why wouldn't cotton duck be washable? did it shrink?
i would think you could buy duck or canvas and preshrink (wash) it and then serge and sew. but that would be a lot of yardage and money to cover a couch. I would try to use something less expensive and i would think if you tailored/ made it fit right then that would look fine....
we have an "Old Time Pottery" store nearby- i know it is a chain, maybe you do too? and i know they always have a large selection of decorator weight sturdier fabric and it is like $3.99/yard or something. they don't cut it tho, you buy all that they have....but i always see lots of patterns that might be enough yardage for a project that large. or maybe you could use ticking...i saw some in the home-sew catalog that is cotton and all white, so it could be dyed....cna't remember the price i think it was around $6/yard...
i personally would probably use a blanket or make a quilt
....or at least do it first with sheets or muslin... before i would attempt making a tailored sofa cover out of expensive fabric
i would be quite daunted by that and afraid it would be messy or sloppy or not fit (right)...but it certainly can be done and i bet there are good tuts or maybe you already have those skills
anyway, good luck- i hope you do it well and are pleased with the result!
i am NO expert; just noticed you hadn't gotten any replies. again, good luck!