Say you like the idea of flat rappel knots, as they seem likely to slide easily over the terrain when you pull down the ropes — but also you remember hearing reports of flat knots rolling along the tails until the ropes came apart. Here’s a suggestion for securing your favorite flat knot without needing any extra equipment… using only the rope:
The two tails often tilt in the direction of one rope or the other: make a half hitch in the rope in that direction and close to the knot, and pull the two tails through it. Snug up the half hitch against the flat knot, and carry on with your rappel, that’s all there is to it.
The joining knot is still nearly as ‘flat’ as without the half hitch, while the hitch curbs any tendency of the knot to invert under tension. This was proven in testing to about 10X the weight of a climber, or some 15-20X the load that a rappel knot would normally withstand.
More information is available in the Safety page articles, under Resources.