I started this once but stupid IE locked up...
If you are putting together your gear list for hiking or possibly next Spring's thru-hike and you are trying to pick a light weight solo tent, maybe I can save you spending $150 on that sexy solo tent.
The obvious benifit is weight, and maybe this post should go under "Going Light" but it isn't just about saving weight. There are a lot of advantages as well as some disadvantages to a tarp, so IMHO this becomes more of a gear selection question than just a weight comparison. In Roland Mueser's book "Long Distance Hiking: Lessons Learned from the Appalachian Trail" the average hiker spent the majority of time in shelters, hostels and hotels; they only spent about 20% of the time in a tent. Why carry something weighting 3 1/2 pounds if you are only going to use it 20% of the time? Even Earl Shaffer ditched his tent on the Long Cruise for a poncho/tarp.
So what are the benifits? The first and most obvious is weight. While a solo tent like the 2002 SD Light Year has a pack weight of 58 ounces, a simple 6'x8' sil-nylon tarp in the same campor catologe only weighs about 13 ounces when you add 6 stakes and some cord. So weight wise the tarp option is less than 1/4 the weight of the tent.
Pack size. Well a tent will take about 4"x18" of pack space but a tarp will take about 1/4 to 1/3 the space. This means a smaller pack needed and a lighter pack needed.
Price. That SD Light Year costs about $150, while the Equinox sil-nylon tarp is only $50. At 1/3 that cost, maybe trying one out before spending a bank roll on a tent is a good idea. And if it works, that is another $100 for a good sleeping bag or some other piece of gear you want. If you want to go really high end you can get a poncho/tarp for upwards of $100.
Size. A CD Light Year has 20 square feet of space in it. While 6'x8' doesn't sound very big, it translates to 48 square feet - over twice the space of the tent.
So that is it for the actuall numerically comparable reasons, but there is more.
Versatility. A tent is always a tent. Some people even need that extra space of a tarp and spend extra money (and weight) for a vestibule to add to their tent. If you stay in a shelter and you need more protection you may be able to put your tent up inside, but ot always. But with a tarp you have a vestibule, tent, vapor barrier, extra shelter wall, etc.
1. It's 15 degrees and you have a 30 degree bag, You are sleeping in a shelter. Use it inside your sleeping bag (not outside for Pete's sake) as a vapor barrier.
2. You are staying at Blue Mountain shelter and the wind is blowing into the shelter. Use the tarp inside the shelter to make a trapezoid or diamond pitch to protect yourself.
3. You staying in a shelter with a doorway instead of an open face in a bad snowastorm. If the shelter has the right design or you have other people with tarps you can add a wall.
4. You are stealth camping and a bad rainstorm with slanting rain is comming. There are numerous pitches you can use like the trapezoid that gives you the same protection as a tent. Just check the ground first for run off direction. BTW, even with a tent and built in floor you will have to do this.
5. You hiking around lunch time and the rain is one of those steady downpours and there isn't a shelter for miles. In 1-2 minutes you can pitch a dry spot and eat in comfort.
6. Your pitching camp in a normal rain storm - no blowing rain in the deep woods. You just want a vestibule, the whole tarp is you vestibule.
7. If you want to spend a little extra, you can get a poncho/tarp that also serves as a rain top and maybe even a pack cover.
But tents are "better". Tarps have disadvantages right? Lets cover some.
Floors. Tents have floors, tarps don't. Of course, but with a tent, you are supposed to carry a groundcloth slightly smaller than the tent bottom to prevent abrading and keep it clean, So if you have the CD Light Year you need a 9.5' x 3.5' groundcloth. With a tarp you just need a groundcloth big enough for your gear and sleeping pad - with a little extra, or about 7' x 3' So with a tarp you still get over on size and weight.
Bugs. A tent has nets and screens to keep the bugs out but a tarp doesn't. But for the majority of hiking seasons on the AT you don't need a bug net unless Grandaddy Longlegs freaks you out. There are ways to fight this - mainly tie in nets or bug bivies. Both these options weigh about 7 ounces but allow you to leave it behind when you don't need it.
In driving wind and rain the tent is better, right? Maybe. Depending on the tent design, you better make sure the right end is pointing into the wind or it may get destroyed ar collapse. A tarp is the same way, it just takes a little practice just as any other piece of gear does. But from personal experience, I have been through some very bad rainstorms and snowstorms in nothing but an Army poncho/tarp.
A big difference in storms between a tarp and a tent is ventilation. In a rain storm a tent can get wet inside, but it isn't usually from the rain - it's from the perspiration and exhaled breath condinsing on the inside. Often the thought in such a storm is to batten down the hatches and zip up, but ovenight a quart of water condenses and runs into the floor. To illustrate this, imagine someone pouring a liter of water inside your tent while you are asleep - not a fun picture. To solve this there have been lots of double wall designs, ventilation strategies, and plain old unzipping of the stormproof flaps and taking the chance on rain getting in anyway. While tarps don't completely solve this, they do help because there is a lot more ventilation. But while tents try in vain to develope solutions while tacking on weight and price, tarps just give up and give in by doing the best they can with what they have. The secret is to not to touch the tarp wall on the inside while there is condensation on it. And just in case yu have not actually ever hiked in the Appalachians, you will deal with humidity and condensation 99% of the time while out there.
Speaking of condensation and rain. The average tent can add 1/4 of it's own weight after it gets wet from rain and condensation. A Sil-nylon tarp adds almost none - just shake it out and stuff it.
Now that is a lot of stuff to say, and I'm sure I've not hit on all the areas. There are some that have sworn by tents and will never give them up. And there are some that have tried tarps and decided to go back to tents - find one and get their tarp cheap LOL.
Tarp = 1/4 the weight, 1/3 the price, 1/4 the pack size, twice the room, more versatility, and all for good storm protection.
Now let the comments fly.