Recommended tarp floor material for snow

I've never found that total waterproofness is important on snow. I've used old polyethylene groundsheets many times that had lots of pinholes in them, with no issues. I suspect this is because although the surface of the snow may melt slightly under your pad, the water drains away into the snow below so there is very little hydrostatic pressure to deal with.
Slipperiness can be an issue, but I've never had it be a big deal, just annoying sometimes.
What I like best is a very thin foam pad – I got mine from Suluk46, he used to sell them as "Padded groundsheets" – 1/8" thick EVA. Mine is 30×76, weighs 2.65 oz. No sliding around, and gives you a little bit more insulation.

I found that a polyethylene sheet works OK as a ground sheet on snow. Since it is slick, it won't freeze and stick to the snow.

–B.G.–

On snow, 30D silnylon will be fine. If there's a puddle under you, and you lay or kneel directly on the floor, some water might be pressed through the floor.

Is cuben less slippery? If so, it might be worth it for that reason.

Good silnylon is waterproof. I assume they use good stuff. You could ask them what the HH is. Should be greater than 1000 mm.

If it's not waterproof or if it's slippery, coat it with mineral spirits:silicone 10:1. If it's just for slipperiness, you can put it just where you sleep. After many nights you may have to recoat.

Any reason you wouldn't go with polycro (window film) or tyvek if you wanted something more durable. Both are cheap and have a long history being used as ground cloths.

I would hate to pay for cuben to use as a ground cloth.

I don't camp on snow often but have used polycro on snow a few times with no issues

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What about a tent footprint? A Big Agnes Fly Creek UL2 footprint is listed at 5 oz (per REI). The material is PU-coated, so it should be waterproof and not that slippery.

I've never used a PU-coated floor material on snow – only silnylon (which I found worked fine).

The 70d silnylon I have seen is coated one side, unlike 30d. It also has a bit of PU in the blend and is double weight the coating at .5 oz Yd squared. So it is less slippery than 30d and more waterproof but about twice the weight. It might be a good compromise if you need more waterproofness under high pressure/extreme wet conditions with more durability.

I often use just a full length closed cell pad, for winter use it augments the summer one like a previous poster mentioned.
The ones I use are 3/16" 20×76 is 5.25 oz and is waterproof to 3500mm. Adds an R value of about .75 too.

Thanks for the responses all.

I guess the main reason I'm looking at the bathtub floors is for it to be versatile for this shelter. If I bring it on trips at late shoulder season and find rain instead of snow that would be nice for splashback. I imagine it may also help to stem spindrift if high enough? The other benefit is that BWD will tailor it to fit the pole structure internal to the SL2 so that I'm not having to wrangle a groundsheet around that.

Paul makes a good point about the water draining back out through the snow, but that argument might actually support having holes in the floor to allow it to drain back out. :)

David's comment about the CC pad is also well taken; I have a Thermarest that in some cases I've taken along with the air pad, might be more than acceptable for just me.

Meanwhile sounds like silnylon should be just fine for me. Now to debate on whether the extra weight of the 70D is worth it over the 30D…