How much slope does a backyard zip line need?

The short answer

Drop the cable 3% of the run between the start and end anchors: 18 inches over 50 feet, 27 inches over 75 feet, 36 inches over 100 feet. That is the maximum for a line that ends in a stop block or a spring brake. Go up to 6% only with a bungee brake that has been installed and tested. Then set the tension so the cable sags at least 2% of its length under the heaviest rider, measured below the lower anchor: 24 inches on a 100 foot line.

The figure most people find is "3% to 6%", from the top of Zip Line Gear's slope guide¹. Further down, the same page says a line without a professionally engineered braking system should not exceed 3%, and that 3 to 6% is possible only with a brake such as a bungee brake, installed and tested¹. Every maker here that gives a percentage sets the same 3% ceiling without a bungee²⁵⁶. 6% is the limit with one, not a target.

Five makers' instructions, side by side

• There are two families of kit. Zip Line Gear, Hyponix and Cable-ride work from percentages, for runs up to 1,000 feet (Zip Line Gear)¹ or 90 m (Cable-ride)⁶. Slackers and Zip Line Fun sell children's seat kits capped at 100 feet and print a table of anchor heights instead.
• The seat kits run steeper than 3%. Slackers sets the start at 132 inches and the end at 78 inches on a 100 foot course, a 54 inch drop, and warns: "Do not add additional slope to the cable!"³ Zip Line Fun's table carries the same 54 inches⁴. Neither relies on a bungee. They rely on a slack cable: the rider starts slowing in the last third and finishes "going slightly uphill at the end"³. Slackers' ZipSpring brake is made for all its ziplines⁷, on heights steeper than the 3% Zip Line Gear allows with any spring brake², so do not mix one maker's brake with another's heights.
• Zip Line Fun's text and table disagree. The written step adds 30 inches at the start for the full ride length; the table beside it adds 54 inches over 100 feet⁴. The table matches Slackers line for line. Ask the maker.

Slope in inches for 50, 75 and 100 foot runs

The slope is the difference in elevation between the anchors, not in how high each sits on its tree. On a hillside the ground supplies part of it. Zip Line Gear's worked example is a 180 foot run where the ground falls 8 feet: the 3% slope rounds to 5 feet, the end anchor goes at 11 feet, and the start anchor at only 8 feet². Slackers does the same in inches: take the fall of the ground off the start height, never below 78 inches, and do not set up on a steep hill³. Site prep and leveling shows how to measure the fall with a string line.

Sag: how tight the cable should be

Sag is how far the cable dips below the end anchor with the heaviest rider at its lowest point. Zip Line Gear's worksheet defines it as the end anchor height minus the cable height under the test weight, minimum 2% of the length². Hyponix gives the same 2%⁵.

That is the second confusion. Cable-ride gives "a sag of 8%" in one line and 2% from the end anchor in the next: at 6% slope, a cable 2% below the end anchor is 8% below the start anchor⁶. Same cable, two reference points. Use the end anchor.

Sag sets the pull on your trees. For a rider at mid-span, basic statics puts the cable tension at about the rider's weight times the length, divided by four times the sag. At 2% sag that is 12.5 times the rider's weight: around 2,500 lb for a 200 lb rider. Halve the sag and the pull doubles. It is a rough scale (it ignores the cable's weight and the slope), but it fits the makers: Hyponix says cables can put 800 to 3,000 lb of horizontal force on an anchor⁵, Cable-ride up to 1,500 kg⁶, and Zip Line Fun wants supports that hold 1,250 lb⁴. The ninja line page describes the same effect on webbing.

How high the anchors go

The seat kits sit far lower, because a child rides a seat a few feet off the ground instead of hanging in a harness above a 7 foot clearance. Slackers fixes the end at 78 inches and the start at 132 inches for 100 feet, low enough that a lost grip is a fall of a few feet, high enough that feet do not drag³. Zip Line Fun sets the low end at the rider's height plus 30 inches, 78 inches for a 48 inch child, raising the line 6 inches per 6 inches of rider height; a rider more than 6 inches off that height waits until the cable is reset⁴.

That height is a fall height. The CPSC public handbook calls track rides upper body equipment needing "significant upper body strength", keeps them off toddler and preschool playgrounds, and puts the handle 64 to 78 inches above the surfacing⁸. The zip lines buying guide covers public units; at home, backyard surfacing and fall height and use zones set what goes under the low point. Slackers points to the CPSC handbook for cushioning, and Zip Line Fun rules out grass and packed earth³⁴.

Trees, posts and clearance

Three of the four manuals that size a tree give 12 inches of trunk diameter, or 38 inches around²³⁵; Cable-ride says 30 cm⁶. Slackers measures 6 feet up³. Hyponix and Zip Line Gear set wooden posts 4 feet in the ground or 10% of the post length plus 2 feet, with guy cables and ground anchors²⁵, and Slackers wants an engineer's opinion on any post because of the side load³. That sideways pull is why the swing set footings in how deep to set legs in concrete and the rules in anchoring a playset do not carry over.

Tree swings explains why anything wrapped tight around a living trunk constricts it as it grows. The kits sleeve the cable in tubing and wrap it in a loose teardrop, the first clamp well off the trunk: 2 feet for Hyponix, 1.5 times the trunk diameter for Slackers³⁵. Torque those clamps as hardware and fasteners describes; both kits give 30 foot pounds for the first³⁵.

The corridor needs side room too: 5 feet each side (Hyponix), 1.5 m (Cable-ride), 6 feet from any structure (Zip Line Fun)⁴⁵⁶, and Slackers wants 100 feet from where any power line could fall³. If that does not fit, see small yards.

Test it before anyone rides

Slackers asks for two tests: 250 lb hung 5 feet from each end to load the anchors, then 250 lb on the trolley, released from the start five times. It must slow past the midpoint and stop short of the first clamp; if it hits, loosen the turnbuckle, or raise the finish or lower the start³. Zip Line Gear puts the stop block 5 feet from the end anchor and warns that a stop block is not a braking system². Re-tension after the speed test, because new cable stretches³, and add the line to the annual maintenance round. Below 32 F, Slackers takes the whole line down and Zip Line Fun the trolley and seat³⁴, alongside the rest of winterizing.

Where these numbers stop applying

These are makers' figures for their own kits, and none cites a standard for its slope or sag. Zip Line Fun says its system conforms to ASTM F1148, the home playground standard, "where applicable"⁴; the standards explainer sets out what that covers. The seat kit tables stop at 100 feet³, and Zip Line Gear's guidance is for lines under 1,000 feet and does not replace an engineer¹. Public and commercial lines are outside these figures. Rider limits run from 200 lb (Slackers) to 250 lb (Zip Line Fun)³⁴, and Slackers tests at 250 lb, above its own limit³; swing set weight limits covers how ratings are set on other home equipment.

What to do next

Measure the run and the fall of the ground. Use your kit's table or worksheet; on a percentage kit, 3% unless a bungee brake is in the box. Mark the anchors, tension until the line is nearly straight, then load-test and speed-test at the heaviest rider's weight and check the sag against 2% of the length. Before the first ride, settle who watches the line, which supervision covers.