tarp / tent material

Unless you use the lightest weight possible of cuben, It doesn't offer much weight savings. However, it is a plastic laminate and nearly waterproof. Difficult to sew, most people resort to gluing or taping. More prone to punctures from small sticks, stakes, etc. About .5-.6oz per yd. Sewing has a way of elongating any holes under stress.

Spinnaker is a bit lighter than nylon. But, is prone to sudden failures. Where you would get some stretching with nylon and a slow leak, you get a tear with spinnaker.
about 1-1.1oz per yard Depends on where and what you get, some is nylon, some is actually polyetheylene. Some is DWR coated. Some is siliconized. I have not seen Polyurethane(PU)coated.

Nylon is the heaviest at 1.1-1.6oz/yd. But, is by far the most durable. I have had one tarp I made over ten years ago…it still works fine. Some is PU coated and is 2-2.2oz/yd for floors. Often they sell this as 1.1oz…this is before treatment. DWR treatment is available. SilNylon is 1.1oz treated with silicon for 1.3-1.6oz.

All weights are after treatment, but treatment can vary a LOT.

Some stuff is only DWR treated. This is lighter, but not nearly as waterproof nor as durable.

For a tent, I suggest sil-nylon, or cuben. Spinnaker is hard to get and yes, it can flap a bit more if not designed correctly.

silnylon stretches

you set up your tent taut in the evening, next morning it's sagging

this is really more aesthetic than functional, although if there's condensation on the inside you could get wet when you get up in the morning

Perhaps the best way to keep a nylon tent (sil or polyurethane coated) is to put an elastic band between the tent guylines and the stake to create constant tension.

I'd do this only if I was expecting a lot of wet weather &/or condensation. With my TT Moment I have had very litle sagging even on damp nights. MAybe it's just part of the Moment's design that keeps it taut.

BTW, most freestanding tents remain taut because the tent poles themselves keep a constant tension on the entire tent (and fly, if double-walled).

Yeah. Good thought. I usually use a hair tie looped to the corners of my tarp. I have mentioned it on the yahoo list a couple times, probably not on this forum, though. Just enough to let a little ventilation to come in, but it also relieves pressure from the winds a lot, letting the whole tarp lean a bit.

To keep silnylon from sagging/flapping in the wind.

Need Prussics on the Tie outs. These allow you to tension the lines and for said lines to retain their tension as reseting stakes usually doesn't work or a rock is in the way or since we are talking tarps you get the drift.

Set it up at night "tight" then after about 30minutes to an hour come back and retighten it. It will be fine all night when you do that I have found. Of course it really depends when it starts to dew as its the humidity that causes said nylon to "stretch" IE SAG.

Cut your lines, and add in prussic knots to your tie outs. You won't be sorry. I suggest minimum of 4 wraps or maybe even 5 especially if you are going with small cord on small cord as proper UL minimalists.

Also if a storm does blow in at night, tightening your tarp becomes very fast with the prussics. Just grab them and pull and your tarp is now tight and you are back snug as a bug in your bed.

Doug,
What you are looking for has been so much discussed on these threads, forgive me for not going there. Backpeddle through the last few months' thread titles and see if you get the kind of info you are looking for.