Build a zip line in your backyard (© Adventure Rope Gear)

© Adventure Rope Gear

The popularity of zip lines -- those thrill rides where riders fly along an elevated cable at adrenaline speeds -- is zooming.

There may now be as many as 600 zip lines nationwide as a growing number of landowners build their own just for fun -- like this one.

Want to build a home zip line? You can -- if you're smart, careful, a little mechanically inclined and you have the time. But you can't cut corners. A zip line can be fun, but it can also be lethal if done shoddily, experts say.

Here are the steps you should take to determine if a zip line is right for your yard -- and how to set one up safely.

Step 1: Can your property accommodate a zip line?

Before you spend a dime, find out if a backyard zip line is allowed in the zoning for your area, advises ZiplineRider.com. Check with your neighborhood about covenants and with your municipality or local government about building codes in your area. (One place to look is PermitPlace.com.)

Does your property have the right terrain for a zip line? You don't need nearly as much slope as you think you do -- in fact, nearly none at all, says Mike Reddish, owner of Adventure Rope Gear, a big supplier of zip line gear, and a consultant who estimates he has built about 50 zip lines.

For a standard zip line (called a "center drop" zip line), in which a rider stops about two-thirds of the way down a wire that's strung between two anchors, you mainly need some stout trees for anchors and a very gentle slope -- probably less than 4%. "And where the low point is [to dismount] we want a flat, smooth area," Reddish says. "The ladder has to be on stable ground" where the rider dismounts, he says.

For a popular kind of zip line in which riders coast all the way to the end of the wire (called an "end drop" zip line), the ideal terrain is two hills -- one at the start that's taller than the one at the end, by about 5% to 6% of the distance between the hill tops. For this "our recommended slope is 6% -- that's six feet of drop for every 100 feet of run," says Aaron Roper, sales and marketing director at ZiplineGear.com, an online retailer that sells premade kits to businesses and private individuals.

Once you find a run, you must "clear it of trees, branches, bushes, old cars, live ordnance and pedestrians," -- 10 feet on either side of the line, Reddish writes in his cheeky, highly informative PDF booklet, "Zip Line Construction Guide." (ZiplineGear also has some instruction and safety manuals here.)

Step 2: Anchors are huge

A good zip line lives or dies (and so might you) on great anchors at both ends. Know this: The load on the anchor is many times more than a rider's weight, so your anchor has to be beefy. Trees can be great anchors. Look for trees that are strong, healthy and that grow straight up or even slightly angled away from the zip line run, Reddish says.

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Here's a good rule of thumb: If you're considering using a tree, make sure the branch or trunk you'll wrap is at least 10 inches in diameter plus 1 additional inch for every 100 feet of zip line, Reddish says. Consult an arborist if you have any doubts.

Step 3: Gather your parts

You'll need several things for a basic zip line:

Cable. Use 3/8-inch, 7-by-19 (a gauge of the number of strands) galvanized aircraft cable for most applications, Reddish advises. Use 1/2-inch cable for runs over 600 feet. Don't skimp. Never use rope. It will stretch and get worn by weather.

Trolley. A pulley that rides the cable. Only buy one with steel wheels, to match a steel cable. Make sure yours has a backup system (usually a second wheel), Reddish says. Trolleys should be rated for five times the expected load of the rider.

Tether. To attach your harness to the trolley. Be sure to get a strap that can adjust to least 22 to 40 inches long, Reddish says. Tethers are usually made of webbing or a static climbing rope, with 5,000 pounds minimum breaking strength.

Harness and helmet. Helmets, harnesses and safety tethers should be used on almost all zip lines, Reddish says. The only exception might be a kiddie zip line that only goes 2 or 3 feet over the ground or water. Even zip lines over water often start out high over land, where a fall could be painful or worse. Plan for the worst, so it never happens.

At minimum you'll also need: a few locking carabiners or D-rings to link it all together; several drop-forged cable clamps (wire rope clips) to hold together the cable at either end; tubing to keep the cable from biting into the tree; a sturdy, tall ladder (or two); and a "tensioning kit" that makes lifting and tightening the cable possible. You may also want wood to build a launch platform.

Daunted? Don't be. Companies also sell kits ranging in size and elaborateness, from ZiplineGear's basic $240 Viper kit that comes with just a handheld trolley for very modest backyard zip lines, to $635 for Adventure Rope Gear's Advanced Zip-Line Kit with a braking system, 500-foot wire and extra handle bar. Warning: Look closely at the components before you buy, to see that they're up to snuff. "Some of the kits aren't safe," Reddish says.