Keep in mind, that the goal for all anchors is "SERENE".
- S - Strong (or Solid) - The stronger the better
- E - Equalized - Anchors should be constructed so that each component of the anchor carries an equal amount of the load
- R - Redundant - Anchors should consist of multiple components in case one or more components fail
- E - Efficient - Anchors should be as simple and timely as possible without giving up any of the other SERENE qualities
- NE - No Extension - Anchors should be built so that if one or more of the components fail the remaining components won't be shock loaded
To anchor off a tree, there are a few rules of thumb you might want to keep in mind. These include (1) make sure the tree is alive, (2) keep the anchor as low as possible, (3) be sure it's at least as large as your thigh, and (4) be sure it's firmly rooted in dirt (not 4" of dirt on top of solid rock!). Before you commit yourself and your partner/s to a tree be sure to push on it and make sure it doesn't move or break. Be careful not to fall over the edge if it gives way while you're pushing it! Also, check for sap and any sharp rocks that might be near the tree. You can use an old piece of carpet (or your partners favorite sweatshirt) to protect the rope, slings or tree.
Note - if you're Top Roping off a tree anchor, add another carabiner (preferably a locker) at the master point. These pics only show 1 for clarity. Also, when tying in with a clove hitch, be sure to orient the "load strand" toward the back of the 'biner for maximum strength. And don't forget to lock the 'biner and tighten the clove hitch before you lean into it.
Tools needed: A couple of locking 'biners, slings, and a rope.
Option #1 - Double Length Sling: Wrap a double length sling around the tree and close it with a locker. This is OK. It's Strong (assuming it meets the "rules of thumb" above), it's Efficient, it's Equalized but it's NOT redundant. I would add a 2nd double length sling if I had one. You can also girth hitch two single length slings together.
Option #2 - Single Length Sling: I guess this would work in a pinch if that's all you had. But it's bad for all kinds of reasons. It cross loads the 'biner, it's high on the tree (because it was too short to go around the base), and it's not redundant. Better add another sling.
Close up showing the 3-way cross loading of the 'biner. This dramatically reduces the strength of the 'biner. If you forgot to lock the gate, the 'biner could blow apart in a hard fall. Not good.
Option #3 - Add a 2nd Single Length Sling: Again, this would probably work in a pinch but it's still got some serious issues. It's not redundant and it has a "pulley effect" the way it's threaded through the 'biner. The pulley effect puts unnecessary force on the anchor.
Option #4 - 2 Single Length Slings Girth Hitched: These 2 slings are girth hitched together. Much better but still not redundant and the slings put a bit of cross loading on the 'biner.
Option #6 -Open Sling or Cordellete: If you have 1" tubular webbing or a cordellete, you can simply open it up and tie it around the tree. Use a water knot on the webbing and an overhand knot in the Cordellete. I recommend 8 millimeter Cord or 1" tubular webbing for this purpose. It's rated at over 3,000 lbs. Note - Don't use 1/2" webbing. It's only rated for 1,000 lbs. - which sound like a lot but a top rope fall could easily generate 1,000 lbs. of force. This setup is good but still not redundant. If you had a couple more slings, you could add them both.
Option #7 - Doubled Cordellete: Finally some redundancy! This meets all the SERENE requirements and rules of thumb. It's quick and simple. Just wrap the Cordellete (or 1" tubular webbing) around the tree twice and tie an overhand knot or Figure 8 knot in the end. If any single line were cut, the whole thing would hold together.
Option #8 - Rope: Take a BIG bight of rope, wrap it around the tree and tie it into a bowline. Don't know how to tie a bowline? Better learn before you climb in an area that requires tree or rock anchors. Practice it in your backyard and have the neighbor kids pull test it before you head out to the rocks.
Close up of the bowline knot. Note - this is turned around 180 degrees from the pic above (sorry for the confusion!). This setup would be even better if it was tightened and had a stopper knot tied behind the loose end. This setup is efficient but uses a LOT of rope. To get a redundant master point (aka power point) in the rope, simply tie an overhand or Figure 8 knot in both ends of the rope between you and the bowline knot.
Option #9 - Tree Wrap: So what do you do if it's late, you're tired and can't remember how to tie a bowline in the dark (much less remember your own name)? Just walk around the tree, tie a Figure 8 knot in the end of the rope and clip it back into your belay loop or harness. I know what you're thinking, "But it's NOT redundant!" Well it doesn't need to be redundant, it's ROPE! The rope can literally hold you, your partner AND your truck without much trouble. If you have enough rope, you can add a Master Point by bringing both ends together between you and the tree and tying a Figure 8 or overhand knot. Assuming you chose this scenario because you can barely remember your name (or were nearly out of rope) you can also belay directly off your harness. Often times, simple is best.
Stay safe and have fun!