Mobile home construction – At the factory

Mobile Homes built before 1976 were constructed to much lower standards than those built later. Homes built prior to 1976 had only one or two inches of insulation wrapped around the walls, floor and ceiling, 2" x 2" or 2" x 3" studs, uninsulated air ducts in the floor and ceiling, no ceiling vapor barrier, and jalousie windows. In addition, some homes built between 1967? and 1971 were constructed with aluminum wiring. Mobile homes manufactured in 1976 or later were built to much higher standards required by the HUD (US Government Dept. of Housing and Urban Development).

Modern mobile home manufacturing plants are marvels of organizational efficiency. Approximately 150 production workers will produce 10-12 floors per day. Floors is an industry term meaning 1 singlewide or 1/2 of a doublewide. The work is routine, fast paced, and physically demanding. Workers are rewarded for speed and penalized for errors in ways that encourage teamwork and put pressure on them to work as a team. These bonuses are also frequently tied to attendance to avoid problems resulting from absenteeism on Mondays and Fridays.

In the following paragraphs I will go through the construction process. There are variations from factory to factory but in general the homes are all built the same way. Everything is done with jigs from a limited number of plans. The plan to plan variation is not great and anyone who has worked at the factory for more than a few weeks has all the variations memorized. The following description is for a singlewide factory. Just for fun I have attached a list of the defects a new homeowner might want to check for before accepting deliver of home.

Frames Belly paper 16' wide is unrolled for the length needed and stapled to the wood frame that will run around the perimeter of the home. Insulation bats as laid on top of this and electrical wire pulled to the points it will come up through the floor and coiled in place. Pre-assembled water and sewer lines and the ductwork for heat/AC are dropped in. Pre-cut floor joists are then nailed in place. With the joists in place the lines are fastened into position using pre-cut forms to make sure they have the right slope. The result is that all the water, sewer, and electrical lines in a finished home are above the insulation where they will stay warm from the heat in the house. Five or six people can do all this in an hour or less.

Floors. The completed frame is lifted, moved over, and set down on top of the axles and wheels that have been assembled elsewhere. Liquid glue is spread on the top of the floor joists and frame members and sheets of tongue and groove plywood or OSB are nailed in place. Holes have already been cut in these sheets for the water and drain line penetrations. Since the plywood is square this step really determines how straight the walls of the house become. If the grooves are seated properly and care is taken to make sure the panels are lined up before they are nailed in place the materials themselves produce nice straight edges. If things get to rushed, materials are badly warped, and the overhangs get trimmed with a router the edge of the home may look like a snake. In any case, with the hundreds of nails driven in by the air nailers and the glue set, there isn't going to be any way to change it later.

Floor Prep. The completed floor is then rolled to a prep area where one person gets under the home and drives lag bolts through the metal frame members (outriggers) and into the floor joists. This is what connects the home proper to the metal frame. These same bolts are what can be adjusted after the home is set to try and stop floor squeaks. On top of the floor there is a lot of activity. The areas of the floor that will be covered with vinyl get sanded. Holes for the floor registers are cut, ducts installed and glued/stapled into place. The water heater and furnace are fastened in place. The vinyl is put in place and stapled around the edges (no Adhesive). The vinyl is then covered with a protective mat or plastic sheeting. Finally, toilets are set in place.

Walls. At the same time the floor is getting nailed down the exterior and interior walls are being assembled. Like the floor, precut studs are laid out on a jig and nailed in place. Insulation is unrolled between the studs. The edges of the studs are coated with liquid glue and precut panels which will become the inside wall of the house stapled into place. Openings for windows and electrical outlets are cut at this time. The entire assembly is lifted with a hoist and moved over to where it can be set onto a floor. You can imagine that an 80' wall made of 2 x 4's butted together has LOTS of flex in it and how straight it becomes depends on the straightness of the floor it is attached to.

Assembly. One of the exterior side walls is lowered into place on a bead of caulk and fastened into place. The end walls are fastened into place and interior walls placed. The interior walls are pre-assembled, with paneling on only one side so the inside is accessible to electrical. As fast as the interior walls are placed the pre-made cabinets, counters, etc. are put in place, wall mirrors hung etc. Finally the 2nd exterior wall is lowered and fastened into place.

Ceilings. Along with floors and walls, ceilings has been busy laying panels out on a jig and setting pre-assembled trusses in place. Trusses are nailed to perimeter 2 x 4's and held in place by wood strips at the centerline and at a couple of other places. The point where the bottom of the truss rests on top of the ceiling panel is sprayed with a quick setting foam adhesive. The ceiling is sized to go from outside edge to outside edge of the house so the edge of the panel rests on the wall. Do we begin to see how repairing or replacing a ceiling panel can be a challenge? If the ceiling is to be textured, it is picked up and moved to a spray/paint area where someone can walk around under it and apply the appropriate coating(s). Finally it is moved to an area where the holes for electrical outlets, ceiling fans etc. are cut and the electrical boxes installed.

Electrical and Assembly. As soon as the walls are placed electrical can beginning pulling the lines that were dropped in place by floors. Holes are drilled through the bottom of the wall and the floor material and metal sleeves hammered into place. The wire that was coiled in the belly is pulled through these sleeves and up into the walls of the home. The main panel box is installed and breakers etc. wired into place. Someone uses a router to cut notches into all the exterior wall studs and wire for the wall outlets is stapled in these notches. Sometimes, depending on the siding that will be applied, the wire is covered with shields to prevent a nail from being driven through it when siding is applied. At the same time the plumbing people are installing showers and tubs and the cabinet and mirror people are fastening their things in place. Additionally, the ceiling is lowered onto the home and fastened in place with metal straps. Electrical also runs wire from ceiling outlets, fans, bathroom vents etc. Installing electrical boxes and running wire is the process most likely to disturb the insulation. Batts get pulled to the side so boxes can be installed, they come loose etc. If this happens and is not corrected the walls have voids which are going to reduce the efficiency of the heating and cooling systems.

Roofing. A layer of rockwool insulation is blown in on top of the ceiling. Then, if the house is getting a shingle roof, plywood or OSB decking is is nailed over the trusses. The decking is covered with tar paper and shingled. If this is to be a metal roof, there is no decking and a large roll of metal is unrolled over the house. It is fastened at on end and then stretched from the other. When stretched, the second end is fastened down and then the sides are folded over, caulked and stapled.

Siding. While the roof is being done others are fastening siding. This may be hardboard, metal or vinyl.

Trimout This is where the mistakes get found and, hopefully, taken care of. Trim strips can cover a multitude of things quickly and cheaply. This is when protective mats are taken off the vinyl and any damage discovered. Carpet is installed, windows are caulked, interior doors are hung etc.

Testing. The home is hooked up to power, water and gas and everything should get tested. Inspectors are looking for water leaks, dripping faucets, improperly connected light switches etc. This is the point where the some critical management decisions get made. If there was to much pressure for production in spite of absent people or inexperienced workers there will be a much heavier load for the testing and trimout crews. Homes that are not quite ready to ship will start to back up in the yard. The temptation will be to ship them anyhow. However, this is a very temporary solution because the dealers or customers are going to find these problems on the lot or after it has been set. Warranty repairs require huge amounts of travel time and things that would have been simple to fix during construction are much more difficult to handle. For example, sheet vinyl is just laid down and stapled in the factory. If a service technician has to replace it he has to cut it to fit and probably put down quarter round along all the edges. Work that took 15 minutes in the factory becomes a two man, two day job for service.

Transport. Lots of interesting things happen during transport. Drivers are paid by the mile so they like to make time. Police in Albuquerque recently clocked some homes going through a construction zone at 65MPH. During transport the bumps, vibration, and other forces twist and torque the homes and frequently crack panels, loosen connections etc. Drivers occasionally rub against traffic signs etc. Mobile home tires are inflated to 2.5 times the usual pressure so blowouts are not uncommon. If a couple of tires go at once and the frame hits the ground it can be bent making future leveling impossible.

Setup. Setup can raise difficult issues because of the number of organizations involved. The factory is responsible for the home itself, the dealer is responsible for making sure the home was ordered properly and any optional features are installed properly, the setup company is responsible for setting and leveling the home, and hooking up the water, gas and electricity. If air-conditioning is needed that is probably done by another company. Now suppose an electrical breaker blows whenever the AC comes on. Is the main electrical panel, done by the factory, at fault, did the AC installer make a mistake etc. The opportunity for the parties involved to blame someone else are almost endless.