I wanted to be able to let my cats out into our fenced back yard without worrying about them jumping over the fence or other cats getting in. My wife saw a system for sale on the Internet but I made my own for 1/4 of the price. Here's how I did it.

Step 1: The Theory Behind the Fence

In theory, cats will not climb a reverse overhang or jump out and over something when they can see straight down. I found the first part to be true so far, but my neighbor's cat definitely disproved the second part. The unwanted visitor managed to get in my yard, but he could not get out until I let him out and has not been back since.

By attaching plastic mesh garden fencing to my wooden fence, I have been able to keep my cats in and the neighbor cats out (for the most part).

Step 2: Supplies Needed

Green Zip Ties (Fred Meyer) 4 for every garden post.
4' Vinyl coated garden posts (Lowes) Count the number of 4x4 posts that are holding up your fence and buy that many.
40" Green garden fencing (Lowes) This comes in 25' rolls, buy the same length as the fence you are going to attach it to.
2' of 2x4 boards (scrap boards will do)
1' of 2x2 board (also scrap)
1/2" Plastic leg tips (Home Depot) One for every garden post.
Level
Hacksaw or Saber Saw With Metal Cutting Blade
1/2" Electric Drill
1/2" Drill bit
Electric or Manual Staple Gun with 1/2" or 9/16" staples
Tape Measure
Chalk Line

Optional
Drill press
Chop saw or Radial Arm Saw

Step 3: Make Your Drilling Jigs

First, I made a couple of guides for drilling into the fence.

1. Cut two 10" to 12" lengths of 2x4 and the same length of 2x2. If necessary, cut a 2x4 in half.
2. If you have a chop saw or a radial arm saw, cut a 6" wedge of scrap wood at 45 degrees. If not, use a compass or other device to mark 45 degrees on the wood and cut it by hand.
3. Place the wedge under your drill press with your 2x4 pieces on top of it and drill a 1/2" hole completely through the board as shown.
4. Nail the 2x2 to the remaining 2x4 creating a 'V' shaped board.
5. This next step is pretty tricky to do, but if you're patient you can do it. Create another 6" wedge from scrap lumber, except make this one 62 degrees.
6. Put the wedge under your drill press with the 'V' guide on top of it and drill through the joint where the boards meet (through the middle of the 'V').

Some drills have bubble levels built into them. If you have a drill with a built-in level, you can use that instead of a drill press as long as you are very careful to drill straight down.

Step 4: Snap a Chalk Line

Snap a chalk line about 30" down from the top of the fence, all the way around the yard. Using the flat guide, drill a hole into each fence post. Use the 'V' guide to drill holes in the corner posts.

Step 5: Insert Posts

Using a hammer or your 2x4, gently tap fence posts into the holes you drilled.

Step 6: Cut Off the Posts

Using your level, mark the posts you just inserted even with the top of the fence and cut them off with your hacksaw or saber saw. Cap the cut end with the leg tips.

Step 7: Attach the Fencing

Attach the top edge of the garden mesh to the posts using the green zip ties the whole length of the wall or the length of the mesh, whichever is shorter. Make sure to overlap the ends enough so that the bottom reaches all the way to the corner.

Starting in the center, push the green mesh fencing against the wooden fence and attach it to the post near the bottom using another zip tie. Work your way to both ends, one side at a time.

Finish up by adding two more zip ties to each post evenly spaced so that the fence is attached to each post with four zip ties.

Using your staple gun, affix the green mesh that hangs down below the posts to the wooden fence. I put four staples into every fence board, but you may choose to use less.

Step 8: Happy Cats!

The cats love being able to go outside again, even though they are all big babies and wouldn't last ten minutes in the real world.

I wanted to get this published for a friend, but I will be adding directions for being able to open your gates. I'll show how I did it, but I'm sure someone with more imagination can figure out a better way to do it.