I am going to replace my comp roof this summer and it is on top of shiplap boards. Some of the boards are cracked, nails from the boards have worked through some shingles and in a few places pieces of the boards are missing, albeit small pieces. What if I cover the boards with 1/4 plywood? The plywood would be nailed to the rafters and the shiplap boards.
I would use min 7/16
You have the exact same amount of labor to install that 1/4" wood, which will wind up cracking anyways, so why not spend the proper amount for 1/2" CDX plywood and be done with it the right way.
Regardless of your choice, you need to chalk lines where the existing rafters are located underneath the newly installed plywood and ensure that the nails bite into the rafters and not between the spans.
I would cover it with 7/16 as well. You wouldn’t need plywood though. You could get by with 7/16 OSB. It would be cheaper.
Unless you happen to have the 1/4" plywood on hand for free lets say. Then I would choose the 1/4" plywood.
Yes you can do it.
Like the others said, it is better to go with 7/16" or better but you don’t have to.