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Old 02-02-2011, 08:13 AM   #16
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Can you put new plywood decking over the existing plywood?


I reroofed my house this past summer. We used 7/16 osb sheathing as an overlay of the existing 3/8 plywood. The reason was because 3/8 inch plywood on 24" spans with cheap aluminum sheathing clips just wasn't a solid roof. It was very squishy and my buddy even put his leg through one section during the shingle tear-off. I also added eaves and gable overhangs to my house so the new stuff tied the roof together.

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Old 02-02-2011, 10:03 AM   #17
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Can you put new plywood decking over the existing plywood?


I agree that replacing it due to nail holes would be silly.

My fear with having two layers of plywood would be trapped condensation, but that may be irrational. I'm not sure what code says, because we usually just plan on tearing the old off.
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Old 02-02-2011, 12:39 PM   #18
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Can you put new plywood decking over the existing plywood?


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I reroofed my house this past summer. We used 7/16 osb sheathing as an overlay of the existing 3/8 plywood. The reason was because 3/8 inch plywood on 24" spans with cheap aluminum sheathing clips just wasn't a solid roof. It was very squishy and my buddy even put his leg through one section during the shingle tear-off. I also added eaves and gable overhangs to my house so the new stuff tied the roof together.
I can see the concept.But I would not do it.Thats just my thoughts and opinion.I would not knock someone who does.
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Old 02-02-2011, 12:44 PM   #19
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Can you put new plywood decking over the existing plywood?


[quote=Slyfox;583039)
The added weight is dictated by the square foot, not the roofing squares of 100 square foot.
When you calculate the added weight per square foot there is no weight barring issue.[/quote]

I guess I am a goober because on a 30sq.overlaying plywood (7/16")I just thought that 4500lbs was 4500 pounds?Feathers and bricks.
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Old 02-02-2011, 02:53 PM   #20
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Can you put new plywood decking over the existing plywood?


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I can see the concept.But I would not do it.Thats just my thoughts and opinion.I would not knock someone who does.
I researched a lot before I decided. The one main worry I read about was moisture getting trapped between the 2 layers. To me, that is like worrying about moisture getting trapped between the sheathing and the underlayment, you don't. Plus with the addition of soffit and ridge venting, I was even less concerned.

Several houses in this area have been done in the same way and they seem to hold up fine.
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Old 02-02-2011, 08:34 PM   #21
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Can you put new plywood decking over the existing plywood?


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I guess I am a goober because on a 30sq.overlaying plywood (7/16")I just thought that 4500lbs was 4500 pounds?Feathers and bricks.
I was in a hurry when I typed that, had an emergency repair to get to, and I left my statement incomplete, you can do it per 100 square feet or individual.
You should also include the weight of fasteners, underlayments, possible weather issues such as snow if you live in a snow region, etc.

You figure the true weight, roof slope & structural design of the trusses and then determine if the added sheathing is/will be a weight bearing issue.

If you live in a home in which your roof can't handle the additional weight of a layer of 5/16" or 7/16" sheathing you should be concerned with that structure.
Ask you local county engineer about that.
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Old 02-02-2011, 08:37 PM   #22
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Can you put new plywood decking over the existing plywood?


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I agree that replacing it due to nail holes would be silly.

My fear with having two layers of plywood would be trapped condensation, but that may be irrational. I'm not sure what code says, because we usually just plan on tearing the old off.
I have done it in Florida, Texas, Virginia, PA and Ohio on jobs that had on site consultants that inspected our work daily "national nursing home group" and code was not an issue.

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