PING Pat

this is what I was talking about with the over hang

http://www.fanciers.com/cat-faqs/behavior.shtml all the way at the bottom
Keeping your cat in your yard
Cats are very good at scaling fences. But if you have a yard that is
otherwise fenced in, you can try keeping your cat from going over the fence
by attaching corrugated fiberglass to the top of it. There is then no
purchase for the cat to pull itself up. It is even possible to find
different colors of the fiberglass to keep it inconspicuous. Keep in mind,
though, that many cats are clever climbers and high jumpers and may
circumvent anything short of a yard totally enclosed and roofed over with
chicken wire.
You can try making an overhang on the top of the fence, if you cat cannot
jump directly to the top of it. Use large bookshelf type angle brackets and
drape netting or screening on it, to create an unstable barrier.

There are some "invisible fence" products for cats, where the perimeter of
the yard is marked with a wire that will activate an electric collar on the
cat. Do not use these without supervision, and ideally they should be used
in conjunction with a visible fence that the cat can use as a visual
reminder of its constraints. This seems to work well with some cats and not
at all with others.