Questions on making a tent footprint

[searust]

I want to make a tent footprint. I have a piece of tarp and am wondering how much larger than the bottom of the tent to make it. Now I am also wondering if I make it larger than the rainfly, aren't I just making something that is going to collect water under my tent? I see a lot of people making them out of Tyvek, but that also is waterproof, (correct?) and is the same problem as my tarp material. So what's the answer? do I make it smaller than the bottom of the tent, and if so, how much smaller?

[moytoy]

I would make it the same size.

[Hooch]

A tent footprint should be slightly smaller than the floor of the tent.

[bigmac_in]

Hooch is correct. It should be slightly smaller than the tent bottom. Tyvek is an excellent material for a footprint.

[moytoy]

A tent footprint should be slightly smaller than the floor of the tent.

I'll buy that!

[Tipi Walter]

A tent footprint is needed depending on the denier thickness of the tent floor. Many ultralight tents use very thin floors in the 20 to 30 denier range, and these tents need footprints---not to keep water out but to protect the thin floors from holes and abrasions. My MSR Fury tent has a 30 denier floor and so I have to bring a footprint along to protect it.

BUT---it's not a waterproof footprint and allows water to pass thru which is important during deluges and gully washers. Waterproof tent footprints are useless when camped in a gully washer with sheeting ground water and pooling ground water---the "lake effect". Then the water can get between the tent floor and the footprint and form pockets of non-draining pools. This will happen no matter how large or small the footprint is.

Decent tents have decent beefy floors---my Hilleberg has 100 denier floors with triple coated urethane---and so I use a doubled silnylon tarp INSIDE the tent to protect my inflatable sleeping pads from pinhole thorns on the ground, and just in case any water does come in it is sandwiched between the floor and the tarp---important during the winter with in-tent condensation.


This fotog shows what I'm talking about---an in-tent footprint to protect the Exped downmat and to minimize floor moisture.

[garlic08]

Like Tipi says, it's difficult to get a ground sheet to do what it's intended to do. In heavy driving rains, you tend to get puddling on top of the sheet, which kind of defeats the purpose.

You can either fuss around with a ground sheet, folding and cutting and tucking to try to get it to work, or you can find a well-drained campsite where you don't need a ground sheet. There are areas where the latter is not feasible or desired, like if you really want to camp near AT shelters, in established campgrounds, or near lakes or streams. But in my experience, certainly on the AT, I've never had to hike more than an extra mile to find a really nice tent site.

This isn't exactly answering the question, but it's something to consider anyway. This is from tarptent.com's FAQ section. Tarptent is perhaps the most common lightweight tent (made of silnylon) seen on the long trails.
Should I order a groundsheet?It depends on the conditions you expect to encounter and your style of camping. The sewn-in flooring is remarkably tough and does not usually require a separate groundsheet. We just never see floors come back for repair (emphasis provided). Tyvek groundsheets are very tough and great for sleeping out or taking a break but generally heavier than you need just for floor protection. For use on very rocky ground and desert conditions where puncture wounds are possible, a light--2 mil plastic is fine--floor protector will do the job.

[Bags4266]

I keep mine smaller on 3 sides than the tentfloor. However I have a front vetibule and extend the footprint about 18 inches for something to neel on when entering or exiting the tent.