You know dogs can't climb, and cats quickly learn that and take no notice. Sound waves so - so, but no good if cat is determine or deaf... Yep still to the obvious.... An electric fence. Mine runs around the top of loft and fences. A £10 battery charger is actually better as it is constance, where as a E/Fence works of loops... but still the best which ever you use. http://www.ehow.com/how_5008062_make-electric-fence-car-battery.html. But the charger has to really be the way to go I believe. You can buy one for less than a tenner.