Perhaps the most commonly used rock climbing rope is the classic single. This will in general be anything from 8.9 mm up to 11 mm in diameter and is designed to be used as your one and only single rope for protection in the event of a fall. These are particularly suited to modern sport climbing and to naturally protected routes, where the line of the climb does not wander. At the thinner end of the range, the ropes have the advantage of lightness, but are not as durable as the thicker models. Any abrasion or nicking of the sheath makes a much bigger impact on the overall strength and reliability of these thinner climbing ropes. The fatter ropes up to 11mm are easier to grasp, can sustain more punishment and have significantly more resistance to cutting than the thinner versions, but have significantly more weight.
The evolution of the use of double thinner ropes was probably driven mainly by British climbers. This came about, in part, because many lines on our smaller British crags have intricate wandering lines, as the UK lacks the soaring buttresses and mountains of the USA and Europe. Our sport had to make the very best of a much smaller surface area of rock. Double ropes were used initially, in order to reduce the rope drag that could occur with a single rope that was clipped in to the protection first on one side and then the other side of the route, as a climber picked their way up an indistinct line. The use of two ropes meant that each rope could be slightly lighter; travel a slightly smoother path up the route and led to the development of specialised climbing half ropes around the 9 mm mark. Each half rope in the double rope system is rated to take a significant fall by itself. Another advantage, particularly useful on Scottish winter climbs and other large mountaineering undertakings is that, if a retreat is necessary, climbers can abseil the full-length of the rope, rather than half a rope length at a time. This is quicker, more efficient and reduces the number of potentially dubious anchors that the climbers have to rely on.
As climbers and manufacturers strove for lighter equipment, for more extreme situations, twin ropes appeared on the scene. These are the ultimate skinny bits of climbing string. The reduction in weight and diameter produced climbing ropes that had to be used as a pair at all times. Each protection point that was passed had to have both ropes clipped in because the strength of the rope was much less than a half rope. The elasticity of these thinner ropes also meant that a climber fell much further before being brought to a halt and even though he or she would suffer much lower impact forces, the extra distance significantly increases the risk of injury through hitting something as you fall. Used as a pair, the ropes offered as much protection as a single thicker rope in fall situations and gave the advantage of being able to abseil full rope lengths rather than halves. They did however have the same disadvantage of rope drag when the line wandered from side to side; added to by the fumble factor of having to clip two ropes into protection.
The final category of climbing rope is the simple hill walking safety rope which is intended as a reassurance for members of walking party or scrambling group but not as a leading rope. These are typically around 8mm diameter and are the type of rope that walking leader could use to secure members of his group as they tackle steep drops and even when descending simple but steep grassy slopes. These types are clearly marked as being unsuitable for out and out climbing use.
There are a couple of really critical points here. Twins really should NOT ever be used as half or single, simply because of the extension when they are loaded – they’re stretchy little blighters! Although each half rope is rated to take a big fall on its own, it is poor practise to use them as singles, because they are much less resilient than a fully rated single rope. These days it is also good practise to check that the rock climbing rope you are buying carries the appropriate UIAA rope marking label, heat shrunk onto the end of the rope.
Additionally, despite the huge strides in rope technology of recent years, it is absolutely imperative that you check visually and by touch the whole length of your rope every time you uncoil it or take it out of your rope bag. Any cuts or tears in the sheath are causes for concern and need closer checking. If the sheath is torn to such an extent that the core of the kernmantle construction can be seen, then at the very least, that section of rope needs to be cut away. Inspection by touch involves running the rope through your hands and feeling for any lumps or bumps in the core that could indicate damage or degradation of your rope. If you have any doubt about the safety or integrity of your rope, then maybe that’s the day put off the project and go shopping for a new rope instead.