One of the most critical elements in the specification of any rope is its measure of elongation: how much the rope stretches under varying degrees of load. Elongation is not a bad thing. For example, one good way to reduce force in a rigging system is to use a rope with greater stretch. For climbers, in the event of a slip or fall, rope elongation helps absorb impact energy that would otherwise be transferred to the climber, which could potentially be a source of injury. However, it is important to point out that even our most elastic arbor climbing lines are still low stretch, by definition.
The often-missing number for an arborist in the field is the load. Without a load referenced, the elongation percentage is all but meaningless. There are two key metrics for evaluating rope elongation with a load:
Percent elongation at 10% of MBS: Works well for evaluating elongation in rigging lines.
Percent elongation at 300 lb load: Best for evaluating climbing lines.
Elongation Categories, as defined by the Cordage Institute:
Low Stretch: A rope whose elongation is greater than 6% and less than 10% at 10% of its minimum breaking strength (MBS).
Static: A rope whose maximum elongation is less than 6% at 10% of its minimum breaking strength.