If Jim is thinking about coated rip stop nylon, the problem will be More so when the nylon is put away. If stored wet it will have a tendency to have the coating soften and start peeling, same with other coated nylons like pack cloth or cordura. It's important for coated nylon to get proper air drying before being stored. 
Also, not impervious to mold and mildew...

****************
Cheri LePage
GFAB Manufacturing Ltd
www.theskylightguy.com
604-740-1018

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------------------------------------------------------> on the road again soon

*Remember to check your skylight seals and make sure you drive with it closed...we've already replaced 6 in March for driving into low garage entrances!

On 2012-04-16, at 3:28 PM, Dave Mcneely <mcneely4@COX.NET> wrote:

It would be absolutely water permeable.  Rain would flow through like through a sieve.  mcneely

---- Jim Felder <jim.felder@GMAIL.COM> wrote:
> Has anyone sewn one up out of ripstop nylon?
> 
> Just wondering,
> 
> Jim

--
David McNeely