Horse trailers can be a costly investment, but they are a necessity if you have horses. If you ride them frequently, you are limited to riding them closer to home unless you have a horse trailer. In addition, in an emergency, you may have to have a way to transport a horse. You can save money on a horse trailer by building one yourself.
Choose the Chassis - Choosing a sturdy foundation will provide strength for a solid framework. Lowboys, flatbeds or utility trailers can make good foundations for 4-horse trailers. Both are available with reinforced steel or aluminum frames. Be sure you get one that will hold the weight of the frame and the number of horses you will be hauling. With a little reinforcement, you can also use a boat trailer to make a 1-horse or 2-horse trailer. You will have to weld a few pieces onto the frame to make it square and strengthen it with some cross pieces. It would be helpful to know what size boat the trailer carried so you would know how much weight the trailer handled to start with.
Building the Frame - Pressure-treated 2-by-6 inch lumber works well for a floor. Add rubber mats made for horse trailers to prevent slipping. Frame the walls with treated 2-by-4 inch boards, then use ¾-inch marine plywood for the panels to box it in. You can add Plexiglas or fiberglass windows in the panels for air circulation. Glass is not recommended because it may break.
Building the Roof - There are a couple of options you can use for the roof. You can build the whole trailer like a box, making the sides long enough in height to put a flat roof on it. An alternative would be to build the sides about 8 feet high, then build an A-frame on the top to accommodate a piece of sheet metal or aluminum. Bowing the sheet, fasten it to the top and the sides of the A-frame.
Building the Door - You can accomplish two things at once if you build a door heavy enough for the horses to walk on. Make the door so that it hinges on the bottom of the trailer and fastens at the top or at the sides; when you lower it, it provides a ramp for the horses to walk up on. Reinforcing it with some steel rods on the outside, or using cross pieces of 2-by-6 inch boards will strengthen it.
Finishing Up and Maintenance - To hook up lights, splice into your truck's back tail lights so that when you hit the brakes on your truck, the lights on your trailer show up as well, to let people know you are stopping. You can also hook up trailer brakes that work with your truck brakes and help you stop the trailer. Be sure and move your trailer about once a week to prevent the tires from getting flat, and check them periodically. If possible, keep the trailer parked under some sort of shelter to protect it from the weather. Even if you never use your horse trailer, having one is a comfort. You know that in the event of an emergency, if you have to transport one or more horses, you will have a way to do so.