What are you using for tent floors?

I used Sheild Silnylon from thru-hiker.com in bivy floor on wet ground – doesn't leak.

I've used generic 2nds silnylon before and it did soak through on wet ground.

the 70d PU coated nylon from DIYgearsupply is cheap, durable, and completely waterproof. Not a huge weight penalty, and can forgo a groundcloth as well

For the tent I'm making right now I'm going to try this: http://jwbasecamp.com/Articles/Silnylon1/index.html

The Shield silnylon is good, but am going to try 1.26 oz Cuben purchased from Zpacks.

If it takes a pointy body bit to get water to go through, and you're sleeping on the pad, what's the problem? Especially if you're using a bivy. I once pitched my Hexamid over soggy ground. A really urine poor job. I stayed dry because I was on my pad and in my bivy. I would use silnylon.

Building grade Tyvek is tough and surprisingly water resistant.

If I'm considering using 30d silicon coated nylon as a tent upper, would it be possible to use housing grade tyvek as a bathtub floor? How would you connect the two fabrics? Is stitching the only possible way?

I've sewed household tyvek. Didn't work too well. It then tore easily, sort of like a perforation. But I gave up quickly, maybe would have worked…

Maybe just have tyvek flat, as groundsheet, not sewn to walls. Condensation dripping off walls would just land on ground. And if the weather's good, just skip the tent

I used silnylon for my tent floor then used the plastic film used to cover windows in the winter as a ground sheet. It weighed 3oz for a 2 person tent I believe and was totally waterproof. I really liked having a ground sheet because I could pack up the tent on top of it without rolling it up in the mud. One sheet lasted an entire AT thru-hike and the weight penalty isn't much different than using a heavier nylon.

As posted above, I was going to use 1.26 oz Cuben with the heavier Mylar, until seeing a post here about the effects of abrasion on it when used on a pack.
Now the plan is to use the silnylon from Thru-Hiker that Jerry posted about. I used silnylon from T-H several years ago for a floor and it worked well. Their new stuff has more resistance to water penetration, so will be even better.

As for abrasion resistance, that is where nylon really shines, especially if using the Dupont 6.6 with a high quality coating.
The elasticity of both nylon and the silicone coating also help to resist both abrasion and punctures.

The temperature related expansion (read sag) of nylon is not an issue for a floor, and is much less on the better quality material T-H sells than on the cheaper silnylons.
Some do not like the 'slippery' quality of silnylons, but if you are going to be pitching on level ground as I am, and put silicone chevrons on the pad bottom as Nemo does, that has also not been an issue.

Some just don't like the gossamer-like quality of light silnylons. The material slides over the ground and bunches up very easily, not good qualities for a floor. Hence the interest in stiffer materials like Cuben, and Cordura and Chikara nylon (which are currently much less water resistant). The answer to this is to have a floor that is tensioned taut at all of the floor corners. Not just tensioned with thin shockcord, but tensioned tight as seen on self-supporting tents where the pole tips go into grommets or other receptors at the floor's corners – just one of many examples is the Hubba. Using this type of floor design, there is no bunching or sliding. With other designs, a heavier material and/or heavier coating might be necessary to address this concern.

Many on this site express a preference for other designs, but since I use only self-supporting tents with flexed carbon poles, high quality silnylon is the clear choice.