In reply to Ats.Scott:
Depends greatly on the situation. Possibilities include:
- use an intermediate lower-off - leader clips to intermediate loweroff when part way down, unties, pulls rope, threads intermediate loweroff, ties back in, belayer takes up slack, leader unclips from intermediate loweroff, lower continues to the ground.
- If the knot is proper safe, belayer starts climbing the start of the route, when leader gets to the ground, belayer clips to a single bolt, leader unties, belayer pulls rope and uses it to get themselves back down by lowering themselves off the one bolt (or equalise 2 bolts if you have plenty of draws around and are paranoid). Just make sure that as the belayer, you unclip quickdraws as soon as you can on the way up - climb too high before trying to unclip and you can get stuck on the quickdraw, unable to unclip it, then you are in a bit of a pickle.
- if the leader can touch the wall and isn't dangling in space, get them to climb back up slightly, clip into a bolt, untie and pull rope without letting it go entirely then tie back in and lower as before but from half way up, allowing the ground to be reached.
- Shout over to someone else at the crag, they could climb a route next to your leader, lower off (with a long enough rope!), attach to your leader with quickdraws and then you can take your leader off belay and the other belayer can lower the two leaders that are connected together.
- If another rope is available, you could throw it to the leader and, when attached to him/her it can be used to swing them more and more till they can contact the wall, then continue as for the 2nd method above.
- Boulder mat stack and big balls!
- Throw the leader some jumars/prussic loops and tell them to get going while you sun yourself lying at the base of the route and get gently rocked to sleep.
- Tie the leader off on the first bolt/ground anchor (in extremis you could even step out of your harness once you have done this if you can't unload the harness). Then go get another rope from other climbers/the car/etc. Walk round to the top of the crag, make a tree/boulder/etc. anchor in the right place, abseil down and to the leader, clip them to you, release them from their rope (this is going to be darn tricky when free hanging unless you bring a knife) and abseil to the ground. If you had to leave your harness behind and have jury rigged a sling harness this is going to be abject agony enough to persuade you to never ever do something like this again!
bottom line really should be that you are trying to solve a problem of only a small shortage in the rope so you should need a minor fudge. If the rope is far too short, the error should become clear long before you lower the leader to hanging position miles above the ground.
The first option is standard procedure. The second option is something I've done a few times, I think always when knowing the rope length could be an issue. If you find yourself doing the fourth, fifth or seventh, it's not a good sign of your climbing competence. I've never owned a bouldering mat. Amazingly I've actually had to do the first part of the eighth option, making a ground anchor with trad gear on a sport route because the rope, while long enough, had gotten totally wedged in a crack in the ground and there was no way to get it out without escaping the system to collect a nut key from the backpacks to free the rope.