Slackline is different from tightrope, tightwire, slackrope, (etc) in that it uses a dynamic webbing. Although slackline webbing feels flat, and feels static when in your hands, it is actually a tube (that is stretched flat), and once you put your weight on it you can feel it is quite elastic.

There is some semantic discussion amongst slackers nowadays as to what constitutes “slackline” as more and more longline walkers switch to stronger (less stretchy materials). If nothing else, slackline is 1″ nylon tubular webbing. It may include a couple other types of webbing too, depending on how you look at the situation.

The table below contains a bunch of common webbings (closer to the top) and then newer “exotic” webbings being used for longlines and highlines. If the strength rating has an asterisk (*) that denotes that I am providing an actual tested strength that is LOWER than the manufacturer’s rating (as I would rather provide the least optimistic number). A tilde (~) denotes I have approximated a value.

MIL SPEC vs CLIMB SPEC
If you look at the image above, you will see the tennis-ball-yellow (this color is called “citrine”) webbing has prominent horizontal ridges, like a backpack strap, and the red/silver/purple/orange webbings are smoother in their weave. The ridged weave is military spec webbing, the smooth weave is climbing spec webbing. For the most part, military spec is easier to find, and in a larger variety of colors. REI only carries climb spec in khaki and purple. Other colors can be found in climb spec, but usually have to be ordered through an Internet vendor (or maybe your local indie climbing shop has them). For the most part these webbings are identical. Climb spec is something like 6% stronger (19Kn vs 17.8Kn). I use both. Bluewater makes the climb spec webbing commonly found (sometimes Sterling climb spec can also be found, and sterling has a special climb spec version that I have yet to try, called “tech tape” which will hold 20Kn).

9/16″ AND 11/16″
There are tubular webbings available in smaller sizes than 1″ — both 9/16 and 11/16 inch webbings are available, with 9/16 usually being easier to find (also called half-inch sometimes). 11/16 = 13.3Kn (3000lbs). 9/16=9Kn (2023lbs). Either of these webbings will probably hurt if you walk them barefooted, but in shoes they feel fine, and in general they stretch more than 1″, accentuating the “bounciness” of the slackline. I like to walk half inch because it’s fun for bounce and surf tricks, and it looks more like a “wire” to observers who don’t know what slackline is.

2″
Two inch tubular webbing is super strong (30Kn!), but doesn’t wrap well around carabiners because of its width (forget about primitive tightening, it won’t slip through the ‘biners), and it stretches very little. It’s great for jump lines, assuming you can pull it tight enough (ratchet, pullies, etc). Some specialized jump lines (like those from Gibbon) are not nylon tubular 2″ at all, but a flat polyester or similar.

THREADED LINES
One way of strengthening a line is to run a smaller line inside of it. In the image above you can see the red 9/16″ has been threaded inside of the silver 1″. The physics of this are still up for debate — certainly the two strengths are not just added together (!), but anecdotal evidence and pull tests show that threaded lines are stronger. As half inch is stretchier, there have been instances where the outer line breaks and the inner holds. Also, if the outer line frays, the inner will become visible, which can be a nice safety. Of course, threaded lines weigh about 50% more.

TYPE18
This is a flat (non-tubular) type of military webbing that is no longer made, but still available from some retailers. It’s approx 50% “thicker” than tubular 1″ but has the same 1″ width.  You can see it compared to mil-spec 1″ here and here. It weighs double that of tubular one inch, and offers a 6000lb (26.7Kn) break strength. Line locker pull tests done by others have shown it to break around 7200lbs, so it’s likely the 6000lbs is underrated. It stretches about half as much (9-10%) as tubular 1″ too, so keep that in mind. For normal walking it feels great, very supple, but for jumps and whatnot it is noticeably more ’static.’ This webbing works great for highlines, but beware of barehanded catches, it is known for bloody palms.  A word of warning: Type18 can be somewhat hard to find, is not cheap, and there are some retailers selling “knockoff” versions.  I buy mine directly from Yates.

Be aware, the info below applies to people who are rigging 100+ft lines.  On a short (30-60ft) line, any method is fine!

KNOTS
You’ll find there are several different ways to affix a line to another line, or a line to a carabiner.  The most basic is a knot, usually a waterknot.  A waterknot is just an overhand knot tied in two pieces of webbing, and they backtrace each other.  The larger the bend-radius of a knot, the stronger it is, so I sometimes insert a third 10″ piece of webbing to act as “filler.”  (see this pic for example)  Knots generally reduce line-strength to about 50-65% of rated.  If the line is tied in a loop (a “basket” configuration), the strength of the line is doubled, so overall you can expect at least 130% or so of the regular line’s rated strength.

LOCKERS
Line lockers are great — I swear by them.  They generally do not reduce the strength of the line significantly, so just make sure you use nice, fat, solid lockers.  That being said, it is possible to break/bend lockers.  I use a lot of steel rap rings as lockers, many of which I’ve sourced from one vendor.  These are supposed to be 10,000lbs break strength, yet loading a line with way less than 2000lbs of load, I was able to bend one of these rings close to failure; so watch your gear, double up for protection if possible, use backups, and tie things off so they can’t go flying. If you want to know more about line lockers (how to use them, which to buy, etc) … see this post.

STITCHING
A bartack is a type of burly industrial stitching used to fuse webbing to itself (or to other webbing). Each bartack can hold around 700lbs, in nylon webbing, meaning that a series of ten should be stronger than the webbing itself. I used to trust stitching more than any other solution, but recently I had a couple of bartacked lines nearly fail on me, and I have stopped trusting stitching that hasn’t been done (and tested) by large companies. I would still trust Petzl (etc) slings, as I know these companies randomly pull-test their products to ensure safety, but I am no longer trusting anything I’ve had custom made by small vendors. The stitching in question began to pull out at around 1500lbs of load. This is highly unsafe.

Just how much tension can you put in a slackline?
In the first of our tests, here’s adam putting 332lbs into a line barehanded, then ~450lbs with a carabiner for a handle, then almost 800lbs with two pulleys and some cord.