Using Harbor Frieght tarp?

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If this site http://shop.coversuperstore.com/index.cfm?fuseaction=catalog.catalog&categoryID=152 is correct, then it's 3.8 oz per square yard – 2.85 lbs. Not really light weight, but there are some other tarps on the same site (not camo) that weigh in at 1.75 oz/square yard – 1.31 lbs.

I might look at some of the cheaper tarps from Equinox (grommets) or a tarp from alpinelitegear.com (tie outs).

Those blue and camo tarps aren't super heavy, but are definitely heavier than the tarps made out of silnylon, spinnaker and cuben fiber. If your on a tight budget they would work just fine as a lightweight shelter. Those expensive fabrics yield shelters from 4oz solo shelters to 12oz 8×10 palaces and are for when you can afford them, or are trying to shave every ounce. Shipping weight on a blue 7' 4" x 9' 6" tarp is 1.38lbs or 22.08oz from harbor freight. At 22.08oz it is definitely a lot lighter than any tent you'll pick up for $5 for one person. Which is probably why they have been in a few of the UL on the cheap posts. Another thing is that those lighter weight blue tarps like to pull out eyelets under a lot of stress. I use them for car camping all the time as shelters over picnic tables, as gear or bike protection, over cheap tents, or under the same tents in torrential downpours to keep the floor dry. Another downfall to the blue tarps is they don't always pack back down small or at least not as well as some of the other materials.

You don't have to spend tons of cash on a silnylon shelter either. Just pick up a cheap sewing machine and start learning to sew. It's fairly rewarding to make your own gear, and not to mention cheap in almost all instances.

Am I missing something?

William, that tarp is a freakin' dead weight around here :). Take a look at some of the gear and projects discussed on this site and you'll quickly see that a 2+ lb, 1000 denier tarp, may not be too popular.
That being said, you're on the right track as most people are probably carrying a 5 lb tent…so your already lighter then them with your bulletproof tarp!
Welcome aboard.

I see. I'll give this a try and see how it goes. I haven't even gone on any backpacking trips yet, so I don't wanna spend too much and find out I don't like it.

Go for it, there's nothing wrong with what you're thinking.
My first trips with a tarp were also with a hardware store variety. I think there's much to learn through working with what you've got.
The weight isn't that bad and it's an inexpensive way to see if you even like sleeping under a tarp before committing $$$ to fancy fabrics.
And if anyone thinks you're funny looking under your Harbor Freight tarp, screw 'em. I've got more respect for someone that goes out to learn in your style than those that drop $1000 at a gear shop for a first trip.

Hey William,
Don't get me wrong here, if YOU want to use it go ahead. Heck, you can use a piece of plywood if you want to. But, your specific question was…

Is there some reason everyone feels the need to spend 100$ on another material?

The short answer being that the tarp is heavy. But, if you're just getting into backpacking and want to try some cheaper gear out to see if you like it, good idea! Most of us have done the same.

A fantastic tarp for the money spent is the oware cattarp 1.1, this UL tarp (priced at well under $100.00) offers a great value in performance, light weight and quality workmanship. If the 1.1 tarps minimal coverage is not enough for a tarp beginner the oware cattarp 1.5 may be a better option.

I got a couple cheap tarps from Ocean State Job Lot that appear similar to the 1.75oz/sq yard tarps. They seem like a good idea to see if you want to use a tarp. People have also fashioned tarps for backpacking using plastic painting drop cloths.

I haven't used the HF tarps for camping, but the disadvantages seem to be that they are noisier than silnylon in the wind, probably not as strong. Some of the expensive tarps will pitch tighter and not flap around as much.

Try it out. It should be fine unless you camp in a really windy spot.

William, this item number 160320805627 on ebay.co.uk will lead you to 5 1/2 yards of 4' wide olive green 1.1oz high quality ripstop parachute nylon for a mere $7.

It would make a great 8'x 8' tarp weighing around 11-12oz after you wafted a can of silicon tent re-proofer over it.

I'm a little late to the party here, but in looking at the description of the tarp, it is made of polyethylene, not polyester. It may use 1000D yarn in its construction, but this tarp is no different from the ubiquitous "blue tarp" that you can find almost anywhere. The only difference is that it is camo.

Would it work for camping? Sure, I've seen people suggest it as a cheap way for giving tarp camping a try. But, i don't think it is really a viable replacement for a tarp made from more appropriate fabrics (Nylon or poly with some sort of coating- PU or Sil).