How to Make a "Polycryo" A-Frame Tarp
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I’ve been making and selling “polycro” (cross-linked polyolefin heat-shrink plastic) tarps/tents for a few years now and have learned a lot in the process about what works well and how to construct them, so I thought I would share what I have learned.
Materials & Tools:
Duck Double-Thick Patio Door Insulation Kit
Uline double-sided tape
Scotch “Tough” Transparent Duct Tape (their glow-in-the-dark duct tape works well too)
150 lb. 1 mm Spectra cord from http://www.ultralightdesigns.com
Ten 7/8″ nylon washers (1″ works well too)
1/8″ shock cord
Blue masking tape
X-acto or utility knife
Sharpie pen
Measuring tape
Scissors
1/8″ steel rod or tent stake
Propane or butane torch or stove
I use Duck’s double-thick patio door insulation kits because as far as I know Duck is the only manufacturer which makes it in 1.5 mil thickness instead of .75 mil.
I use Uline double-sided tape to “hem” the edges because it has far better adhesion and durability than the double-sided tape that comes with the kits.
I use Scotch “Tough” transparent duct tape because it has excellent adhesion and weather resistance. Colored tapes, especially dark colored tape, get hot in the sun and “creep.” In the past I used ripstop nylon sail repair tape, which is lighter and stronger than duct tape, but it doesn’t adhere well enough over the long run.
I use Ultralight Design’s 150 lb. Spectra-core cord because it is bright yellow, knots well, and is plenty strong enough. I attached a fishing scale to the end of the ridge line cord of a tarp I had set up, pulled harder than I usually do to pitch the tarp, and it measured 25 lbs. of tension.
Step 1 is to clear a large, hard floor area to work on and vacuum it thoroughly. The polycro is static-y out of the box and attracts the smallest motes of dust and pet hair. Then unpack the Duck kit, unfold the polycryo, and spread it carefully on the floor. Use blue masking tape to tape down the corners. You will want to put the tape about 1.5″ from the edge and parallel to the direction of the long edge, to hold the polycryo firmly while you “hem” the edge with double-sided tape.
Next apply the double-sided tape as close to the edge as you can get it. The Uline tape comes with one side covered, so you can press down firmly to make it adhere fully while laying it down. I start about an inch past the edge of the tarp so that the edge will stay put as you pull while laying down the tape. When I’m done applying the tape to the edge I trim off the extra inch.
The next photo shows the polycryo spread out and taped down to the floor, with about half of one long edge taped.
Next peel the cover off the first couple of inches of tape and fold the edge over so the tape is sandwiched between. I like to work from right to left while making the hem.
Start pulling the cover strip off with your left hand while following closely with your right hand. You should peel the cover strip straight off so that you get a running fold. Plant your right pinkie, ring finger and middle finger firmly on the folded portion to hold it in place while following the fold with your index finger and pushing down firmly to adhere the tape. When your fingers are spread as wide as they will go, move your right hand closer to the peeling/folding point, as shown in the next three photos:
When you’re finished with one long edge do the other. When both long edges are done, move the blue masking tape holding down the corners so they are parallel to the end. Proceed to apply tape and hem the ends of the tarp.
When all four edges are hemmed you’re ready to start putting on the tie-outs. Use a tape measure and Sharpie pen to mark the middle of the ends, and to mark the long sides in 1/3 intervals (about 40″). Take 8 of the 7/8″ nylon washers and use the scissors to cut one side of each to make a flat edge that will go into the fold of the tape for the tie-outs, to spread out the forces. Smooth the cut edge with a file or sand paper. The remaining two washers will be used at the ridge line tie-outs and don’t need to be trimmed.
I like to do the corner tie-outs first, then the ones on the long edges. Cut about 10″ of duct tape for each corner and side tie-out. Lift the polycryo and insert the tape sticky side up under the tarp, then press down firmly on the polycryo and rub and smooth it to get good adhesion and eliminate bubbles. Place a washer on the tape right by the corner or edge, with the flat side of the washer away from the tarp, then fold the tape back over on top of the polycryo:
The technique is the same for all the corner and side tie-outs. Once all the corner and side tie-outs are done you’re ready to do the ridge line tie-outs.
The ridge line tie-outs are built up from several layers of overlapping and crossing duct tape because they are more highly stressed than the corners and sides. Start by cutting two 14″ pieces of duct tape. Lift the center of the tarp and insert the tape sticky side up about half its length under the tarp, press firmly on the tarp and adhere it to the tape and remove any bubbles.
Now it gets a little tricky. You’ll need to take one of the two round washers and tie one end of the Spectra cord around the washer, then place the washer on the tape at the edge of the tarp while placing the cord straight down the center of the tape under the tarp:
Finally, fold the tape back over the washer and on top of the tape underneath. Press and rub to adhere the tape and eliminate bubbles. Now place blue masking tape on the tie out so it will be held firmly while you construct the ridge line tie-out at the other end.
At the other end of the cord you’ll need to trim it and tie on the last round washer at just the right length. The right length is a hair shorter than the length of the tarp, so that when tension is applied to the ridge line tie-outs they will pull the tarp taut. Cut another 14″ piece of tape, lift the tarp, and insert the tape sticky side up about half its length under the tarp. Press and smooth. Then place the washer on the tape at the edge of the tarp while pulling gently on the cord. After the washer is in place fold the tape back over the washer on top of the tape underneath.
Once the two ridge line tie-outs are fabricated, you are done with the underside of the tarp. Turn the whole thing over so the ridge line cord is now underneath. Cut two 10″ pieces of duct tape and place them cross-wise centered over the ridge line tie outs you have just made. Press and smooth.
Next cut four 8″ pieces of tape, two for each end. Lift the ridge line tie out, place two tape pieces sticky side up under the tarp on either side of the tie out:
Finally, fold the two pieces of tape back over the tarp on top of the tape below.
With the tie-outs all done you are now ready to melt holes through the tape in the center of the washers. Heat the 1/8″ steel rod/tent stake with a torch or stove, and carefully push it through the center of the washers in the tie-outs:
In the pictures the tie outs are sitting on the floor because I had my camera in the other hand. In practice, lift the tie outs up and hold while pushing the hot steel rod through.
Finally, cut eight 10″ pieces of shock cord and knot them in little circles through the corner and side tie outs. The shock cord loops give a nice tight pitch, but also act as a shock absorbers in windy conditions.
Final weight: 11.2 ounces.
You’ll need to tie another 10 feet or so of the Spectra cord to the ridge line tie outs so you can pitch the tarp. Also, since I often camp in the high Sierra on flat rocks or shallow sand where tent stakes are useless, I also attach about 3 feet of Spectra cord at each corner and side tie out with a little knotted loop at the end, so I can make big adjustable loops to put around rock anchors that I have collected.
The finished product (an earlier version with different tape) pitched:
It is truly amazing to be able to fall asleep while looking up through your shelter at the starry sky. It’s also nice to know whether that critter sniffing around your shelter in the middle of the night is a bear or just something small like a skunk.
Total cost: About $30-$40, depending on where you get your materials.
Total time to construct (including taking pictures): Two hours
I will weigh the next tarp I make at various stages of fabrication. I’m making one for a High Sierra trip this summer, although it’s a zippered floorless tent like in this picture and 12.5′ long instead of 10′. It’s the “regular” thickness instead of the double thickness I normally use because I haven’t been able to find any double-thickness material longer than 10′ and the zippered floorless tent needs more length for headroom and a backpack to fit inside. I will weigh just the material, then at each stage where I add more weight. The weight of the polycryo will be apples to oranges, but the number and size of the tie outs will be the same so we’ll be able to compare apples to apples in that regard.
I’m waiting to build it until I can do the tape adhesion test I mentioned above so that I can use the best tape (although if the Tenacious Tape wins I’ll be using the second best tape because there’s no way I’m spending $75 just for the tape). Just picked up some polycryo at the local hardware store today so I’ll try to get the test done this weekend. I plan to spray the samples with a hose multiple times to see how moisture affects the tapes.
Tape Test Results
I tested four different tapes for tie outs: Gorilla Clear Repair, Scotch/3M “All Weather”, Scotch “Tough Transparent”, and Duck Glow-in-the-Dark. I decided not to test the Tenacious Tape because it’s so expensive I know I will never actually use it. It rained here last night, so it was a good simulation of real-world use. Each tie out was hung with 20 lbs of weight.
Here is a picture of the test rig:
The Gorilla Clear Repair failed almost immediately. Didn’t get to test adhesion because it wasn’t strong enough to hold the nylon washer in place, which tore out in less than 5 minutes.
Next was the Scotch/3M “All Weather” tape. It creeped down about 1/2″ in 12 hours – you can see the creep area as a light gray area just above the tape. Fail. Not a bad thing, because it’s ugly.
Then came the Duck Glow-In-The-Dark tape. It creeped about 3/16″ and started to de-adhere. Fail. The creep shows as a light colored area just above the tape. Too bad, because I really like the glow feature. It lets you see the perimeter of your tarp in the dark so you don’t stumble into it.
Finally the Scotch/3M Tough Transparent tape. No creep, no de-adhering. Pass, which is cool because aesthetically I like it the best – transparent tape on a transparent tarp.
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