I have been using the "Austrian" knot, (ie an overhand knot), for joining ropes together for abseiling off routes for a few years now, with no problems.
I have recently seen examples, on V-threads and fixed gear, of this knot being used to join the ends of abseil cord, (not tape), as opposed to the more traditional double fishermans knot.
Is there a consensus of opinion on this as to whether it's safe or not, is it listed as a valid technique in any instruction books? Or has it evolved from people using it to join abseil ropes together?
In reply to GaryK: As with abseil ropes, the knot will progressively roll and tighten if a loop of cord is subjected to increasing loads. So the same advice would apply - dress neatly and leave long tails - then there will be no reason for the knot to fail by 'rolling'.
Unlike with abseil ropes, thinner cord doesn't have vast safe margins in terms of strength and I am fairly sure that loops tied with a double fisherman's will be marginally stronger than those tied with an overhand.
As such, I'd say an overhand would be a rather poor choice for 4mm or 5mm cord but absolutely fine (when neatly tied c/w tails) for 7mm+ cord. However, given that 4mm and 5mm cord are a poor choice for fixed gear in the first place and should be treated with suspicion regardless of knots, I don't think there is a massive issue.
I have been tying the double overhand knot ever since I changed over from a double fishermans. As for thinner cord at rap points, I won't abseil off anything less than 6mm, preferably 7mm. If I find thin tat then I replace it, or add my own larger diameter cord.
It isn't a question of safe or not. There's a spectrum of safety concerns, with greater strength and security being balanced by various other specific usage considerations.
If, as Jim mentions, you are hanging on with one hand and need to tie a knot, then it goes without saying that a knot you can't tie this way isn't going to be of any use, and so the overhand, whatever its deficits may be, is the only game in town.
If the resistance to pulling a rappel caused by knot drag is important, and/or if concern about the knot actually hanging up and preventing rappel retrieval is a problem, then the overhand knot has been tested to excel in reducing both these concerns, and hence its broad acceptance for joining rappel ropes. A second feature of the overhand knot is that it is relatively easy to untie after loading.
On the other hand, viewed out of any specific contexts, the overhand isn't a good joining knot. It rolls under loading, with cyclical loading being especially worrisome, and there are stronger alternatives that are completely stable.
My take is that if you have two hands and aren't in a desperate rush, then rappel slings, whether tape or cord, should be joined with a double fisherman's. None of the advantages of the overhand apply to knots that aren't being dragged down the rock, the slings are not destined to be untied after use, and so it is hard to see any good argument for using an otherwise inferior joining knot for that purpose.