Cat Proofing
We've got a garden about 10m by 10m (roughly 30 foot square). Luckily there's a fence that's about 160cm (5 ft 6 ins) around the garden. Whilst the cats can scramble up the fence we now have a shelf on top of the fence. The cats can get up the fence but can't get round the shelf!
As you can see from the picture below, the cat-proofing is not particularly intrusive. It's build of wood (softwood and exterior ply) with roofing felt over the top.
It's a war of attrition though - occasionally one of the cats develops a new escape plan. When they do the cat-proofing makes a wonderful walkway round the garden. It doesn't usually take long to plug each escape route though. The netting you can see in the top right of the picture is to plug one such gap - Mhairi discovered she could climb up the top of the willow tree in the corner, from which it was an easy jump to freedom!
The shelf slopes slightly and is about a foot (30cm) wide. The top edge is about 6 feet (180cm) from the ground although where the ground drops away near the house we've kept to top of the cat-proofing level. Here the effective height is about 8 feet.
Here you can see the use of trellis and plastic mesh to fill the gap between the shelf and the original fence. You can also see the shelf support which is made from plywood and 4x2 softwood.