# any advice on adding plywood to roof



## grgewer (Dec 12, 2009)

I'm going to be reroofing an older house with 2x6 rafters and plank 1x sheating. Want to add a layer of 7/16 osb on roof. Longest rafter length is about 14' to the collar tie in a vaulted ceiling (still a 2x6 rafter). 20' wide area. everywhere else we had braced front and back of roof to a wall (truss like) Will this vaulted area carry the small extra load of plywood without sag or any ideas how to stiffen it up keeping in mind the top planes in with the rest of the roof on the front of the house and the bottom side of the 2x6 rafters is drywalled?


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## festerized (May 19, 2007)

OSB on the roof, Wrong


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## Tinstaafl (Jan 6, 2008)

festerized said:


> OSB on the roof, Wrong


 Why?


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## WNYcarpenter (Mar 2, 2007)

How many layers of shingles were/are on the roof?...is there obviouse sag in the ridge?

In this area if we do a tear off with skip plank sheathing, or even tight 1x planks there's almost always three layers of shingles....often time cedar shakes as well. OSB doesn't even compare to the weight of all the layers of asphault.

You braced off the 2x6's which is more than a lot of people would do. If we asked our engineer he'd probably want 1 3/4"x 11 7/8 LVLs sistered every third rafter!!!


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## pinwheel (Dec 7, 2009)

Tinstaafl said:


> Why?


X2. Pretty common practice in this part of the world.


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## jarvis design (May 6, 2008)

festerized said:


> OSB on the roof, Wrong


OSB is pretty common around here, but, I'm with you - it shouldn't be. 

Its as bad as OSB subfloors (IMO)


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## Joe Carola (Jun 15, 2004)

grgewer said:


> I'm going to be reroofing an older house with 2x6 rafters and plank 1x sheating. Want to add a layer of 7/16 osb on roof. Longest rafter length is about 14' to the collar tie in a vaulted ceiling (still a 2x6 rafter). 20' wide area. everywhere else we had braced front and back of roof to a wall (truss like) Will this vaulted area carry the small extra load of plywood without sag or any ideas how to stiffen it up keeping in mind the top planes in with the rest of the roof on the front of the house and the bottom side of the 2x6 rafters is drywalled?


Why are you adding another layer of sheathing on top of the existing sheathing?


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## A W Smith (Oct 14, 2007)

*7/16 OSB dont hold nails for *****

OSB? never on any of my jobs. in fact I even forbid SYP 3 ply 


Before











after










*BUT* using osb on the roof will make stripping off the shingles in the future very easy as OSB doesn't hold roofing nails as well as fir CDX. your future roofer will thank you.


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## Tinstaafl (Jan 6, 2008)

A W Smith said:


> *BUT* using osb on the roof will make stripping off the shingles in the future very easy as OSB doesn't hold roofing nails as well as fir CDX. your future roofer will thank you.


I've never been able to grab a shingle and rip its nails out of OSB sheathing. So what's yer point? :whistling


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## Kent Whitten (Mar 24, 2006)

A W Smith said:


> OSB? never on any of my jobs. in fact I even forbid SYP 3 ply
> 
> 
> Before
> ...


If it was my home, I'd use the same, 5 ply fir. I wouldn't use SYP unless they paid me to use it. 

OSB is not all created equal either. If you use the OSB they throw in the SYP chips in, like Polar Board, you are asking for trouble also. But I have used some great OSB before. And the Advantech IMO, is beyond using the term OSB. It's not in the same class. It's a good product.


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## A W Smith (Oct 14, 2007)

Tinstaafl said:


> I've never been able to grab a shingle and rip its nails out of OSB sheathing. So what's yer point? :whistling



My point is once osb gets thropugh a lifetime of wetting cycles. it loses its nail holding capacity. and it will be easier to strip off. Ive torn off roofs over OSB, Fir, and plank decking In fact ive torn off roofs ive already torn off 25 years earlier. And my experiences have prejudiced me to fir plywood. simple as that. 

http://bct.nrc.umass.edu/index.php/...ing-between-oriented-strandboard-and-plywood/


> Performance is similar in many ways, but there are differences in the service provided by osb and plywood. All wood products expand when they get wet. When osb is exposed to wet conditions, it expands faster around the perimeter of the panel than it does in the middle. Swollen edges of osb panels can telegraph through thin coverings like asphalt roof shingles.
> The term ghost lines or roof ridging was coined to describe the effect of osb edge swelling under thin roof shingles. The Structural Board Association (SBA), a trade association that represents osb manufacturers in North America, has issued a technical bulletin outlining a plan to prevent this phenomenon. SBA correctly indicates that dry storage, proper installation, adequate roof ventilation and application of a warm-side vapor barrier will help prevent roof ridging.
> Irreversible edge swelling has been the biggest knock on osb. Manufacturers have done a good job of addressing this issue at the manufacturing facility and during transportation by coating panel edges. But the reality is that builders don’t limit osb use to full-sized sheets. The edges of cut sheets are seldom if ever treated in the field. Houses under construction get rained on. And if you use osb in an area of very high humidity, like over an improperly vented attic or over a poorly constructed crawlspace, you are asking for trouble.
> Osb responds more slowly to changes in relative humidity and exposure to liquid water. It takes longer for water to soak osb and conversely, once water gets into osb it is very slow to leave. The longer that water remains within osb the more likely it is to rot. Wood species has a significant impact. If osb is made from aspen or poplar, it gets a big fat zero with regard to natural decay resistance. Many of the western woods used to manufacture plywood at least have moderate decay resistance.





framerman said:


> wouldn't use SYP unless they paid me to use it.


Even when they pay me to use it. I refuse. It's what I sell. Fir is only about 2 bucks a sheet more. than SYP 3 ply, if that. Fir however is twice that of common aspen 7/16 OSB, (used to be marketed as Aspenite)


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## grgewer (Dec 12, 2009)

Couldn't use anything larger than 2x6 because roof is on top and drywalled installed on bottom so after talking with the lumber rep here, we ordered engineered beams 6x6 designed for post beam sunroom, they get installed every 4' so I only needed 5. thanks for the idea


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## Tinstaafl (Jan 6, 2008)

A W Smith said:


> My point is once osb gets thropugh a lifetime of wetting cycles. it loses its nail holding capacity.


True enough. But ultimately, that goes for any type of wood. The trick is in constructing and maintaining your roof system well enough that it doesn't have to endure those wetting cycles.

Like it or not, OSB is here to stay. And they do keep improving its resistance to such problems. I would even venture to predict that at some point it will develop into a product that will be significantly superior to all of the commonly available "natural" wood substances available today.

If you really want to be a purist, you should be eschewing _all_ glued-together products. :thumbsup:


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## SC sawdaddy (Oct 15, 2008)

I gotta agree with Tin, osb is very common around here. We've been using it for years and never had a problem with sheathing failure.


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## Advantage (Jan 6, 2010)

Ive used OSB on over 5,000 roofs and have never had a problem.


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## wellbuilthome (Feb 5, 2008)

Every time i use OSB i swear ill never use it again .
I think fir ply wood will stiffen the roof enough to compensate for the weight. 
I hate the bubbles you get with OSB.


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## RMcBride (Dec 21, 2009)

When we strip a roof and uncover water damage or broken boards we replace the damaged sections with new 1x material for small sections, appropriate thickness OSB for the larger sections. We do tend to use Advantec 3/4 or the new OSB with baked on water guard.

If the basic structure of the roof is in good condition but simply has wide spaced boards we will cover it with 1/2" OSB sheathing and we simply use a 4" OC edge nailing pattern and a 8" OC field nailing pattern. That is of course if the roof beams are substantial enough to carry the load. We also install what I call shoulder bracing - might have other names in other places to stiff the roof. We also go as far as to install hurricane ties on all our rafters and joists.

I've stripped a lot of older roofs with OSB that looks fine - and a lot that look terrible. Relying on your substrate to survive being covered in water is silly - that's what shingles are for.


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## greg24k (May 19, 2007)

Advantage said:


> Ive used OSB on over 5,000 roofs and have never had a problem.


In Jersey its not a common practice to use OSB, I never seen anyone use it on roofs, because OSB's biggest downfall is its tendency to swell around the panel edges. It swells faster around the perimeter of the panel than the center. This swelling is irreversible. Once OSB swells, it stays swollen, and expanded panel edges protrude through roof coverings such as asphalt roof shingles.If there is to much moisture in the attic, you could have some serious problems.


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## KennMacMoragh (Sep 16, 2008)

grgewer said:


> Will this vaulted area carry the small extra load of plywood without sag or any ideas how to stiffen it up keeping in mind the top planes in with the rest of the roof on the front of the house and the bottom side of the 2x6 rafters is drywalled?


I had a roofing job like that once. We threw OSB right on top of the skip sheathing. I never even thought about the extra load, but I doubt that will be an issue. Weight of the OSB is nothing compared to the weight from the asphalt shingles, and then the snow load on top of that.


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