The content of this post is built on the premise described on this post.
Thieves are considered to be able to climb any reasonable vertical surface with 100% percent efficiency, so long as the distance climbed does not exceed their stamina. A 'reasonable surface' is defined as any constructed or natural surface that is not, a) polished or plastered smooth and intentionally manufactured without cracks; b) possessing a vertical slope greater than 95º; or c) made of material such as smooth metal or glass, possibly of magical origin.
The total distance that a thief may climb without fault is equal to their own dexterity in feet, +3 additional feet per level. Thus, a 4th level thief with a 15 dexterity would be able to climb a distance of 27 feet (15' + 3' x4) without any danger of falling. This presumes that if the thief is able to rest (remain in fixed position without having to use his or her hands to hold on), the thief would then be able to climb another 27 feet without fault. Thus, imaginably, the thief could climb a 3,000 foot cliff, so long as there was a place to rest every 27 feet or less, without any danger of falling.
If the distance between places to rest is greater than the distance the thief is able to climb safely, then the thief must take a chance of falling. This chance of falling is equal to the standard climb walls percentage found in the player's handbook (table posted here to follow later), up to an additional distance equal to twice the thief's no-fault climbing distance.
Thus, a 4th level thief with a 15 dexterity has a climb walls of 88%, and is faced with a single climb of 80 feet. The thief can easily accomplish the first 27 feet; since the remaining distance is less than twice 27 feet (which would be 54 feet, and the remaining distance if 53 feet), the thief need only succeed at rolling 88 or less on a d100 in order to successfully climb to a place where rest is possible.
If the distance is greater than twice (say, it were 55 more feet), but less than five times the thief's no fault distance (a total of 135' beyond the initial 27'), then the thief must roll his or her climbing percentage twice to succeed in climbing that additional distance (anywhere between 82' and 162'.
If the distance is still greater, then the thief must roll three times to succeed in climbing a distance more than five and up to 8 times their no fault distance (up to an additional 256', or between 163' and 283'). Further distance calculations are based on 13 times the distance, 21 times the distance and 34 times the distance.
Obviously, a different base dexterity and a different level would modify the above numbers, which are all part of the same example.
Modern pitons do not exist in a D&D world, but spikes may be driven into walls or rock faces as often as availability allows. If pitons are used, there is a good chance that they will belay the fall of an individual who has slipped. For the remainder of this document, pitons will refer to spikes, and not to modern mountain climbing tools.
Pitons must be placed by persons with wisdom in order to be at their most effective. Any individual can hammer a piton into a rock; only a wise person will put it in place so as to perform its purpose. Note that proper placement of a piton does not guarantee success - but poor piton placement guarantees failure.
Piton effectiveness is never determined until such time as the piton is actually employed to arrest a fall. At that time, the individual who placed the piton (which may not be the individual who has fallen) must make a wisdom check (roll equal wisdom or less). If the wisdom check succeeds, then the piton has been placed properly and has a chance to arrest a fall. Otherwise, the piton will become unfixed from the rock when weight is applied and the piton will be useless.
The chance of a piton holding (when placed correctly) is equal to 100% minus the total weight of all persons falling divided by 50 lbs. multiplied by the number of 32' distances dropped. Thus, a 150 to 199 lb. climber falls a distance of between 64 and 95 feet, which equals a subtraction of 3 x 3 percent - in which case the piton will hold if 91% or less is rolled.
The distance fallen is determined by the length of rope plus the distance above the piton prior to the actual fall.
If individuals tie together, without the use of a piton, the chance of a resting individual (not holding on with his or her hands) on the rock face arresting the fall of another individual is equal to a percentage of 5 times the anchor's strength minus the same percentage applied above. If this percentage fails, the anchor is pulled off the face of the climb and will fall.
If the individual is not resting, and is in fact also climbing, then the chance of that individual arresting another individual's fall is equal to twice the anchor's strength minus the usual percentage.
Non-thieves are able to perform no-fault climbing up to a distance equal to their dexterity. They are able to danger climb just as thieves do, only their percentage climb walls ability is equal to 70%, subject to race adjustments. This climb wall ability does not increase with level.
I'll be adding thieving abilities to this page throughout the week.