ground tarps

Rain flies help keep the water out from the top, but the bottom of a tent (typically) is not a water-proof material. In a heavy or steady rain, the water often runs under the tent and eventually soaks through the bottom, getting you and your gear wet.
So, you need an additional piece of material to act as a barrier.

What to use?
Some tent manufacturers sell what is called a Footprint - a piece of coated nylon in the shape of the tent body. They attach directly to the poles of the tent, are cut slightly smaller than and in the shape of the tent body so there are no overhanging pieces of material to collect water, they are compact and lightweight. But they can be expensive, and can be limited if you don't have a tent that makes a Footprint.

There are some generic shaped Footprints - like a 6x8 hexagon or a 7x7 square. But if you don't know the dimensions of your tent, this is not always a great alternative.

You can also buy a sheet of plastic to use as your ground tarp. You can get these at most outdoor stores or even the local hardware store. Some people cut them into the shape of their tent body, and others just fold the plastic underneath the tent to match the tent shape. This is an inexpensive option, though the plastic ground tarp can be a bit bulky and slightly heavier.

Where to put it?
Outside: It seems simple enough - by definition a ground cloth goes...on the ground. As I said, Footprints directly attach to the poles on the outside, underneathside of the tent. For a plastic tarp, lay the ground tarp on the ground, and then set your tent up over it. If your tarp is bigger than the shape of the tent, you want to fold the tarp edges downward, and tuck them underneath the tent, so it is just smaller than the tent body. This will help keep water from pooling on the excess edges of the ground sheet and with it folded downward, the water that runs under the tent should follow the curve of the tarp and flow away from the tent.

Inside: However, sometimes even with all this attention to set up, the water still pools on the ground cloth, in between the the tent and cloth or on the edges. In this situation it can be useful to put the plastic sheet inside the tent. Putting the tarp on the inside, acts as a direct barrier from your bag to the outside of the tent. The tent floor may be wet in the morning, but since the tarp is covering the tent floor, you will be dry. It can be a bit noisy to have the tarp on the inside, but overall does a better job of keeping you dry. Note: if the plastic sheet is not as large as the inside of the tent, there will probably still be water seepage into the tent where the tarp does not reach. Just make sure your gear and sleeping pad and bag are on the plastic. In a pinch you can use garbage bags.

One more reason for a ground tarp:
To protect your tent from the ground itself. Sticks, dirt, gravel, sand, etc. all are abrasive to your tent body, and over time can wear holes in the nylon.

Whether you put the ground tarp inside or outside of the tent is largely based on where you camp and a little bit on personal preference. If you are not camping in a place that rain is a big risk, like Joshua Tree, I suggest putting your tarp on the outside of the tent. Conversely if you are camping in a wet environment, like In the BWCA or Cascade Mountains, put the tarp on the inside because rain and wetness are a much bigger factor than damage from the ground. Play around with it and see what you prefer, but remember the ground tarp's primary purpose is to help keep you and your gear dry.

--Deb