Look Up! High-rise window cleaning is pretty amazing. To a passerby, it looks effortless on the part of the window cleaner. But in reality, high-rise window cleaning takes a very stringent set of skills to remain safe and efficient. We have recently been asked by many window cleaners unsure about whether knots are acceptable to use during high-rise practices. In this post, we will address how this common misconception arose, what the truth behind this idea is, and the pros and cons to the alternatives window cleaners have been using in place of knots.
To begin, we will address the notion that knots are not allowed in high-rise window cleaning. This thought is pervasive in the window cleaning industry, and the truth of the matter is, it’s not entirely wrong. Knots can decrease the strength of your rope up to about 50%. This can lead people to believe knots are not allowed in practice because they make your rope weak and unsafe. A figure 8 on a bight, for example, decreases your strength by 20%, a bowline 27%-33%, a butterfly 25%, and overhand 35%-40% (Rescue Knots for Roco Students). These percentages are approximations and can vary slightly from test to test, but the point is that all of these are standard practice knots and show a significant percentage of strength decrease.
…knots are fine, so long as your system maintains a strength of 5,000lbs
So instead of thinking knots decrease the strength of the rope and therefore are not allowed, this idea can be looked at as only half the story. The I-14 states “the securing of a rope to an anchor with a knot is permitted providing the specific knot does not decrease the initial breaking strength of the rope below 5,000 pounds (2268 kg) considering the operators intended deceleration and the reduction of tensile strength over the course of daily use.” In layman’s terms, they are saying that knots are fine, so long as your system maintains the strength of 5,000 lbs. ANSI Z359 used to say that knots weren’t allowed in fall protection. That standard had in mind a construction type worker that would be connecting to an SRL and would not need knots. However, the new ANSI Z359.15 makes allowance for knots. Since rope has to maintain a strength of 5,000 lbs, and knots weaken rope, it’s understandable to conclude that knots are knot okay, see what we did there?
So here is the other half of the story. We know the system needs to maintain the strength of 5,000 lbs and that knots weaken the strength of the rope, but what hasn’t been addressed is the rope itself. You see, if we take a look at the most common knot, the Figure 8 on a bight and consider it’s 20% strength loss, we need a rope of at least 6,250 lbs to stay over 5,000 lbs. Taking the high estimate of 33% on the bowline, we’d need a rope of 7,463 lbs. This is well over 5,000 lbs strength necessary. So if this is the requirement, what is the strength for most rope? Well, KMIII, for example, is 8,000 lbs for 7/16″. That easily exceeds the strength required. The fact of the matter is that most kernmantle ropes exceed the 5,000 lbs strength requirement. And that is the full story.
But what are the alternatives to knots, and how do they compare? One thing window cleaners have used to replace knots in the high-rise window cleaning industry is sewn eyes at the end of their ropes. They will only lose 0% – 15% of the strength, depending on the rope. In addition to being stronger than knots, they are compact, and can’t be tied wrong. Having a sewn eye makes connecting the rope to your anchor incredibly easy. The downside is that they add cost to the rope and usually increase the lead-time by a week or more. Termination plates are also popular. We couldn’t find any data about strength loss on plates, but presumably, it’s better than knots. Termination plates are great because they are strong and are removable, but they suffer from being bulky, heavy, and expensive.
All in all, knots are allowed in the high-rise window cleaning industry. Though the strength of the rope must be maintained at least 5,000 lbs, most kermantle rope well exceeds that, even with knots tied in. Regular rope is also the most economical option. Sewn eyes and termination plates are good alternatives used for knots. They offer safety, strength, and presumably decrease the rope strength less than knots. These alternatives, though, tend to be more costly than simple rope. When it comes down to what’s best, it’s a personal choice. But, the fact is that knots are allowed in the high-rise world. For more information, check out some great resources about knots, how to tie them, and their statistics about strength decrease. As always, this blog is not a substitute for proper training!