I got some window shrink film for a ground sheet and wanted to know what size to cut it down to. I've never really used a ground sheet. Before UL I had a tent and didn't use one. When tarping I have a MLD bug bivy with a heavy silnyl floor and never used a ground sheet. This summer I am going to ditch the bug bivy and do a headnet, bag, pad and poncho tarp. So, I cut the sheet so it's just inside the footprint of the tarp right?

Totally novice question, I appreciate any help before I hack the polycro up.

If you pitch on grass, your bag and pad will be lower than the rest of the groundsheet, and any rain that falls on the outer edges of the sheet could run to the middle to form a lake if it finds a path. I've seen rogue raindrops falling on the edge of my groundsheet flow a long way before pooling.

So, I cut it a bit closer to my sleeping gear than I used to. Sorry for not having advice in inches. I would just recommend cutting an irregular shape that gives you groundsheet where you want it (maybe more around the head end than the foot end?) and keep it a bit smaller than the footprint of your tarp.

There is no rule.

I made a ground sheet out of spinnaker fabric. First, I laid out my sleeping bag on a pad, and I drew an outline roughly six inches outside that, and I cut out a pentagon near that outline (two lines by the head, two long sides, one bottom side).

I turned up the five edges and sewed a corner in five places. That way, rainwater can't flow in over the edges to contact the sleeping bag.

--B.G.--

A nice thing about polycro is that if you cut it too big, it's so thin that you won't even notice it if you have to stuff the excess under your pad.

"Sorry for not having advice in inches." HA! That's quite alright, I was looking for general advise. Which you gave, thanks. I was thinking about cutting it close but then I had another idea.

I'm going to do like you said and cut a 6in outline around my pad/bag but double the material. So when it's folded in half it will be the footprint you describe but 2 layers. If it rains cats and dogs, I can get under one of the layers (I'm not a fan of bivys). This may be some good insurance against heavy rain with low weight cost, like a polycro burito.

Bob, I also dig your bathtub floor idea. I could probabely do that with packing tape on the polycro.

Anyway, a new material to try out, and reduced pack weight, should be fun.

I don't see the need for two layers. This is BPL.

--B.G.--