I have an old house with 3/8 plywood, on joists spaced 24" oc. You can feel the bit of give of the plywood when walking between joists, but its otherwise been fine. I do think its thinner than ideal, given the soft feel, but don’t have the budget to replace all of it on the house, and the contractor wasn’t interested in doing it either, so I’m opting not to replace what isn’t damaged.
However there is a large repair being made with many sheets in a particular area being replaced (and possibly a few scattered ones here and there). With 1/2 being the standard now to my knowledge, I’d hate to go to all the work and expense and put 3/8 right back on. I’d rather just use 1/2 if I knew there were no issues.
I heard a couple roofers say different things. One says you would never even see that 1/8 inch difference after having all the shingles on and should just use 1/2 for anything new. Others say you would see that 1/8, and that the edge could be a problem area. Not sure if any of these opinions came from real experience or just guessing though, and I haven’t found any sort of pictures or hard evidence to see it myself.
Anyone ever done this before, and have any input if it would be better to just stick with 3/8", or transition to the 1/2"? I’m also leaning towards possibly using 7/16 or else using 1/2 but tapering the edges down with a belt sander to make a closer match.