Never saw it before, but 2 houses had it this week. One was a '54 and the other a '72 build. Both 24" OC trusses. Must be more common than I thought.
I probably have been in over 3K attics and have to verify the roof deck thickness for insurance inspections. I would have to say that I’ve seen about (3) houses with 3/8" roof decking…very rare.
3/8" sheathing requires two (2) H clips in between the rafter spans. The alternative is solid blocking that the sheathing seams are nailed to prevent deflection. Ideally 1/2" is a preferred standard for obvious reasons and requires only a single clip.
3/8" roof (or wall) sheathing has been non-compliant in FL for decades.
Probably non-compliant everywhere now, but in the 60s it was compliant and not unusual on lower-quality homes in parts of Colorado.
Rick, this is UBC?