
poor
|
terrible
|
Completion time
|
fast
|
average
|
slow
|
Security from job site
theft
|
excellent
|
poor
|
poor
|
Quality control consistency
|
excellent
|
average
|
poor
|
Average costs savings (for
similar style house)
|
10+% savings
|
||
Interest savings from rapid
completion
(building loan)
|
best
|
variable
|
worst
|
Design limitations
|
minimal
|
minimal
|
minimal
|
Design limitations when
combined with other building systems
|
virtually none
|
virtually none
|
virtually none
|
Limitation of the Modular Building System
Unlike stick-built housing, the modular system creates three
dimensional sub-assemblies that quickly combine into a completed house. But to
create the completed house, these units must be transported to the job site and
there are basically only two methods available. The modular units can be
transported either by truck or by helicopter. With rare exception, trucks are
the most cost effective method. But transporting units over the highway have both
federal and state imposed restrictions. While the restriction may vary between
states, in general truck shipments cannot exceed a width of 14 feet, height of
13½ feet or a length of 65 feet. These size restrictions could certainly cramp
a home buyers options but because the modular building system is such an
advanced building system, the modular home manufacturers have developed
sophisticated approaches to meet the needs of home buyers. With a potential length of 65 feet, there is little concern
about any problems there. Few, if any rooms come close to a length of 65 feet.
Now let's look at the height issue. Of the 13½ feet
available, the trailer used for transportation gets the first claim. The
trailers used by a manufacturer vary but on average the trailer requires
around three feet. That leaves about 10½ feet for the floor, ceiling, interior
height and possibly a specialized folding roof system. The folding roof system
is quite ingenious, so we will get to that later. But now with only 10½ feet
the modules height does not leave much room for error. Since most manufacturers
utilized automated CAD systems, they are able to maximize their height
limitations.
The most apparent restriction is the 14 foot width
restriction, but relying on the same structural materials used by the panelized
and stick-built systems, the modular home manufacturers' commonly build units
that marry together providing rooms that are 27 feet by 17 feet. While this is
sufficient for most people, using the more advanced and costly building techniques used by other building
systems can result in rooms as large as 27 feet by 35 feet, or larger. But once
rooms exceed the more common 27 feet by 17 feet, an experienced builder will
recommend that the home be built using a combination of modular and one or more
other building systems such as post & beam, panelized or stick-built.
The modular building system does have its own unique
advantages and disadvantages but with the right general contractor and modular
home manufacturer, your building choices are virtually unlimited.
The Modular Building Process: Step by Step
With the final blue prints approved by you, the modular home
manufacturer can now begin to undergo the process of building your new home.
Lets take a closer look at the entire production process from start to
finish.
Before and building can take place, the manufacturer
converts the floor plan prints that you approved into detailed production
drawings to be used on the production floor. Every company uses a different
production approach, so the style and detail utilized varies by manufacturer.
Some companies go to great lengths creating detailed drawings of every
individual component, while some manufacturers, especially the custom shops,
minimize the number of detailed sub-assembly plans while relying on the skill
and training of their workforce. Regardless of the approach, the rigorous
quality control system imposed by the third-party inspector ensures that your
home is well built, to code, and according to the plans you approved.
A side note on Blue Prints
As you begin the process of designing your new home, you will find that your house prints wont' be blue. Fear not, for with the increasing sophistication of photocopy technology, the original blue print machines are far too slow and costly compared to the new wide-carriage printers. Today's CAD/CAM systems print directly to high-speed printers, eliminating the need for blue prints. For an average blue print machine, a single one page print can take nearly a minute, from a hand drawn original.
For a single house, a complete set of prints can total upwards of ten pages. This includes front, rear and side elevations, floor plans, electrical, plumbing, heating and ventilation, foundation, details page and cover sheet. All this for one complete set. And more than one set is required. There's one for the home buyer, the general contractor, a production copy, a file copy, several for the third-party inspector, and several for the state. When they are all added up, most homes require over 100 pages, and it can be closer to 150.
First Step: Sub-Assemblies and Pre-cutting
With the production drawings complete, the production of
your new home begins with the fabrication of specialized sub-assemblies to be
used in your new home. This process takes place in the set-up shop. Windows and
doors represent the most common sub-assembly fabricated. The set-up shop also
prepares any specialized components necessary for fabricating the house panels.
The completed sub-assemblies and components are then moved to the panelization
area.
Second Step: Panelization
Essentially, the panelization process is a sub-set of the
modular building process. Modular home manufacturers build the structural shell
utilizing a panelization process identical similar to that used for houses
built using the panelization process. As the panel process begins, workers
build individual panels for each of the walls, floor decks and ceilings
required for each modular unit. And because the production process is conducted
in a climate controller factory, the panels are build at the same time.
Following the production prints, workers build each floor
deck utilizing a specially designed table or jig. The deck jig allows each
floor deck to be built precisely. Workers first place each floor joist upright
on the jig following the prescribed spacing diagram. Holding the joists in
place is a single or double perimeter beam. The perimeter beam consists of long
dimensional lumber that wraps around the entire floor deck. Modular construction
utilizes double perimeter beam while stick-built construction often uses only a
single perimeter beam. With the joists and perimeter beams in place, workers
nail the components together with pneumatic nail guns. The floor deck is then
finished with either plywood or OSB board. To ensure a solid, long lasting fit,
and to help prevent squeaks, virtually all modular home manufacturers apply
sub-floor deck adhesive to the joists and perimeter beams. This step takes more
time, but substantially increases the strength of the floor deck. Before the
deck is marked as complete, many modular home manufacturers sand the floor deck
to ensure that all of the sub-floor panels are even.
The fabrication of wall panels closely follows the same
process used by the floor deck. Beginning with a specially designed wall jig,
workers layout the wall studs, top plate, sole plate (bottom plate) and the end
studs. The wall components are then nailed in place.
Plywood
or OSB
With the introduction of Oriented
Strand Board several years ago a battle has been waging between their
promoters. When it first came out, OSB suffered most from an unstable glue,
and has not fully recovered in the eyes of plywood advocates. OSB fans
contend that the quality is superior to plywood, and that it cost less. And
environmentalists favor OSB. OSB utilizes recycled wood chips, while plywood
relies heavily on old-growth trees to get the large, continuous veneer strips
necessary for layered plywood. The experts disagree, so which one should
you choose. Many experienced builders prefer plywood for the floors and
roofs, while using OSB for the undersiding.
|
While not always, most manufacturers take the wall
fabrication process to the next step and install drywall to the completed wall
panel while it is still on the wall jig. When they do, most companies first
apply adhesive and then screw the drywall into place. Just like the floor deck,
the application of adhesive adds an additional level of structural integrity to
your new home. Another unusual, but stronger approach to securing drywall to
the wall panels utilizes a special foam-seal. Foam-seal essentially is an epoxy
especially designed for the housing industry. As more and more manufacturers
automate their panel production, the foam-seal process will become an
increasingly popular drywall adhesive.
Creating an Open Shell
Before we more on the ceiling and roof systems, the newly
built wall units must be attached to the floor deck. Using overhead cranes,
workers lift the floor deck off the deck jig and onto a transport system. At
this point, the completed floor deck represents the first point in the assembly
line of the modular manufacturing process. Next overhead cranes lift the
completed wall units off the wall jig and onto the floor deck. Workers then
secure the wall sections to the floor deck and the other wall panels using
pneumatic nailers. At this point the open shell is ready for the ceiling or
roof panel.
Ceiling & Roof Systems
For the most part, floor decks and wall panels are
straightforward flat panels. But unlike the floor decks and wall panels, the
type of ceiling or roof system built for a module varies tremendously. Ranches
and the upper sections of two story homes can be built with a special folding
truss system that combines both the roof and the ceiling. Conversely, the lower
sections of two story homes only require a ceiling section. But while there is
a great deal of variability, the ceiling, roof combinations can be broken down
in to a small subset as shown in the following grid:
Ceiling and
Roof System
|
Ceiling
System (no roof system)
|
Roof System
(no ceiling system)
|
|
Floor Load Ceiling
|
Cape
Second floor of
two-story house with expandable attic
|
Contemporary,
or other style where roof system will be built on-site
|
|
Non-floor Load Ceiling
|
Ranches
Second floor of
two-story house
|
Lower/first
floor of two-story house
|
|
No Ceiling System
|
Custom vaulted
interior ceiling
(Requires custom engineering not
be offered by all manufacturers)
|
The last equation that goes into the type of roof system is
the pitch, or angle of the roof. But before we can understand how the pitch is
measured or calculated. Roof pitches refer to the number of feet they rise for
every twelve (12) feet of running length. For example, most Capes built in the
United States have a 12/12 pitch. If your Cape is 24 feet from front to back,
then the peak of the roof is 12 feet from the front or back. With a 12/12 pitch
roof, your Cape has a peak height of 12 feet.
If we combine the various roof pitches available with the
ceiling/roof grid above, the styles and types of roofs available to a modular
home is virtually limitless.
Building the Ceiling & Roof Panels
For most types of ceiling/roof systems, the first step
begins on a specialized jig. And like the floor deck and walls, the
ceiling/roof jig assists in precisely laying out the ceiling joists or trusses.
And like the floor deck, a perimeter beam system is incorporated to strengthen
and stabilize the ceiling/roof system. Once completed, the ceiling/roof panel
is hoisted up off the jig and moved into position on top of the open modular
shell.
To streamline the production process, many manufacturers
have utilized the foam-seal epoxy glue system used on some wall systems to
attach the ceiling drywall to the ceiling joists. Before any joists or trusses
are placed on the ceiling jig, the ceiling drywall is placed face down on the
flat surface of the ceiling jig. The ceiling and roof panel is then built
directly on top of the drywall. With the ceiling panel framed out and resting
on the drywall, the foam-seal is applied along every joist where it meets the
drywall. After a few minutes, the drywall is firmly attached to the ceiling. No
drywall screws necessary! Without drywall screws, your new ceiling will never
have a "nail-pop" and thanks
to the flat ceiling jig, your ceiling is perfectly flat. The foam-seal meets
all building codes, and is considered to be superior in strength to drywall
nails and screws.
Automation
Today most manufacturers utilize non-automated manufacturing
practices to fabricate their deck, wall and ceiling panels. A few manufacturers
have had good success with automating the fabrication of walls, and over time,
we can expect that more and more manufacturers will incorporate automated panel
fabricators into their production process. This should decrease the time and
labor cost to produce each panel resulting in lower costs and prices for the
new home buyer.
Benefits of Using Panels
Before we move on, now is a good time to discuss some of the
benefits from utilizing a panelized system for building the initial modular
shell. The most obvious benefits are the speed and efficiency realized.
Panelized construction, whether it is fully automated, or manually built using
panel jigs takes the guess work out of the building process. As a result, the
walls, floors and ceilings are flat and true. With this precise assembly, the
finished house is built with tighter tolerances and few air gaps in the
structure.
Roughing Out the Shell
With the walls secured to the floor deck, and often before
the ceiling system has been installed, the plumbers and electricians begin
installing their systems. This process is virtually identical to that used by
the stick-builders and panelized house builders. The primary difference lies in
the fact that modular building system builds a structure from the inside out.
Once the plumbing and electrical have been completed for the
exterior walls, workers install the insulation. They are now ready to install
the exterior sheathing. Just like the
interior drywall, the sheathing is first glued, and then nailed onto the wall
studs. With the sheathing installed, some modular manufactures add an
additional step to further minimize the level of air infiltration. Some individually
caulk all of the exterior plywood seams while others install a vapor barrier
wrap such as Tyvek. While both are fine, experts disagree as to which is the
best approach for a modular built home.
At this point, the individual modules of your new home
represent one of the strongest, most airtight forms of house construction
available. With both the sheathing and drywall glued as well as nailed and/or screwed to the wall
studs, a modular house has an extremely high level of cross-bracing rarely seen
in a stick-built home. While this added cross bracing goes far beyond what most
home buyers will need, the unfortunate home owners in south Florida could well
have benefited from this high level of structural integrity. And between every
wall stud is a near hermetically sealed air chamber filled with insulation.
Benefits of Construction in a Climate-Controller Factory
While the modular building system closely resembles the
stick-built process, the ability to build the individual modular units within a
factory provide special benefits unavailable to stick-built and panelized
houses. These benefits derive from the modular systems ability to build a home
from the inside out. As the wall units were built, the interior drywall was
first applied. The exterior plywood sheathing will not be install for a few
more steps.
This may seem like a minor issue, but take a look at the
installation of the rough electrical process. With the drywall already
installed, the electrician marks where an outlet box will be, and quickly makes
a precise cut. If you have ever had to install drywall, you'll remember how
time consuming it was to mark where the outlet or light switch on the drywall
before you installed the drywall. With the modular building system, the
drywallers can rapidly install the drywall without stopping to take switch and
outlet measurements. On an entire house, the time savings add up resulting in a
lower price to you.
But there's more. In most homes, one of the biggest
contributors to heat loss in a home comes from the electrical outlet boxes. The
outlets are always larger than the boxes, providing opportunity for outside air
and temperature to enter the living space. To reduce this air infiltration,
most modular companies seal around the exterior wall outlets. This added level
of sealing helps to further reduces heating and cooling bills. Since most other
forms of house construction install the outside sheathing first, sealing around
the wall outlets is virtually impossible with other forms of house building.
Folding Roof Systems
With the delivery restrictions placed upon the modular home
manufacturers by the various departments of transportation, designing and
building the ceiling and roof systems poses perhaps the greatest engineering
challenge for modular home manufacturers. When a manufacturer ships a house to
a general contractor, they want the home to be weather tight before the end of
the day. Manufacturer's warranty every home they build, and the less exposure
to weather, the lower then the chances the manufacturer will be called upon to
make adjustments. To solve this problem, the modular home manufacturers have
developed numerous methods for incorporating special folding roof systems into the design of their modular units.
The easy way out for modular home manufacturers would be to
simply not build the roof system, forcing the general contractor to build the
roof on site after the house modules had all been set on the foundation. And in
some cases, this is the only viable alternative. But for the majority of houses
built using the modular building system, manufacturers have developed numerous
methods for structurally sound folding roofs.
As we saw earlier in the chapter, there are several
different roof types, and for each type the manufacturer has a solution. While there are different styles of roofs,
including mansard, gambrel, hip, the occasional flattop, and the traditional
sloped shed roof. Sloped roofs are found on Capes, colonials, Victorians, two
stories, ranches, and split levels to name just a few. And for these homes, the
distinction is the pitch of the roof. Let's take a look at how the roofs of
some of these homes are designed.
Moderate Pitched Roofs: Ranches, Two Stories, Split Levels
Most of these homes are built with a 5/12 to 8/12 pitch
roof. In designing moderate pitch roofs, the lower the pitch the easier it is
to design and build. The 5/12 pitch roof is most often built using a special
engineered folding pressed truss system meeting all building codes. These
pressed trusses are no different from the standard trusses used on many of the
stick-built homes except for capability for the truss to fold down. For most
houses, the folding 5/12 pitch roof can be designed with a single hinge point.
The slightly higher pitched 8/12 roof comes with an equally
longer distance between the ridge and the eaves. To build a complete roof
system, manufacturers have developed several methods of accommodating the
increased roof size. The first is to build a roof with two separate hinge
points. For the 8/12, the most common place for the first hinge point is at the
eaves. This allows the first section to lie flat which enables the second
section to flip backwards on top of the first section. In rare cases, the
second roof section folds forward and down, but with the 14' road restriction,
this can only be used when the individual modules are less than the maximum 14
feet.
High Pitched Roofs: Capes, Saltboxes, Contemporaries
For most houses, the design of a high pitched roof closely
follows the same process used for the 8/12 pitched roof. The first hinge point
starts at the eaves with a second hinge point designed to either fold backward,
or in rare cases, forward and down.
Roof Panels
In certain circumstances, the manufacturer determines that
roof panels may be necessary to complete the roof. This could be for small
sections of a complicated roof, an entire section of a roof, or perhaps as an
extension of a folding roof system of another module.
Completing the Ceiling and Roof System
With the sheathing completed, the module moves down the
assembly line, to the roofing station. Here the workman install any additional
structural components necessary to complete the roof system and then they
install the roof sheathing. With the sheathing
in place, the roofing felt paper, ice and water barrier, and then the roof
shingles are installed. For roof systems requiring a second roof fold,
depending on the manufacturer and the specifics of the roof, the second section
may have shingles applied. If they are usually one or two courses, or rows, of
shingles are left off to be installed at the job site after the roof is up.
Finishing the Exterior
All that is left to complete the exterior of the module is
to install the windows, doors and siding. With the precision built window and
door frames built by the panel sub-assembly shop, the windows and doors install
quickly. The next order of business is the installation of trim around the
doors and windows. The door and window trim is not standard with all
manufacturers, so you will need to check on this and have it added if you so
desire. And finally, whether you selected cedar shakes, a wood siding, vinyl or
aluminum, the last step to complete the exterior of the module has workers
installing the siding.
While it would be ideal to have the entire exterior of your
new house sided inside the factory walls, a closer look tells us that the
finished product would look unnatural. To better see this, consider the
building of a popular two story home using the modular building system. At both
gable ends, each of four modules, plus the raised sections of the folding roof
will all need siding. If each of these units were sided independently, they
would look more like a grid.
Interior Details
With the exterior being completed, workers continue the
process of completing the interior living area of each module. Once the rough
plumbing and electrical have been installed, the few remaining walls requiring
drywall have it installed. The house is now ready for the application of joint
compound. As with panelized or stick-built homes, a team of "mudders"
apply the joint compound to all of the joints, seams, and corners of the house
where drywall has been installed. Applying joint compound is more of an art, and
the best "mudders" are so good that only minimal sanding is
necessary.
Textured Ceilings
A cost cutting trend in home building has been the use of textured
ceilings rather than a smooth ceiling. They're less expensive, so you get a
lower price. But if you ever want your textured ceiling to be finished, expect
a big job. Either you have to remove the texture from the entire ceiling, or replace all the
old drywall.
Mudding a ceiling is one of the more difficult jobs, so when
a ceiling can be finished with a textured surface, builders can get by with a
fast, imperfect mudding job. First they do a quick job of mudding all of the
drywall nails and screws, joints and corners. Perfection is not necessary here.
Once it has dried, a joint compound mix is sprayed onto the ceiling using a special spray gun. Because it is sprayed, the joint compound is slightly thinned out with water. When that dries,
ceiling is ready to be spray painted. If you have ever had to fixed a textured
ceiling and were unable to match the texture, now you know why. Next time, just
add more joint compound. Textured rollers just don't match joint compound that
was sprayed on. Better yet, if you can - avoid a textured ceiling.
Smooth Ceilings
If you have the opportunity, your best option is a smooth
ceiling. In fact, many modular home manufactures proudly promote their near
perfectly smooth ceilings. How can they do this? Remember how the ceiling was
built on the ceiling jig. If the manufacturer was utilizing the latest
foam-seal technology, the first step was to lay out the ceiling drywall. This
drywall was then foam-sealed to the ceiling joists resulting is a near
perfectly flat ceiling. Any imperfections in the ceiling joists or roof trusses
were filled in by the foam-seal. The foam-seal also eliminates drywall screws which
reduces the finish work.
Painting and Trim Work
Once the drywall has been fully mudded, workers prepare the
interior to be painted. Any exposed doors, windows, trim work, plus anything
else to be protected is taped or papered off. The interior is then spray
painted with a base coat of flat primer. Some companies may add a second finish
coat, but most stop after the primer coat. But even though this is only a
primer coat, most modular home manufacturers do such a superior job that a
finish coat may not be necessary. Some manufacturers install the trim work
before painting, while others install the trim work after the rooms have been
painted. Either approach is acceptable as long as the end result meets your
expectation.
Brand Image
To maintain their reputations for superior quality and
workmanship, the majority of modular home manufacturers select well known and
respected brands for installation into their homes. In the end, this works to
your advantage. You get a well built homes surrounded by brand name, high
quality doors, windows and fixtures.
Finishing the Interior
The modules are now ready for the final finishing touches
before it is ready for shipment to the job site. The module still requires the
finish electrical work, plumbing, bathroom and kitchen cabinets, any appliances
if ordered, and finally the floor coverings.
Electrical
Our electrical work has been on hold once the rough
electrical work was completed. Now that the interior finish work is mostly
complete, electricians finish the job by installing all of the switches and
outlets. With all of the units in, quality control can inspect the system before approval.
Workers install the cover plates to complete the job. At this point the
individual modules are ready for the final hookup at the job site.
Kitchens and Baths
Before the plumbing system can be finished, the major
fixtures and cabinets have to be installed. Because the modular building
process builds from the inside-out, most likely the large bath fixtures were
already installed during the building of the modules shell. As the unit moved
down the line, they were covered and protected. With the unit nearing
completion, the protective covering is removed and the final plumbing
connections made.
Workers next install the kitchen and bath cabinets. And just
like the homes built using the stick-built or panelized method, tradesmen at
the modular home companies go to great pains to ensure that the cabinets are
installed professionally. With the cabinets installed, workers next install the
counters and sinks. The kitchen and baths are now ready for the finished
plumbing.
Plumbing
Workers then move in to finish off the plumbing in the
kitchens and bathrooms. Sinks are installed, faucets and fixtures installed and
water and waste lines connected. In the bathrooms, workers install the toilet,
sink, shower and tub connections. All that remains are the final connections at
the job site once the modules are placed on the foundation.
Flooring
All that is left for the manufacturer to do is the flooring.
Most, but not all manufacturers install a variety of floor coverings.
Wall-to-wall carpeting was once popular, but wood floors are now more popular. For the kitchens and baths, a
resilient floor covering is most often installed, but many people today want
tile for the bathrooms. While modular homes manufacturers could offer tiled
floors from the factory, tile styles are a personal and individual choice that
can be better accommodated by your local general building contractor. If none of the manufacturer's choices appeal to you, simple work
with the general contractor to find a local source to meet your needs.
Quality Control and the Third Party Inspection
While the plumbing and electrical staff are finalizing their
connections, it is good to know that all of their work has been performed in
compliance with the applicable building codes, inspected by the quality control
department and approved by a third-party inspection company.
With this level of attention to quality and code compliance,
you have a high degree of assurance that your new home is well been built and
according to code.
Insulation, Air-infiltration and Ventilation
House for house, a house built using the modular building
process is the tightest form of housing available. And when the manufacturer
uses insulation and insulating windows and doors, a modular home has one of the
lowest levels of heat loss during the winter (or loss of cooling during the
summer). But what is the right level of insulation. The experts sometimes
disagree, but if you live in an area where the loss of heating or cooling is
important, at a minimum you should have the following insulating values for the
walls, ceiling and windows:
Component
|
R Value
|
Walls
|
19
|
Ceiling/roof
|
38
|
Floor (between basement)
|
19
|
Windows
|
4
|
Windows
Windows represent one of the most expensive components for a
house, but if inexpensive windows are installed to save money, they become a longer term expensive. Many people chose lower quality, inexpensive windows
to save money, but quickly find out that their choice ended up costing them
even more. It is not uncommon for someone to install storm windows within a few
years of building a home with inexpensive windows to help stop the heat loss.
When you add up the cost of the inexpensive window and the storm windows (plus
installation) high quality windows are a better value. If you need to reduce
costs, have the second bathroom plumbed, but unfinished.
Today's quality windows are double-pane with special glazing.
Some brands, such as Andersen use argon gas between the panes to further reduce
the heat loss.
Transportation to the Job Site
With all of the interior finish work completed, your new
home is ready to be transported to the job site. With the foundation ready, all
the manufacturer and general contractor need is a fair day for setting the
home to minimize exposure to the elements.
