I'm a pretty experienced sport climber getting into trad and was wondering if people could help me out with this. I really want to get a rope that's light but will still take a few falls. I'm wondering if a 9.0mm rope marketed as a half would work as a single. Please discuss and let me know what you think.

Are you under 50 lbs?

I wouldn't use a half as a single ever, but that's just me. If you want something small and light, one of the smallest diameter ropes still rated as a single is the Mammut Serenity at 8.9mm.

Twins and halves should only be used as a pair. I think though that you'll find the peace of mind that a thicker rope will give you will more than offset the extra weight; especially when you're lugging a rack up the wall anyway. Just my opinion. Good luck, have fun, and be safe.

Unless you are in the Alpine where weight Reallyt matters, than I would say no.

my rule: twins as singles get ONE lead fall.

ETA... and as per Jace above, that's for alpining- i wouldn't take a single twin to the crag.

You should buy my buddy's rope, it's 9.4mm.

Seriously, he's selling it in the forums.

no. you could pick up the Mammut Serenity at 8.9mm thats a single line if you're that hell bent on saving weight.

the rest of us usually land in the 9.4mm to 9.8mm range, depending on our preferences.

Sterling 9.2 nano
Light and as long as u are leading an following handles great. If toping hangdoggers it gets twisty. But that rope has at least 10 lead falls over 15 feet and still has such a soft catch it makes a screemer seam obsolete.

10.5 is where it's at

you should definately fuck around and use a half rope. hell you might as well go thinner, or just use some cordelette. Its a half rope for a reason bud, those options for a single in the same range are golden for sure.

Half Ropes are nearly ALWAYS fallen on as a single cord. They are designed to survive at LEAST 1 "standard" UIAA single rope fall (FF 1.78, 80kg etc) . They test with the lower 55kg weight to get a better idea of how strong they are "above" the single 80kg Drop.

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Also, a rope's rated Thickness isn't an accurate indicator of "lightest". You should be more focused on the weight/meter rating.

That said, I'm not sure WHY you'd want to use such a thin cord for Trad work. You'll be buying ropes more often, many skinny cords have higher impact forces (with Mammut being REALLY high) and the risk factors increase (edges that cut ropes always seem more common in the trad realm).

Unless you're doing a lot of pitch linking and running a long rope with that (70m+) the weight savings can be found elsewhere without compromising safety (biners, choice in cams, etc).

Do what you want but realize that really skinny ropes and Trad climbing is REALLY rare and with good reason...

Take a bigger shit in the morning and use a 9.5 or bigger so you never feel the need to say "take".

Responding to the OP - back in the 90's, I actually seriously considered buying a 9.0mm half-rope to use as my sport redpoint rope. I already had two 10.5mm rope which I was using for trad and sport and there were only a small number of sub 10mm single ropes available back then.
A successful UIAA testing of half-rope is 55kg weight drop 5 times (FF of almost 2.0) without the rope breaking. I weighed about 52kg back then so I figured I should be OK with a 9.0mm half rope. I asked around and was not able to find someone else who had tried the same and lived! :) So I got a Mammut Galaxy 10mm instead; one of the best ropes I have ever used. For the life of that rope, I never felt any failed redpoint attempt was because the rope was too fat or heavy. Since then, I have stuck to 9.8mm to 10.2 mm ropes. I have come to realize that the doubt that comes with using a super skinny rope more than negates any weight and rope-drag advantage that it may offer.

I've been beating on an Edelweiss Performance 9.2mm 70 (now 63ish) for over three years. I'm considering replacing it just now. Just be judiscious with it.

Aside from rope diameter, I'm going to suggest that you should perhaps reconsider your approach to falling while learning trad.

First, there's the issue of how good your gear is -- which you won't really know for sure for a while, and should probably refrain from finding out the hard way.

Second, there's the issue of ledgy falls. You may be a 5.18 climber who only needs to step it down to 5.11 to learn trad, or -- like many people, you'll dial it way back to easy routes to concentrate on learning gear. Most easier routes are far less steep than even their corresponding sport grade, let alone harder sport. Falling on less-steep routes is less good of an idea.

Just something to consider ...

@JSH...

That's exactly what I've been doing is jumping on 5.7 and placing a lot of gear to get use to it. But these cracks are steep, but juggy. I've also been taking falls on the gear 10ft off the ground with a crash pad to test placements. Could you suggest another method to get into traditional climbing?