# Can you put new plywood decking over the existing plywood?



## Roofmaster417

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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## jmiller

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 :yes:


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## Slyfox

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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## Roofmaster417

Slyfox said:


> 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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## boman47k

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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## Roofmaster417

boman47k said:


> 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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## boman47k

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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## Roofmaster417

boman47k said:


> 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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## Gary in WA

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


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## boman47k

GBR in WA said:


> 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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## Roofmaster417

GBR in WA said:


> 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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## Gary in WA

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... 

Gary


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## Slyfox

Roofmaster417 said:


> 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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## Slyfox

Roofmaster417 said:


> 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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## housegsx

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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## jmiller

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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## Roofmaster417

housegsx said:


> 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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## Roofmaster417

Slyfox;583039)
The added weight is dictated by the square foot said:


> 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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## housegsx

Roofmaster417 said:


> 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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## Slyfox

Roofmaster417 said:


> 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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## Slyfox

jmiller said:


> 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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## roofing pro

Roofmaster417 said:


> 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?


saves customer in labor costs and works. (2) sheets of 7/16 is not much heavier than using 3/4.


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