Not sure what kind of fencing you have, but mine will get up onto the top rail of 4' high chain link fencing, even with BOTH wings clipped! Board fencing, the same thing... if they think it's wide enough to support landing on, they'll take a huge jump and then boost themselves up with whatever wing power is left.
My solution was to thread thin bamboo poles at intervals along the fencing so they extended a foot above the top rail of the fence. Then I looped baling wire around the bamboo poles and onto the next one so there was a line of wire 6" higher than the top rail, all along the fence. You can use string if you don't have baling wire.
It's too flimsy-looking for my chickens to want to land on. That's the key.
The strangest thing is, I don't have high fences around 90% of their yard. Most of their area is surrounded by 2' green wire rabbit fencing. I think it's the color. They can't see it from a distance because it's green wire against grass and forest. And once they get up close to it, it doesn't look strong enough to support their weight so they don't try to hop over it. But if there's a branch hanging nearby or something they can stand on to launch themselves over... the fence is way behind them! Opportunists.
Hens go broody when you don’t want them to… and won’t go broody when you do. 
