Beyond expense and containing your dog, there are other factors to consider before choosing which flooring to use in your outdoor kennel or run. You will want something that is easy to clean, comfortable for your pet and easy to maintain. How the floor looks also may be important to you if you're setting up a backyard run. If one type of flooring isn't meeting your needs, you can also consider using more than one type of flooring in the kennel or run.
Concrete flooring has a lot of benefits that make it a popular choice. It can easily be cleaned daily with a quick scooping of feces and then a spray with the hose. If you have sick dog, concrete can even be disinfected so that disease transmission is reduced between pets. It lasts for years--even decades--making it very cost effective. The biggest drawback is that it is not comfortable for dogs to lie on and can get very cold in the winter. Also, dogs that habitually run around their kennel can wear down their pads on the concrete, causing injury and infections to their paws.
Grass gives a nice, natural substrate for any outdoor kennel or run. It is comfortable for the dog to lie on and stays cool in the summer. It is inexpensive to lay down sod or grow from seed and easy to rake up feces. However, it does not stand up to the dog that loves to dig and will not survive great amounts of dog urine. Grass is best in large dog runs or used as part of the run. In small areas it is not likely to survive very long.
Many dog owners choose dirt simply because it is what they have available. However, it usually isn't a good idea to use dirt as the flooring for the entire kennel or run. If the run or kennel is in an area that gets wet, the dirt quickly becomes a muddy mess. Mud mixes with the dog feces and you have the start of a cesspool that your dog is going to be lying in and playing in. Unless it is just one part of the dog run and can be kept dry, dirt is not a good idea, and it can even be unhealthy for your pet.
Rubber mats can be used on top of any kennel or dog run floor. They are easy to remove to clean and can even be disinfected. They are great to keep dogs off hard concrete floors and can be a temporary solution to cover muddy ground.