Roof Deck - what thickness and grade of plywood?

I apologize if this is not the correct forum for this question.
What thickness and grade of plywood should be used for my roof decking?
I own a bungalow that was built in 1960. My 25 year shingles have just about fried off (in 14 years) and every time I go up there to check things out the roofing plywood is so weak I think I am going to end up in the living room. The boards are not rotten, they are just too thin or too weak or both. I have decided to replace the roof, plywood, shingles and all. Is there any advantage to using plywood over OSB? Will I get better cost vs performance with OSB?
thanks,
Andrew

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Originally Posted by Andrew Thuswaldner

...What thickness and grade of plywood should be used for my roof decking?...

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What does your local building codes call for? When my shop was built, it was decked with 1/2" CDX ply, but the requirement may vary with the local code authority and may depend on pitch and rafter spacing. I don't have enough experiece with OSB have a valid opinion on that subject.

Depending on your rafter spacing most roof sheathing these days is 7/16ths osb.

It's quite possible your current decking is 3/8" plywood. That was fairly common in the post-WWII construction boom, particularly houses built by National Homes. As for the plywood vs. OSB debate, oh boy, here we go again. I'll tell you that I don't think that of the hundreds of jobsites I've been on around here, I've seen anything other than 7/16" OSB used for roof decking.

I like 5/8" plywood or OSB but I am not the guy that has to carry the heavy stuff around. The guy that did my roof used 5/8" OSB after removing 7/16" OSB that had a lot of moisture damage from no attic venting. The 7/16" that was removed was swollen to nearly 3/4" in some areas. The OSB will last as long as it is installed correctly.
The cover over my front porch was 3/8" plywood. It was very wavy and moved a lot when walked on. It is now solid as a rock.
The guy that did my roof installed a lot of ice shield around all edges. Two runs around the bottom edges, a single run on the sides and a double on the peaks.

I would use OSB if I were doing it. OSB that is made now holds up better than the old stuff used to. Your local code may require you to use 5/8" or some other thickness. If there is no such requirement 7/16" - 1/2" is fine if your rafter centers are 16". I would use 1/2" with clips in between the rafters if your centers are 24". You will have a little less waste with OSB since you can lay it in any direction where as with plywood you have to lay it perpendicular to the rafters.

1/2" CDX plywood is on my newly redone roof. The roofer originally spec'ed OSB (not sure of the thickness but 7/16" is typical) but I didn't feel good about that so I spent a little bit more and got plywood. You'll find varying opinions (some quite strong) on the OSB vs. plywood debate for sheathing.

Cost wise, I think for my ~2400 sq. ft. of roof, the CDX was $400 more over OSB. Not big enough to not consider plywood.

Thanks everyone for the input!
Andrew

Andrew -
If your existing roof deck is not rotted, just overlay it with 7/16 osb.
Make sure you use proper fasteners and follow correct nailing schedule.
You'll be fine and its cost effective.