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Working on reshingling my roof. The roof deck turned out to be 3/8" plywood which is starting to break down due to the number of times the roof has been replaced.

The deck is not rotting out except for a few spots along the drip edge, which we are cutting out to replace. The main issues are that you can hear the plywood cracking and feel it give under you as you walk around. There have been several times we were afraid we were going to put a foot through it, though it has not happened yet.

The question is, can we just lay a second layer of sheathing over the existing deck to stiffen things up? We are thinking of nailing 7/16" OSB over the existing plywood and making sure the the joints between the layers do not line up.

Anyone aware of code or other issues inherent in this approach? I have not been able to locate anything that addresses this point.

We are located a bit south of Saint Louis, roof pitch is 5 in 12 if that makes a difference.

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3/8" roof sheathing? you gotta be kidding me. wow. This question requires a fairly long detailed answer. Will try to respond when I have a bit of time. Maybe Evil can help you out here. –  shirlock homes Mar 24 '12 at 13:22
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I am sure, thought it was half until I got vent covers off and was able to measure. As a side note my new shingles say 3/8s is the minimum thickness allowed. –  Rozwel Mar 24 '12 at 18:51
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Even if you can cover it safely and adhering to code... On any project like this, when I uncover something nasty I'm always a fan of a remove/replace approach. Easier to maintain/work-on in the long run. –  Randyaa Mar 26 '12 at 18:41
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2 Answers

It hard to say without seeing the structure but it sounds like the decking was under engineered for the weight of asphalt shingles to begin with. You could put the OSB overtop but by the time, you clean that surface up (pull nails, etc) and adhere the new surfaces together, you might as well strip it.

You're also leaving extra weight on the roof and if you are already feeling degradation between the rafters, you may ultimately end up with the same problem just thicker.

If you are going to go with asphalt, you might want to upgrade to at least 7/16th OSB depending on your rafter spacing.

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I had a 60's house that also had 3/8" plywood as roof decking it was okay except for a few places.

I'm also a fan of tearing off, but unless the plywood is rotten (in the falling apart sense), I would probably just skin over it.

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