anchoring to tree for rap

I have a bunch of slings and lockers to set up a rap station at a tree, no loose webbing or cord though.  Is the best way to girth hitch the tree or to wrap the sling around the tree and clip both ends of the sling with a biner.  I am weary of the girth hitch, as it can generate crazy torque forces at the hitch.  I see a potential for loading the biner in three spots if I do the second option, but only if the sling is too short (I would be using double length).  Thoughts?  Or do I just need to pick up some webbing and tie around the tree?

 Well, they all can work just fine done properly. But each has some advantage.   A girth hitch cuts into itself and the tree, as you mentioned and weekends it. But even at a 60% reduction in strength, you're still at eight or 9 kN, which is more than adequate for a rap.   I am assuming you will double it up anyway for redundancy. 

Alternately, the basket hitch which is clipping both ends of the sling is stronger for the sling and probably preferred if the sling is long enough   I wouldn't worry too much about weird loading on the biner especially if you are using two. No one has died from tri-axle loading on a biner from repelling loads.

 If this is a permanent rap station, then webbing and some steel links are probably best. Very cheap, cut whatever length you want, wider band puts less stress on the tree, less likely to get stolen. 

I've had to make (or replace) a few rap station on trees. I either use a cordelette or tie webbing, but slings work if that's all you have.

If using a sling, I'd prefer a longer one and to tie an over hand or figure 8, then clip a biner through that point. The reason for tying the knot is redundancy. If one strand fails (unlikely but possible), the other will hold. This is not the case if you just put the sling around and clip it together. I would not girth hitch; it's bad for the tree and I can't see any benefit to doing it, in this case.

Cordelette is cheap to replace and can be used on larger objects, and still tied with a "master" point for redundancy. Webbing is better for a living tree, or so I've heard, because it distributes the force over a wider area and does not cut into the trunk as much.

Webbing is also cheap. I buy it by the roll. I've recently replaced my chalk bag belt with webbing, so I have some if I need it. On multipitch I sometimes carry webbing and a rap ring. I've used that setup once to create a rap station. My preferred method is to wrap the webbing around the trunk twice (6' of webbing seems a good length) whilst threading it through the rap ring, and then tie a water knot with a good 3"-4" of tail on each end. That won't create redundancy in the wraps around the trunk, but it better distributes the load. Make sure that knot is tight and has plenty of tail.

If there's a doubt about the integrity of your setup, you can build a separate anchor (if the tree is solid, you can tie off to it separately), and use that to back up your rap station rig for the first person to rappel. The idea being to fully weight the setup you intend to leave, but have the second one in place with minimal slack in the event the primary fails. If the primary worked for the first person, it will likely work just fine for the second.

Mathias wrote:

If using a sling, I'd prefer a longer one and to tie an over hand or figure 8, then clip a biner through that point. The reason for tying the knot is redundancy. If one strand fails (unlikely but possible), the other will hold. This is not the case if you just put the sling around and clip it together. I would not girth hitch; it's bad for the tree and I can't see any benefit to doing it, in this case.

This would be the best option, although depending on the tree a double length sling might not be long enough. If that's the case I would girth hitch 2 slings together and then do this. Yet girthing slings together reduces their strength but it won't matter because you're rapping. If you're setting up a permanent anchor rather than bailing, I would suggest getting some webbing and 2 rap rings and doing this. 

#1 Choice basket.  As mentioned, no strength reduction so it's stronger, but requires a longer sling or smaller tree, so nature might make this choice for you.

#2 choice girth.  Still plenty strong, and gives you extra length.

The problem with both is redundancy of soft goods.  You can live with that (plenty do), use 2 slings (fine), rap directly off the tree (dick move as pulling the rope damages the bark, but doable in emergencies) or, my preference, is to use a cordelette and tie both ends into a figure 8.  Quick, easy, bomber.  You may be able to do this with a long sling but I usually either find the slings too short to fit the tree or the trees too thin to act as a single piece anchor if the sling does fit.

From a cost stand point, you're going to spend a lot less with 1" bulk tubular webbing and a rap ring/quick link than you will leaving sewn slings and a locker. Unless these are planned to be recovered later?