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What follows is a collection of information on materials, coatings, and construction techniques. This collection of information is not a definitive work on all fabrics, coatings, and construction techniques, but may help in understanding or confusing why things are made the way they are. As a design engineer by trade, sewing entrepreneur by obsession one of the sayings I have come to latch on to is "for every feature there is a compromise". Meaning every time you try to include a new feature or benefit in a product, you may end up compromising another feature and or benefit. Keep this in mind as you wander through the following technical musings.

What cloth/fiber should be used?

Two basic synthetic material choices used today are nylon and polyester. Some basic characteristics of the two materials should be noted.

Nylon melts at 460 degrees F. where Polyester melts at 480 degrees F. Nylon and polyester will both reach a critical point at 350 degrees F., where both materials will begin to degrade. (Fires will destroy both equally, fire retardent treatments will not keep a tarp from degrading after 350 degrees. Fire retardent treated fabric will not keep the fabric from melting at the materials melting point, it will slow down the flame, the oxidation of the material.) If your tarp is subjected to these temperatures they will degrade or melt. Further information on ignition and flammability of synthetic fibers link to www.fibersource.com/f-tutor/HEALTH.htm.

Polyester has a greater coefficient of friction than nylon, not a large differential, but maybe the difference between abrasion or a slipping condition to those things the fabric comes in contact with as well as internal interaction of the yarns in the weave of the cloth. Polyester will abrade faster than nylon.

The specific gravity of materials shows that nylon is lighter per equal volume; nylon has a specific gravity of 1.14 while polyester has a specific gravity of 1.38. Nylon fibers weighs less than polyester fibers of equal size. Nylon is stronger than polyester equal size to equal size. There are two types of nylon on the market nylon 6 or nylon 6.6, the nylon 6.6 is stronger than nylon 6.Typically at time of manufacture the polyester fiber has a penalty of having 10 percent less strength than a equal denier nylon cloth. If the nylon has not been treated with proper UV inhibitors, that nylon will after prolonged exposure will deteriorate to the point where it is not as strong as the polyester.

Polyester does not elongate at the same rate as nylon. Polyester rope has 40 percent less stretch than an equal rope made of nylon when subjected to equal loads. When the materials are under load polyester will hardly stretch, while nylon will stretch as it absorbs the force of the load. This will mean that polyester cloth will keep its shape better under a load. It also means that is less likely to give and absorb the force when it is shock loaded with gusty winds. Polyester will also break at a lower force than nylon.

Polyester absorbs less moisture into the base material than nylon. Polyester tends to keep its size better when exposed to moisture and humidity. As nylon absorbs moisture into the base material it tends to elongate. Your tarp will sag when a rain starts, if either the tarp or the ropes that support the tarp are nylon.

What does denier, thread count ounces and all those numbers mean?

After selecting which fiber has the best characteristics for your needs the next thing to understand is how you identify all the numbers and terms about the cloth.

Both nylon and polyester start out as a hot liquid of petroleum based chemical compounds. These compounds are squeezed out of a very small opening called a spinneret. The size of the opening of the spinneret will determine the thread size. The size of those threads are measured by a unit of measurement called denier. A denier is the weight in grams of 9000 meters of that thread. So 9000 meters of 70 denier fiber will weigh 70 grams. Usually the larger the denier value the more a cloth will weigh, that will also depend on how many threads per inch make up the cloth. The higher the threads per inch the tighter the weave of the cloth, a higher thread count. Typically the tighter the weave the stronger the cloth is, as well making the cloth easier to coat for water resistance.

Typical construction of 1.9 ounce cloth is 98 threads by 98 threads. That is 98 threads per inch running the length (warp) of the fabric, and 98 threads per inch running the width (fill) of the fabric. Typical thread size of 1.9 ounce/ square the thread is 70 denier. Another nylon cloth used in lighter applications is a 30 denier ripstop nylon with a thread count of 127 x 132.

Cloth weight is usually given in how many ounces per square yard before coating and treatments are applied to the cloth. So a 1.9 ounce per square yard ripstop will often actually weigh at 2.8 ounces per square yard after all the coatings are applied. (more about coatings later) Cloth then is sold by the running yard, which is a yard long piece of cloth and whatever width it was manufactured at. I have seen 36", 48", 60", 65", 72" wide cloth on the market, 60" being the most common.

What does water do to my tarp?

Spray Test.
It is measured by a spray test (AATC Test Method 22-189). In the spray test they look for how much distilled water adheres to the material after 250ml is sprayed on the material. A rating of 100 indicates that there has been no sticking or wetting of the upper surface. Any number less than 100 indicates sticking or wetting of the surface. Coatings are added to the base material for a variety of reasons. There is a coating called DWR which stands for durable water repellency. This coating makes the cloth less likely to absorb water, to wet out. You want a fabric to start out at a rating of 100. After use water repellency is degraded, this can be restored with a number of different manufactureres after market products. Often there is a UV inhibitor included in with these DWR coatings for greater protection against UV deterioration.

Water resistance is measured by government standards in a unit of measurement called Mullens (Federal test method standard #191A method 55112). Mullen measures in pounds per square inch of how much pressure the cloth can withstand before allowing water to pass through. A value of 30 would mean at 30 pounds per square inch of water pressure the cloth allowed water through. Government standards say that the fabric must resists 40 psi to be waterproof. What is waterproof or not is subjective to the situations one encounters. A heavy downpour may not exert as much pressure as a driving rain with buffeting winds. (Sealing the the seams is another story)

Urethane coatings are the most commonly used coatings for water resistance. These coatings have been improving steadily over the last several years. Urethane coatings can be a solvent or water based coating. The coating people hesitate to get into a discussion on which is better. That probably stems from they are all heading towards water based coatings as the solvent-based coatings have a number of environmental issues to work through. Being stored away in a wet condition can damage a water-based coating by a partial softening of the coating itself. Always dry the tarp well before any storage, moisture can also invite mildew to grow on the urethane coating, or on any dirt that found its way onto the cloth. Urethane will release trace amounts of Formaldehyde.

Silicon coating is another type of coating that makes the cloth very slippery,spray resistant as well as very water resistant. It is lighter than urethane, for equal effectiveness. It can betougher to seal. (Seal with 100 percent silicon two sealant as a bead (difficult), or spray with a silicon based waterproofing designed for nylon cloth. Such as the 3M product ScotchGuard.) Another advantage of the slipperyness of the Silicon coating is that it increases the strength of the cloth. The fibers slip on each other rather than being bonded to each other with the urethane coating.

Silicon coated 1.9 oz 70 denier nylon ripstop has a tongue tear strength of 12.9 lbs. on the warp and 16.8 lbs. on the fill, whereas Urethane coated 1.9 oz 70 denier nylon cloth has a tongue tear strength of 5.52 lbs. on the warp and 5.58 lbs. on the fill. (1.1ounce 30 denier silicon coated ripstop has a tongue tear strength of 12 lbs. on both the warp and fill.) This means once a tear has begun, a silicon coated fabric will need two to nearly three times more force to continue the tear as it would take urethane coated fabric of equal denier.

In a Ripstop cloth there is a square pattern of heavier thread spaced roughly every ¼". This helps to stop the spreading of an accidental rip or tear of the cloth. The ripstop pattern improves the touge tear strength of the cloth.

Typical grab strength of the 1.9 oz. 70 denier nylon cloth is 145lbs. on the warp, 140 lbs. on the fill. That is the amount of load a 3" wide piece of the cloth can withstand. Coatings seem to have little effect on this strength.

Nylon thread is my preferred choice as the breaking strength of #46 nylon thread is 12.5 lbs., where as #46 polyester breaks at 11 lbs. Sewing with nylon thread is tougher on sewing machines as it is more abrasive than polyester thread. I prefer a stronger thread as well as a like material that will expand and contract the same as the body of the tarp. A nylon tarp will grow in wet conditions, where polyester thread will not grow at the same rate, nor will it have the elasticity to take shock loading like nylon. To keep the tarp in an equilibrium condition with the thread, the tarp and the thread should be of similar materials.

Edging the tarp is required to increase durability, as well as ease of handling and setup. One can edge a tarp by folding over the material into a rolled hem and top stitching it down. This will give the tarp’s perimeter strength and prevent the raw edge from unraveling. Better yet is to fold the edge over and top stitch down a piece of lightweight nylon tape (note same material as tarp body and thread). The width of the nylon tape serves two purposes. The greater the width, it will allow the rows of stitching to be further apart so that the {wind loading} of the tarp will be distributed over a greater area.

As the overall width of the nylon tape increases, the strength increases as well. ¾ " breaks at 400lbs; 1" breaks at 525lbs; and 1 ½" breaks at 900lbs. (Breaking strength of {mi/spec} nylon tape.) Commercial tape is not guaranteed to be as strong, and can vary from lot to lot as the specifications are up to the manufacturer, and often are subject to change without notice.

Rigging a tarp without any designed in tie points is difficult but can be done by the rope around a fist technique. That is where rope is tied around an object wadded up on the other side of the tarp. This technique will work but puts a lot on unnecessary abrasive forces to work against your tarp, and is usually considered jury rigging.

Securing ropes to tarps to set up as a dining fly is most commonly done with brass grommets. All grommets rely on material that has first been pierced, breaking the strength of the cloth. Then the cloth is supposed to be squeezed by the brass grommet, this is to secure the ends of the fibers by squeezing them between the metal sides of the grommet. (The grommet works best if the material to be have the grommet installed is relatively thick and compresses easily under load, but will not be severed by the grommet itself. The grommet itself works only if it has not been compressed to much while being installed; as it relies on elastic deformation of the metal so that it will continue to exert a force against the cloth. If it has been compressed too much the metal has little or no elastic forces to exert against the cloth.)

As the tarp is being stretched by tension caused by setup and/or wind load, the cut ends of the fabric hole in the grommet want to be pulled out from under the grommet edges. The grommet at the same time wants to move towards the anchor point. Spur grommets have points that pierce the fabric in and attempt to hold the fabric under the outer rim of the brass grommet. Rolled rim spur grommets also have a rolled edge to reduce the cutting action of a raw edge of that in a plain grommet, as well as provide opportunity for greater elastic forces against the cloth.

Sewing rope to the tarp as tie outs is not a strong option, as the rope doesn’t provide enough contact space to the cloth. You cannot put enough stitches in through the rope and cloth in the small contact area without damaging the cloth to keep the rope from being pulled off the top of the cloth.

Nylon tape loops sewn to the surface are preferable over grommets, as there is more distribution of the stitching area to the tie out to make a strong bond. Better yet is the strongest attachment method that I have found. That is stitching the nylon tape loops between the nylon tarp and a nylon tape edging with nylon thread. This provides the strongest possible way to bind the loop to the tarp, as the nylon stitching is anchored to both the tarp and the nylon tape.

When you sew the nylon loop into the tarp, it should be folded at an angle to allow for the greatest ease of threading a rope through the loop. By not having the edges line up it is easier to start the rope through the nylon tape loop. Pushing ‘in’ the loop will cause the loop to open up, allowing for easier threading.

Tie off loops should be spaced about two feet apart around the perimeter of the tarp for the greatest versatility in setup. Tie offs should never be sewn to just a single layer of fabric, as they will often rip the tarp as they pull out. All tie off loops in the interior of the tarp should be sewn into seams otherwise they will rip the cloth they are sewn to. These interior loops need to be sewn into multiple layers of fabric.

The use of a center pole requires a heavier cloth to be sewn to the lighter fabric of the main tarp. This fabric serves to protect the body of the tarp from being subjected to the abrasive forces of the pole, as well as to distribute the point loading of stress at the pole. With a CCS tarp they all come with a center reinforcement patch as well as a Quad Loop system. The Quad loop allows the pole to be secured to the tarp as well as to the ground. This system designed by CCS will prevent the pole from moving from its secured position, protecting the tarp from damage from the pole moving off the reinforcement. It also protects the people around the pole from being hurt from a pole suddenly being dislodged from under the tarp.

Proper use of the Quad loop is important. First take a 12 foot long cord and tie a loop on one end of the cord so the loop is at least 12" long. Next put the rope loop through each of the 4 nylon tape loops that make up the Quad Loop. The end of the rope loop will now be next to the knot that made the rope loop. Next pull the loose end of the rope through the end of the rope loop. This will form a girth hitch (a cinching knot) that includes the Quad loops. Insert the pole in the center of the Quad Loop and the girth hitch. Next wrap the pole with the rope 2 to 3 times around. After wrapping the pole stake the end of the rope to a stake put in at a 45 degree angle at the base of the pole. This will keep the base of the pole from lifting off the ground and becoming a hazard to those around.

Do not fold a tarp always the same way for storage, you will wear the coating off along the fold lines. Do not store any tarp in a greatly compress state, it may take a set. Always store away dry and out of sunlight even if it comes in through the windows. Always store it so it is ready to be used the next time; clean, seams sealed, with ropes and stakes.

Tie ropes for tarps. Rope should be one with a tight pick. A rope with a loose pick will get snagged continually by anything and everything in the woods. A bright color is a good feature, so people can see it in reduced light, and so it does not get left behind. Cord of less than 1/8" diameter often is difficult to tie and untie knots. Check for breaking strength, 225 lbs. is standard for 1/8" cord. The stronger 1/8" cord is 450 lbs. breaking strength. (Remember knots reduce the strength of rope from 20 to 40 percent, and different knots reduce the strength of the rope differently.) Also remember nylon will be a more elastic rope than polyester, and nylon will stretch even more as it gets wet. I would not have less than 80 feet of cord dedicated to set up a tarp, with at least another 80 feet packed away for just in case the camping site may require the use of more rope. .

Rope is necessary to set up a tarp / dining fly, but approaches uselessness unless you know how to tie knots that assist you in tensioning the tarp. Get Cliff Jacobson’s book "Camping’s Top Secrets" second edition and learn the power-cinch knot on page 107, as well as check out his section on how to set up tarps for some useful hints.

When you travel best to have your tarp in a stuff sack near the top of a pack. You never know when it would be nice to take shelter from the sun, wind or rain.

In conclusion the best tarp is the one that you have along to use, but don’t have to because the weather is too good. May all your camping days be like that.

© 2007 Cooke Custom Sewing