Below is an excerpt from the book on Predator Proofing a Chicken Coop.
If you have predators in your area, including bears, foxes, raccoons, and members of the weasel family (minks, martens, ferrets, etc.), then of course you’ll want to keep them from eating your chickens. Most predators hunt at night, which is when chickens are roosting in the coop. You MUST keep your coop secure. While you’re at it, you might as well seal the coop against any rats, mice, or other rodents.
The first, simplest way to secure your coop is to follow my suggestion in Thing 4 above (the advice I didn’t follow myself and later regretted). That was where I suggested that you consider making a walk-in run area. If you have a predator-proof, fenced run that is large enough, you can pop the indoor coop inside of it. And as long as the fenced run keeps out the predators, the coop itself needs no additional security.
However, assuming that making a huge run is not possible or affordable for you, then your chicken coop will be exposed to possible predators. You'll need to make it impossible for these critters to get in. Predator proofing consists of two main tasks: tightening the coop’s construction and putting good locks on any doors and windows. Even a pre-made coop that you spent several hundred dollars to buy may still have some flaws when it comes to security.
To tighten up the coop, first look for any holes. Weasel family members and rodents can get through almost any gap. Move your hands around the outside of the coop, gently pulling apart any pieces that join together, and see if larger holes form. If so, add some additional screws or other hardware at more frequent intervals.
If there are any remaining holes, including windows, vents, and gaps you cannot close with screws, consider screening them with quarter-inch hardware cloth mesh. Do not use office staples to secure this; get a staple gun from the local hardware store along with the longest staples (probably 12-14mm) that it holds. These will be hard for any animal to pull out, especially if you staple on the mesh from the inside. Make sure not to leave any jagged edges, though, since you don’t want the chickens to cut themselves.
The second task is to secure any doors or windows that you plan to close at night. My chickens have a window in their coop that stays open during mild weather, but it is permanently screened with quarter-inch mesh. Use good quality latches, even going so far as to buy better ones to replace the cheap quality latches on a brand new coop. Some animals (especially raccoons) are very good at opening things, so every latch needs to either require brute force or else be complex enough to fool them. Usually, if opening the latch requires two or more separate functions (e.g. pull out, then turn a half circle and pull again), that is enough to fool my raccoons. Multiple latches on different sides of the door/window also helps. I hope your predators are not any smarter than mine!