There are typically three parts to what's under your barn – the foundation, the footings and the flooring. Foundation and footings are what hold your barn up, keep it from shifting in cold and heat, and provide the stability to keep it from moving in high winds. Those decisions are best left to the professionals. An Extension Service engineer can take a look at your proposed building, the site and the soils and advise you on the proper footing depth and wall sizes. You may want to hire a professional to pour the concrete walls or floors, especially when working with floors with drains or plumbing.
But flooring is what you and your horse will be standing and walking on. The easiest and least expensive is just leaving everything dirt. However, horses in stalls can start pawing the dirt and can eventually make quite a substantial hole. In addition, if you have a high water table, a prolonged rain or melting snow can cause your stall and aisleways to become a muddy mess.
A better option is installing several inches of gravel, sometimes called screenings or 5/8 minus. You want small, irregularly shaped gravel - the roundness of pea gravel can cause it to shift too much and large stones can bruise a horse’s hoof. Once the gravel is installed, compact it down. You can rent a compacting machine, but if you just have a few stalls, you can also just spray the gravel with water, and then use a hand compactor to pound. Repeat a couple of times, letting it settle a few hours between compacting.
Another common flooring option is concrete. It is expensive, but easy to clean and disinfect. It can also be hard and cold on the horses that are standing on it. Many barn builders use concrete in feed and tack rooms to help prevent rodents from burrowing in. Concrete is also common in aisleways and washracks where it is easy to keep dry and clean. However, concrete can be slippery so many people opt to "score" the concrete with lines to make it less so.
The addition of stall mats (like those by Classic Equine Equipment) can help keep horses from pawing dirt floors and can add additional cushioning with gravel or concrete floors. Classic Equine Equipment also offers Tru-Step® Pavers for a safe, comfortable and nonslip surface in horse stalls, aisles and wash racks.
Your flooring is a critical part of your barn, and one that is difficult to change once it's done. Be sure to install electric and water lines BEFORE putting down the flooring