Page 24 - Bob Holtzman "The Field Guide to Knots How to Identify.."

The working load is usually in the range of 15–25 percent of the breaking

strength. This may seem overly cautious, but in fact it’s merely prudent. Most
rope becomes weaker when wet, and many knots reduce the strength of rope by
half, so as soon as you tie a knot, the rope’s margin of safety (the difference
between its actual breaking strength and the working load) could drop from 4:1
to about 2:1. The working load also allows a reasonable safety margin
considering the age of the rope, its wear, and, to an extent, any shock loads
(sharp jerks) it has absorbed. Any rope that has been subjected to a really heavy
shock load should be retired from use where safety is an issue, and used
henceforth only in non-critical applications.

Ropes in long-term outdoor static use (such as securing tarps over boats or
stacked firewood) are exposed to sunshine and frequent cycles of wet/dry and
freeze/thaw, and they pick up blown dust. All of these shorten a rope’s lifespan.
Cheap polypropylene rope is particularly subject to degradation from UV
exposure, which can make the fibers brittle in a matter of weeks. These kinds of
commonplace uses are a good way to re-use high-quality rope that has reached
the end of its safe working life in more critical applications.

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