Canvas shrinks. I knew that. So when I designed the roof pattern for the yurt, I should have allowed for shrinkage but didn't. Oh well, nothing that can't be fixed with a little more sewing.
Not only didn't I allow for shrinkage in the roof, I also didn't realize that the canvas shrinks unevenly, more along the length of the material than across it. Something to be conscious of next time.
Now that I think of it, I doubt that I could plan for shrinkage in any sort of exact way since the shrinkage is non-isotropic. I suppose I could just make a roof cover uniformly oversize, then when it shrinks, regardless of whether it does it evenly or not, it would still be large enough to do the job.
For now, not having tried to it yet, I am leaning toward pre-shrinking the canvas. Unroll however much canvas I need, hose it down, dry it in the sun, then cut it up and sew it into whatever shape I need. Everything I haven't tried yet sounds like a good idea. Whether it actually is, doing it will tell.
Meanwhile, some before and after shrunken roof cover pictures.