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Old 01-31-2011, 06:15 PM   #1
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Can you put new plywood decking over the existing plywood?


I am about to re-roof my house.I wanted to re-deck the roof because this would be the 3rd time i have replaced the shingles.The decking looks to be in good shape,however, with this being the 3rd set of shingles i would think there would be too many existing nail holes in th ply-wood.The roofer i talked to suggested to just put new plywood over the existing plywood and then shingle it.What do you suggest. Thanks

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Old 01-31-2011, 11:01 PM   #2
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Can you put new plywood decking over the existing plywood?


In my opinion it would be a waste of money and add a ridiculous amount of weight.I believe a single sheet of 7/16" OSB weighs in at around 50lbs.
Multiply that by 3 per sq.30sq.house equals about 90 sheets of plywood,multiply that by 50lbs.That adds an access of 4500lbs to your roof.Now if you are replacing your shingles with a 3 tab shingle,they weigh around 60lbs per bundle so thats 180 per sq.Multiply by 30 is 5400lbs.And that is for 25yr shingles.30yr obviously weigh more.The additional weight of the plywood alone comes close to the weight of a second layer.That is the last option I would choose.I have roofed the same homes over and over and never had a question pertaining to roofdeck life expectancy.Its your call but I would not choose that option.It has too many negatives than positives.But that is my opinion.

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Old 02-01-2011, 07:38 AM   #3
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Can you put new plywood decking over the existing plywood?


We've only ever decked over 1x skip sheathing when shakes are removed. Standard 1x6 sheathing I'd just replace with 1x where needed, and ply vs. ply would be out of the question. Too easy to just rip the old stuff off
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Old 02-01-2011, 08:43 AM   #4
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Can you put new plywood decking over the existing plywood?


I have installed new plywood over existing numerous times with out problems.
The new plywood will be thinner than code because when combined with the existing you will still be exceeding sheathing requirements/codes.
Make sure your roofer staggers the new plywood joints so it off sets the existing.
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Old 02-01-2011, 01:58 PM   #5
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Can you put new plywood decking over the existing plywood?


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Originally Posted by Slyfox View Post
I have installed new plywood over existing numerous times with out problems.
The new plywood will be thinner than code because when combined with the existing you will still be exceeding sheathing requirements/codes.
Make sure your roofer staggers the new plywood joints so it off sets the existing.
Just for the heck of it,I am curious what was the reasoning in stacking the plywood over existing decking? I am confused with the logic with adding an additional amount of weight to the roof structure?
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Old 02-01-2011, 02:39 PM   #6
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Can you put new plywood decking over the existing plywood?


Do you roofers think this should be a tear off and redecked if the existing is in good shape, even though this will be the 4th shingling?

And if it is covered with thinner decking on top of the existing, aren't you defeating the purpose of a new deck not to mention the extra weight? I mean will there be enough meat in the decking for the fasteners?

I guess that depends on the thickness of the new decking on top of the old?

Is it customary to replace decking on the 4th roofing?
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Old 02-01-2011, 03:29 PM   #7
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Can you put new plywood decking over the existing plywood?


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Do you roofers think this should be a tear off and redecked if the existing is in good shape, even though this will be the 4th shingling?

And if it is covered with thinner decking on top of the existing, aren't you defeating the purpose of a new deck not to mention the extra weight? I mean will there be enough meat in the decking for the fasteners?

I guess that depends on the thickness of the new decking on top of the old?

Is it customary to replace decking on the 4th roofing?
In 2005 in Florida I roofed the same house 3 times within 4 months.In my opinion if the decking is not saturated,rotted,deteriorated,spongy,warped or anything other than suitable decking then I would replace anything within these guidelines.I don't think any limit or certain number of reroofs automatically require a redeck.In my opinion I feel stacking plywood is a questionable practice.I cannot find any "logical" reason to add a substancial amount of weight to a roof structure.I also think it could possibly be an insurance risk.I think the insurance company might frown on the "Risk" assesment.I feel it would be the same as too many layers for coverage.But stranger things have happened.
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Old 02-01-2011, 03:35 PM   #8
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Can you put new plywood decking over the existing plywood?


Kinda makes me wonder if the roofer is just wanting a bigger job ($). But, I really don't know, never gave it any thought. Interesting.
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Old 02-01-2011, 04:04 PM   #9
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Can you put new plywood decking over the existing plywood?


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Kinda makes me wonder if the roofer is just wanting a bigger job ($). But, I really don't know, never gave it any thought. Interesting.
If I were to even consider something like that rather than stacking,I would pull the old and install the new.I would feel more comfortable as a professional by doing that.I am not implying that anyone who uses this method is not a pro. I feel that its a questionable practice that I would never do nor have any of my crews do.
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Old 02-01-2011, 05:48 PM   #10
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Can you put new plywood decking over the existing plywood?


The existing plywood would have to be in very bad shape to warrant covering with new plywood. A nail penetration in plywood is far different than a nail penetration in solid wood, which has a grain to split. Just changing the exposure of the first course of shingles would offset the nail holes enough to be safe. To be safe.... from putting a nail into another existing hole- how often would that happen and what would be the short-coming? Like chances of winning the lottery, I would think..... The roofer could look at the starting (horizontal) pattern of the existing shingles and change it up while even using the same elevation course.... I would find a roofer that will change it up and not waste plywood for the added weight and expense as said. My 2 cents, not a roofer.

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Old 02-01-2011, 07:03 PM   #11
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Can you put new plywood decking over the existing plywood?


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Originally Posted by GBR in WA View Post
The existing plywood would have to be in very bad shape to warrant covering with new plywood. A nail penetration in plywood is far different than a nail penetration in solid wood, which has a grain to split. Just changing the exposure of the first course of shingles would offset the nail holes enough to be safe. To be safe.... from putting a nail into another existing hole- how often would that happen and what would be the short-coming? Like chances of winning the lottery, I would think..... The roofer could look at the starting (horizontal) pattern of the existing shingles and change it up while even using the same elevation course.... I would find a roofer that will change it up and not waste plywood for the added weight and expense as said. My 2 cents, not a roofer.

Gary
I had basically the same thoughts on the exposure and or layout.

Not a roofer per se, but I have worked with a little with a crew. Didn't take long before I knew I couldn't be associated with that crew/company.

It is basically out of business now. The owner is very very lucky not to be in jail. There are some things that were done that could bite him in the backside years from now, I would think.

P.S. Not to be taken as commentary on all roofing companies and or employees. Just this particular company.
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Old 02-01-2011, 08:16 PM   #12
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Can you put new plywood decking over the existing plywood?


Quote:
Originally Posted by GBR in WA View Post
The roofer could look at the starting (horizontal) pattern of the existing shingles and change it up while even using the same elevation course.... I would find a roofer that will change it up and not waste plywood for the added weight and expense as said. My 2 cents, not a roofer.

Gary
Unfortunately that could work if it was only the second reroof.Between the holes made for previous shingle applications and plywood penetration by felt installation not hitting any previous nail holes would be impossible.If the plywood was in bad/poor condition by any deteriorated issues,what could someone possibly gain by covering up a roof deck that has had any moisture,leaks,saturation problems?If the decking is rotted then its logical that the new plywood that you are installing over troubled decking would suffer the same fate.Rott creats rott.Or am I missing something?
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Old 02-01-2011, 11:06 PM   #13
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Can you put new plywood decking over the existing plywood?


Now that I read mine again... "The existing plywood would have to be in very bad shape to warrant covering with new plywood." My brain was thinking; replace the bad sheathing, no reason to cover any of it with new if only because of some nail holes in the old.....

Thanks for catching that...

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Old 02-02-2011, 07:53 AM   #14
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Can you put new plywood decking over the existing plywood?


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Originally Posted by Roofmaster417 View Post
Just for the heck of it,I am curious what was the reasoning in stacking the plywood over existing decking? I am confused with the logic with adding an additional amount of weight to the roof structure?
Less land fill waste, less labor 'cheaper for home owner' and if installed properly with a lighter weight/thickness the two together will be stronger than required codes.
(Edited to add: The times I have laid new sheathing over old was due to improper installation of the original, not over rotted sheathing.
This home owners situation was not improper installation of the original but also not rotted issues.)
So why not?
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.
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Old 02-02-2011, 07:59 AM   #15
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Can you put new plywood decking over the existing plywood?


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Originally Posted by Roofmaster417 View Post
Unfortunately that could work if it was only the second reroof.Between the holes made for previous shingle applications and plywood penetration by felt installation not hitting any previous nail holes would be impossible.If the plywood was in bad/poor condition by any deteriorated issues,what could someone possibly gain by covering up a roof deck that has had any moisture,leaks,saturation problems?If the decking is rotted then its logical that the new plywood that you are installing over troubled decking would suffer the same fate.Rott creats rott.Or am I missing something?

I didn't see any mention of rotted sheathing in the home owners post, still don't even after re-reading it.
That would be a horse of a different color.
You can't cover rotted sheathing because of mold/insect issues.

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