Why static rope rather than webbing for toprope anchors?
I'm signed up for an AMGA single-pitch instructor course (to learn how to teach other people single-pitch climbing). One of the pieces of gear we're supposed to bring is a 30 meter static rope "for setting up big 'v' anchors." I don't own one, and the instructor says I can borrow one, so it's not a problem for me, but it makes me wonder why this would be considered such a fundamental piece of gear.
If I'm understanding the application correctly, the idea would be to extend the anchor so that a toprope doesn't have to go over the lip of a cliff, or allow the construction of anchors in which the pro is widely spaced. I would normally have used 1-inch webbing for this purpose.
Why would one prefer a static rope rather than webbing for this purpose? It's more expensive, and a 30 m length would not be useful for other applications such as canyoneering. What are the pros and cons?
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In addition to the other answer I'd like to add that ropes are way safer than webbing in a scenarion where it actually comes in contact with rock. This is the case in top rope anchors when you have to tie the rope back over the edge of a cliff.
Ropes are designed with a protective layer (mantle) and a load bearing inner part, also when moving, they only touch the rock with s amall part and they can 'roll' over the rock while webbing under tension gets severed easily.
Regarding knotting webbing: For nylon webbing this is ok and accepted quite everywhere, but dyneema is too slippery to be safely knoted.
PS: These do not answer your question but provide intersting insights into using knots with webbing /slings:
http://dmmclimbing.com/knowledge/knotting-dyneema-vid/
http://dmmclimbing.com/knowledge/slings-at-anchors/
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It's not specifically answering your question (I don't use either rope or webbing in my anchors, yet) but I really wanted to say that I'm a huge fan of having a chunk of static rope near my anchors. It's one of my most unexpectedly handy pieces of gear.
The main reason I got it was for rappelling to anchor stations to set top-ropes (I don't lead climb, so I have to get to the anchors from above). I used to loop my climbing rope around a tree, rappel to the anchor, then pull the rope off the tree to set it for climbing. It almost always was a mess–dragging the rope through leaf litter, bushes, twigs, and almost always ending up in a snagged tangled mess which sucks to work on off the ground.
So now I anchor the static rope back from the edge; rappel on the static line and leave it in-place to avoid the mess altogether. The static line stays up while we climb, occasionally being used as a backup anchor or a directional, and then occasionally being used to clean the anchor and ascend back to the top.
There've been a few occasions that it's been used to rescue gear (tangled/stuck ropes, trad gear) and one climber (they slipped and got their foot super-stuck in a crack). It's just one of those things where you look at a problem and say "easy!"
fyi - I'm currently using a 20M length of 10mm rope. So far that length has been great, although I regret not spending a little more for something a bit skinnier and lighter.
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This is not a complete answer to my own question, but the following may be relevant.
The book by Long and Gaines on climbing anchors says this:
For toprope setups, most professional guides use static rope when tying off huge boulders and blocks, since it is more abrasion resistant and less likely to jam in pinches than webbing. Static rope is also somewhat handier to work with and easier to knot, and it allows you to use a wider range of knots than would work with webbing or sling material.
I also recently took the AMGA single-pitch instructor course, and we used static ropes extensively for anchors. The big advantage I can see of using a static rope for this purpose is that you can use one end of it as an instructor's tether when you're managing things from the top. The basic idea is that you make an anchor in the shape of an "N." That is, you have a "V" going to two sides of the anchor (either of which could itself be a multi-point anchor constructed with a separate cordelette), and then you also dangle an instructor's tether from one side. Then as you're coaching or helping your student, you can securely operate near the edge. You tie a knot in the end of the tether, and you clip in to it with a grigri.
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Static rope may not be that much more expensive than equivalent tape and is certainly a lot more versatile. In particular rope gives you a lot more options for reliable knots which are also be familiar from climbing rope use. Many people consider any knot in tape to be a bit suspect.
The range of knots may come into play when rigging more complex anchors or when you want to back up the main anchor. In the context of the this specific question it may be that there are some specific techniques that the course is demonstrating. It's worth bearing in mind that single pitch anchors can end up being more complex than just joining two pieces of protection and specific knots may be important for equalisation.
Long lengths of tape can also get in a horrible tangle especially in terms of keeping it flat whereas rope is a bit better at untwisting itself, being round rather than flat.
Also for a very long anchor chain webbing may have more stretch than a static rope and while this is not automatically bad it can add an extra element of unpredictability.
Of course there is also a lot of weight behind what people know to work, especially in terms of safety, so it may be that this course specifies rope because it is tried and tested for they way they do things.
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