20x20d vs 20x50d SilPoly

For a shelter, I would go with one of the PU4000 coated fabrics from RBTR, either the 0.9 oz 15D silpoly/PU400 or the 1.1 oz 20Dx20D silpoly/PU4000.  I think the less stretch the better, but if you want stretch, then silnylon is a better choice over silpoly.

thanks for the input

That is the opposite advice I would give. The PU4000 fabrics might be good for floors, but the heavy PU-coating weakens the fabric a lot (in tear strength). See page 2 of this thread:

https://backpackinglight.com/forums/topic/how-hard-is-it-to-make-a-pyramid-shelter/page/2/#comments

I think there will be little to no practical difference between 20Dx20D vs 20Dx50D

I don’t think you need to worry about the strength of the fabric. With Si coatings the fabric will be quite strong enough. The rest is in the quality of the  construction and the seams.

Kyle is right about the problems with bigger threads. They do make bigger gaps between the threads and seriously reduce the HH rating. It’s a known problem with the larger threads and ripstop threads. My tear-strength testing showed that the thicker threads did not really help anyhow (for the silicone coating, NOT for the PU coating).

Cheers

thanks guys – went with 20x20d silpoly =D