Trailer Construction

trailer construction



Before ordering a trailer, it will help if you have a basic understanding of how a trailer is built.

Most factories use an assembly line process. As the trailer is built, it is moved into another area where a different team completes another part of the trailer.

Although they can vary depending on manufacturer, the most common production areas are separated as follows:

  1. Main frame and axles
  2. Wall frame
  3. Painting
  4. Wiring
  5. Floor and inside walls
  6. Exterior metal sheeting
  7. Doors (side and back)
  8. Roofing
  9. Final finish.

Frame and axles

Everything starts with the frame. Just like the foundation of a house, if it starts off wrong, everything afterward will be a problem and sometimes so bad that the trailer cannot even be completed.

Frames are normally built on a jig which is a template for construction. A jig is a custom-made table used to control the location and placement of the steel. A jig’s primary purpose is to provide repeatability, accuracy, and interchangeability in the manufacturing process.

The outside of the frame is built along with its crossmembers and the trailer tongue.

Once the parts have been welded together the axles are then installed. If axles are not properly placed, it will results in the trailer not pulling straight and excessive tire wear.

Important: When measuring to see if axles have been installed in the location, measure from the center of the tongue where the ball sits, back to the axle. Some people mistakenly think that the measurement should be from the front corner of the trailer box back to the axle.

This is incorrect. The trailer is pulled from the ball hitch, not the corner of the trailer.

Any other steel is added for items that will be installed later in the production process like E-track and D-rings.

The frame is then removed from the jig.

Wall frame

Many manufacturers use a separate jig to build the wall frames. The wall frames are normally built by separate workers while the frame is being built. When a wall is completed it is taken off the jig and put aside until the frame is ready.

When a frame is completed, it is taken into the wall jig area where the proper wall is lifted onto the frame and welded. Roof bows are added to complete the frame and support the walls upright.

If care is not taken to make sure corners are square and measurements are correct, it will make installation of the floor and exterior more difficult, and waste materials later on in the construction process. This can also result in the trailer being warped or twisted, which can make the trailer unsafe to tow.


Most painting is done by using a hand spray gun. The painter should position himself so that the paint covers the entire trailer, underneath and on top.

Because this is a tough job and the painter normally is exposed to unpleasant working temperatures and conditions, many areas of the trailers may not get painted very well and some not get painted at all.

Unfortunately, it is common with factories to cut corners intentionally and not paint areas that will be hidden from view, such as the wall studs and the tops of the roof bows.

This trailer is ready to enter the wiring department.

Trailer framed for walls


Wires are often pulled through holes made in the crossmembers and ran along the inside of the frame. Some higher end manufacturers use wire tubing to protect the wiring and run the tubing along the inside of the frame, not through holes in the crossmembers.

If the trailer has brakes, the brake wires are spliced into the wiring. They are then pulled down or up the back of the trailer for the back lights, along the top inside corners of the walls for splicing in the exterior side lights, and then finally ran to the front of the trailer where the vehicle plug is attached.

Factories differ somewhat on where and how they run wiring. Ask you dealer if you need wiring details.

This is an example of how some manufactures rout wiring through the crossmembers.



Floor and inside walls

The bottom of the plywood/OSB should have been pre-painted to help protect the wood from exposure to the elements such as water and snow. This is normally done in the frame painting area and then moved to the flooring area when the paint has dried.

As you may know, bare wood can soak up a great deal of paint. For the reason, the paint may look somewhat thin after it is soaked up by the wood. Applying enough coats of paint to completely saturate the wood would increase cost which would in turn, be passed on to the buyer.

Some higher end manufacturers use special flooring which is already been waterproofed and does not require painting. While this type of flooring has obvious advantages it will increase the price of the trailer.

The flooring material, plywood or OSB is cut to fit the floor and attached to the crossmembers either with screws or special nails. The plywood should fit as tight as possible to keep water and other road debris from entering the inside of the trailer. Although the inside of the trailer corners should get caulked, the tighter the floor material fits the less amount of caulk needed.

Not all trailers are built with plywood/OSB on the inside walls. If so, the wood is installed in this same area as the floor. Normally, if the walls will be covered with other types of materials such as carpet or plastic sheeting, it is taken to the options department for installation

Notice the plywood is being attached to the wall studs and crossmembers.

Plywood installation on walls and floor

Exterior metal sheeting

Most factories use sheet aluminum for the exterior. This metal comes in large heavy rolls and cut at the trailer factory to fit different size trailers.

The metal is attached with sheet metal screws unless screwless installation has been selected by the trailer buyer.

If the screwless method has been chosen, the exterior side of the wall studs must first be carefully prepared. A chemical is applied to remove and oil or other residue and the studs are brushed using a handheld electric or air tool with a wire brush attachment.

Once the wall studs have been prepared, double sided very high bond tape (VHB) is applied to the wall studs. The aluminum panels are then carefully placed into position and then pressed against the tape.

This type of VHB tape works excellent and is very strong, if the preparation has been done properly. If not, the aluminum sheets can come off the trailer while driving down the road.

Screwless exteriors provide a smooth and attractive finish. It also makes installation of wraps or decals easier and much better looking. This screwless installation is only for the large external aluminum sheets. The external trim is still fastened using screws.

In addition to appearance, a properly installed screwless exterior will hold up better than screwed exteriors and reduce wrinkling in higher temperatures. Screws tend to not allow the aluminum sheets to expand and contract, thereby causing it to wrinkle more easily. In addition, the screws sometimes cause spider cracks to appear over time due to vibration of the trailer.

Due to the added work and cost of screwless exterior trailers, many factories only offer screwed exteriors.

Back ramp doors or barn doors, and side doors also get installed while the trailer is in this area. Some factories may wait until the trailer enters the final finish area to install them.


Aluminum exterior material

This metal comes in different thicknesses and different colors. Common gauges used .024, .030, .040, and occasionally .050. To keep costs as low as possible, many manufacturers use     .024 aluminum but offer thicker gauges as an option.

Because this gauge is very thin, it is very easy to bend or crease during installation. This is one item where trailer buyers should consider increasing their budget and opt for a thicker gauge.



The trailer is then pulled between two scaffolds for installation of the roof. The top of the roof bows are prepared with putty or caulking. Roof material comes in large rolls and is pulled from back to front of the trailer. Screws are used to attach the roof material and some even apply another sealant such as tape or putty.

There are two types of metal used for roofs. Aluminum which has no seams or coated products like the brand Galvalum which has seams every 4 feet. Galvalum is coated with a unique aluminum and zinc alloy. Due to cost, Galvalum is commonly used by manufacturers although some offer seamless aluminum as an option.



This area is where trailer construction is completed.  Hardware and lights are installed along with outside trim/molding, caulking, labels and stickers, and the trailer is cleaned. New tires are now installed, lights tested, and the trailer is inspected to make sure nothing has been forgotten.

Because sheet metal screws are very abundant in many areas of production, junk tires are used on the trailer during construction.

If the trailer is going to have special features “options” added such as floor coverings, cabinets, a/c, generators, and many others, it is now moved into the “Options” department. Due to the complexity of adding these extra features, completion time will vary.

If no Options will be added to the trailer, it is moved into an outside holding are where it awaits pick up by the buyer or a transporter.

Note: The above description of the production of a trailer can vary depending on the manufacturer.

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