Most people I talk to are confused about using a tarp for backpacking, instead of a tent. So, here I'd like to relate some of our anacdotal experience with tarps, some of their advantages and disadvantages.
Now, Jess and I both really like being outdoors, we go backpacking because we like to be in the woods. That's why we're out there. So, for us the primary purpose of a shelter is to keep dry in rain while sleeping. Secondary purposes include keeping us dry while cooking food, keeping mosquitoes off us in bad mosquito country, blocking wind in cold weather, a privacy screen when in tight camping quarters, keeping snow off us, an emergency bivuac (think vapor layer), and to lower condensation.
This list is probably not comprehensive, but it's close to it for us. Many people use shelters also as sort-of a place that feels like "home". 90% of the time Jess and I will sleep without a shelter. On the AT we only pitched it if we thought it was going to rain. So for us, we're already in the 10% case once we're using the tarp at all!
What is a tarp anyway
When I'm talking about a backpacking tarp, what do I mean anyway? I'm actually referring to basically any large piece of plastic. Jess and I have nice silnylon tarp with grommets (1 tarp each). Also note that when I speak of a tarp I'm also referring to its matching groundcloth. Water doesn't just stay still when it hits the ground, it moves, primarily downhill. Covering above you is the first step, but then you have to deal with the running water as well, and possibly damp ground. A groundcloth steers water away from the sleepers.
So, here's a quick list of some of the things a tarp gives you:
Open air, view, and circulation
Depending on pitch of course, but sleeping under a tarp feels much more "outdoors" than sleeping in a tent. To us this matters a lot. We love to be outdoors.
Additionally this means that if it's a steamy summer night, you won't end up soaked in your own sweat, the airflow is free around you, so it's just like sleeping under the stars (you can even stick your head out and do just that if you like).
Robustness in simplicity
It's just a tarp, no poles to break, if it tears, tape it... it's still a tarp. You get used to ways to pitch the tarp, Jess and I got 1 that we used 95% of the time.
In any case, every pitch is a little different, and you get very used to sortof hacking something together each time. At 2am when I feel a sprinkle on my face in the night, the two of us could get up and pitch the tarp in probably about 3 minutes.
The great thing about this is that if something goes wrong, it's just like normal. Your pole breaks, well, grab a stick, tie to a tree, whatever. We tore a grommet out of Jess' tarp. No big deal, we just tied it off by sticking a rock against the tarp and tying a rope around that.
If the floor isn't attached to the roof, you can remove it. This is great for cooking indoors, cleaning house etc. Part of the reason it feels more like the "outdoors" is that there isn't that process and annoyance like a tent. With a tent any dirt you bring in, is now "in" the tent. With a tarp, you just brush it off the ground-cloth. You can put some items off the ground-cloth, but under the tarp, like muddy boots. This is a wonderful feature after hiking in the rain all day.
Another advantage of a movable floor is you can put down your stuff and then pitch a tarp OVER it. Earlier I mentioned pitching a tarp in the middle of the night. With a tent, you'd need to pitch the tent, then climb in. With a tarp, you're stuff stays dry *while* you are pitching it. This can easily save you 5 minutes of rain on your stuff.
And now for the downsides:
No bucket floor
This is a big one. There are a lot of workarounds it turns out, but it's tricky and none are as good as a bucket floor on a tent. That is, the bottom part of the tent going about 6" up the walls that's seem-sealed. This will let you virtually put your tent in a stream, and still stay dry in it. With a tarp you have to be significantly more careful about water-flow in a rainstorm, to make sure it ends up under and not on your ground-cloth.
In the entire 1500 miles of the AT I actually had a real problem with this once. Every other time we were able to make the groundcloth work. On that night, I should add, everyone in the campground with a bucket floor (having assumed they would keep water out) had pitched on ground a few inches lower, and most of their tents filled completely with water. My tarp was *also* hit by a falling branch, and there was about 3 minutes straight of light due to lightning... So my flooding tarp wasn't really my problem anyway :).
No mosquito netting
If it's bad mosquito country... good luck. I hiked from Georgia to Massachusetts on the AT sleeping under a tarp for rain, and without ever using mosquito repellent. There are a number of tricks for reducing mosquito and no-see-um bites, and I hope to go into that in a later post. In the end though, when it gets really bad, it's just bad. There are workarounds, but they are annoying.
Stability and wind protection
A tarp is never going to be as stable in the wind as a good mountaineering tent. Though it should be noted that due to ease of futzing with the pitch it's FAR more stable than a low quality or non-mountaineering tent.
On a mountain I twisted my ankle, we were on an open meadow. The wind picked up to the point where had we opened the tarp in the air, we would've been unable to hold onto it. We *did* get the tarp up, and it didn't tear. We slept that night. It was extremely loud due to being somewhat loose which made sleeping hard, and took a long time and a lot of work to set up without losing the tarp.
So, think about sleeping out sometime, no tarp, no tent. If you like it try using a tarp. It's a lot safer and easier than you think. DO do some research into how to pitch a tarp first though! I intend to write a new article on that soon, in the meantime see our article on www.smalladventures.net/backpacking .