What do you prefer to use for a tarp ridgeline? Stretch would seem to be an issue to me and it seems that a non-stretch cord may be better. Also want something not too heavy and bulky but not so thin that it will become a tangled mess.
I guess the default would be para cord, but it stretches a lot.
You might want to look at the nylon cord sold at fabric stores for blinds. It is typically a bit under 1/8" in diameter and well over 100 pound test. It has a braided sheath and core strands. I carry a 25' hank with me just to have some "rope" with me. The hank weighs less than an ounce. You can get it at Joanne's Fabric or Hancock fabric for a few cents a foot. It is not particularly stretchy and it holds knots very well (but not too well).
I bought some dyneema from an outdoor store that looks identical to the blinds string everyone is talking about. But it is not nylon so it doesn't stretch. I think it is about 1 mm cord thickness. I think I paid $6 for 500 ft. The string really didn't have a brand name. I used it this weekend for my adjustable tarp ridge line. I put two prussik knots on either side to hold the tarp on. They were easy to adjust, and held great. I also tied tautline hitches to the pull outs and they held as well. Oh, and I used a figure 9 to tighten it. It was the smallest figure 9 I could find, and the string didn't pull through. I am very happy with it. Of course, I couldn't tell you where to get it, unless you were in Utah.
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While you certainly can use nylon and pull it tight, it does stretch. In most cases stretch is ok, but for instance winter camping snow load may drop your tarp onto you if tied up with nylon, in which case spectra, dyneema, or the relatively inexpensive pre stretched poly rope sold in marine supply stores would be better. Check on ebay. Personally I use 4,000 pound test spectra rope because its easier to work with than string, but still extremely light, amsteel would be overkill even beyond spectra rope. Jim
These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.
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I've found that having the tarp ridge line of diameter and the prussik another helps the knots to grip better.
Of course, it might just be my imagination.
175lb braided trot-line works good for the prussiks. I found some other cordage in the marine dept at WallyWorld that is polypro and doesn't stretch and it comes in orange and lime green so that I can see it (you might prefer less obvious colors, but I don't do a lot of stealth camping.) It's not super light weight but it's not exceedingly heavy either. I've been using it for the ridge line and it works well.
If you think you can, you can. If you think you can't, you can't. Either way, you're right.