The fence around a chicken run must be tight enough to keep out predators that jump, dig and slither. In addition, predators must not be able to reach inside and snag unwary chickens through the holes. The fence should be tall enough to discourage avian escapees, though some chicken breeds may need to have their wings clipped to stop them from flying out. A fence, by itself, will not deter hawks and owls, but you can tie ribbons or flags on the top of the fence to discourage aerial predators.
Dig a 3-foot post hole with a post hole digger at each corner of the chicken run. Place an 8-foot long, 4-inch wide round or square wood post in each hole. Pour a bag of concrete mix in the hole and fill it the rest of the way with dirt. Tamp the dirt down firmly with your feet or a piece of scrap wood.
Tie a piece of twine to one corner post about a foot above the ground and run it around the outside of the other corner posts. Tie off the other end. The twine acts as a straightedge marker so your fence lines don't become crooked.
Drive 7-foot T-posts into the ground every 8 feet along the fence lines using a T-post driver. Drive each post 2 feet deep, leaving 5 feet above ground. Make sure the flat faces of the posts all face the same direction.
Dig a 6-inch-deep trench along the fence line. Burying the bottom 6 inches of the fence wire discourages burrowing predators from digging under the fence.
Staple one end of a roll of 5-foot tall, 1-by-2-inch wire net fencing to one of the wood posts adjacent to the chicken coop to which the run is attached. Make sure the bottom 6 inches is resting in the trench. Leave enough extra to span any small gap that might exist between the fence post and the wall of the chicken coop and staple it to the wall.
Stretch the roll of wire around the outside of the fence posts and staple it to the corner posts; 1-by-2-inch wire net fencing can be stretched by hand without mechanical help. Cut the fence wire to length with wire cutters and staple it to the chicken coop wall to close any gap between the coop and the last wood post.
Fill in the trench with dirt or gravel to bury the bottom of the fence wire. Tamp it down with your feet or a piece of scrap lumber if using dirt.
Hook the fence wire to the T-posts with fence clips and clamp the ends of the clips down with fence pliers. Three or four fence clips per T-post are sufficient for a sturdy fence.
Tie lengths of ribbon or surveyor's tape to the top of the fence every few feet. The lengths should be long enough to blow in the breeze, which disconcerts aerial predators.
You can add a gate to the fence if desired, but the gate will always be a weak point since there are gaps around the hinges and the bottom can't be buried. It's better to have an entrance to the run from inside the chicken coop whenever possible. Some edging tools and small garden tillers have a trenching attachment that reduces the labour involved in building the fence considerably.
No outdoor chicken run is completely secure unless it is fully enclosed on all sides, the top and bottom, which is expensive and impractical for large runs. If chicken losses are a continuing problem, consider a livestock guardian animal such as a dog, goose or llama.