First of all, what is a round pen?  A round pen is a useful tool in the training and exercise of your horse. Ideally it is an area separate from the arena with footing that is suited to the heavy duty activity that the round pen will be used for. Doing this kind of work in your arena or pasture could cause permanent damage to the base material or to the footing which is on top of the base. Indoors or out, the pen must be constructed as if the horse will be in direct contact with the outer walls, because the very nature of a round pen means that the horse is constantly on the outside rail, right next to the wall. As far as the difference in constructing an indoor vs. outdoor pen, it is quite difficult to construct a round, or even semi-round, building. One easy way to do this is to first construct a sixty by sixty square covered arena, and then build the round pen inside it.

It is crucial to start with a properly applied base material, such as any locally available gravel that will compact to provide a firm, permanent surface that will not sink into the ground that it is built upon. This is the same principle that is used when building a riding arena. An ideal size would be a sixty foot circle with posts eight feet apart.  You may want to leave one of the posts out to enable the initial footing delivery by a large truck, or better yet make a post and two wall sections removable for future footing maintenance. There are many widely available metal round pens that are made up of sections that are bolted together, which you can then affix plywood to, or the entire structure can be made of wood. Many people prefer a wooden round pen due to the fact that wood is usually regarded as safer than metal, with no sharp, unforgiving components to injure a horse.

The gate should be five to six feet wide to enable not only horse and trainer to enter and exit comfortable and safely, but also to allow footing maintenance with a small garden tractor. For larger equipment such as delivery trucks or tractors, a removable wall section is very handy, as noted above. The height of the round pen walls should be solid to a minimum of five feet, with a single wooden 2x6 or equivalent rail above the solid wall at about six feet. Any hardware such as gate latches or training equipment hangers must be on the outside of the pen, or at the very least flush with the surface of the inside wall.

The footing is usually applied last, to avoid any problem with finding lost fasteners or other construction related implements in the finished footing. It is also much easier to set posts, pour concrete, and establish a consistent base line without having to deal with the footing already in place.