I’m a long time camper, but a lot of what I do is just the conventional wisdom or what someone told me. Now that I’m reaching the age where I’m telling other people how to do stuff, that shallow knowledge isn’t getting me far with the follow-up questions. So:
What’s the point of a groundcloth? The tent has a bottom, and it doesn’t come with a groundcloth.
If the tent has a sewn-in groundsheet, it’s probably not necessary to use another, but many tents are just open at the bottom, so you need a groundsheet to stop moisture wicking up out of the ground into anything absorbent (such as bedding) that you put down inside the tent. Also, a bare earth floor can turn quickly to mud or dust depending on the moisture.
The groundsheet should be smaller than the footprint of the tent so that when it rains, the water runs off the fly sheet and drips straight onto the ground - if the groundsheet were to protrude beyond the flysheet, it would collect and run inside.
If you use only one groundsheet, do you put it in the tent to keep your sleeping bag dry for forty days and forty nights, or do you put it between the tent and the ground to protect the tent floor from dirt and abraision, albeit lesser sleeping bag protection from water due to the tent floor wicking water into the middle from the exposed edges?
Personally, I’m a fan of the light plastic sheet innie along with a foam pad outie, but that’s just personal preference.
Some tents do come with groundcloths (aka “footprints”) or they are optional. Mine is made specifically for my tent and, unlike the bottom of my tent, the footprint goes under the vestibules as well. The footprints add extra protection to your tent floor (giving your tent some added lifespan), protecting it from wear and tear of rough ground. If you’re car camping on grass, there’s not much of a point. If you’re on a desert expedition and pitching your tent on a rocky outcropping, the footprint helps mitigate the effects of general abrasion.
Some footprints are also coated to a greater degree of waterproofing than a tent floor.
It has to be slightly smaller or there is the risk that rain water will be channelled between the footprint and your tent bottom. Before I got my proper footprint, I used a smaller tarp, but one day the tarp shifted a little, it rained, and you could feel the moisture working its way through the tent bottom because it practically got sucked in between the tarp and the tent bottom.
I use one because I’m lazy. I put a ground cloth between the tent and the ground. When the final day of my trip comes, I don’t want to spend the morning cleaning the bottom of the tent, and waiting for it to dry, and it’s a pain to reset the tent up when I get home, to let it dry there. With a ground cloth, all the mud/plants/squished worms are on that, and I can clean it off well at home, or toss it away and cut a new one. Due to rocks and sticks, we go through a ground cloth every couple of years, but the tent is still looking good at 15+ years.
The purpose of the groundcloth is mechanical protection of the tent fabric. For some tents a groundcloth is offered as an option from the factory (the “footprint” referenced above).
So as not to collect rainwater.
Some people use groundcloths to minimize water ingress through the floor. Some also use tarps on top of the tent to minimize rain ingress through the top and sides. However, a well-designed high-quality tent in good condition (which includes having been properly seam-sealed) will not leak in either of these fashions. I have weathered hellacious rainstorms in tents that did not get a drop inside.
Come to think of it, I’ve had my tent for about 12 years and the bottom is still in excellent shape. And luckily, they still make it (although the newer version are much better for venthilation), so I can still get new footprints for it.