In reply to Willi Crater:
> Remind me not to let you belay me.
> I'm currently projecting a 28m route that I sometimes lead from the ground, sometimes work on top-rope. Includes a fairly knarly dyno off a mono at around 8m that I only make about half the time. I'm trying it on a 'stretchy' 9.2mm rope, with a belayer about 75% of my weight, giving me a slackish rope (I hate tight ropes once I'm more than a few metres up). On the many occasions I 've come off this move on top-rope I 've never come even vaguely close to the ground. That you think it's normal to nearly hit the ground on a 25m route from 12m up is a bit worrying. Suggests sloppy and inattentive belaying to me.
I was not belaying on the incident described! Between the three of us on the day we had the best part of 150 years of climbing experience, we were aware, attentive but still shocked, for an instant, it was as if the rope had been cut.
I can assure everyone I am an attentive belayer ( I can even read a guidebook at the same time, 1984 Scafell guide photo )
Predicting rope stretch depends on far more factors than a lot of climbers realise.
It will probably be at a maximum if;
the rope is fully relaxed ie not weighted for at least a day or so
there is no tension in the rope (or even a bit of slack)
the rope is free running with little resistance from quickdrawers, rock edges etc
the belayer is standing out from the rock and the rope loops ( catenary ) in a bottom belaying/top rope scenario.
The rope stretch on your project was probably reduced by weighting the rope on the dog up and lowering off before you weighted it on top rope.
Looking up some rope specs, a typical extension on 80 kg weight is 9%. However this is a slow application of weight from 5 kg to 80 kg.
Weighting a rope is an instant increase from 0 to 80+ kg on the crag. The speed of the falling climber will increase the extension in the real world.
Repeating what I wrote in my first post , be aware.