We are considering replacing some of our roofing to open up airflow in our cathedral ceiling . . . we currently have plywood sheathing. Which would be better to use, plywood or OSB, or does it matter?

 

CharlotteAnn,

Plywood on roof, OSB for wall sheathing

Hope this helps!

 

Doesn't matter.

Both are engineered structural sheathing products and designed to do the same thing.

http://www.umass.edu/bmatwt/publications/articles/osb_vs_plywood.html

 

manhattan42,

Your article is good but it also reflects what I am stating here.

Both may do the same thing, structurally, but plywood will perform better than the OSB. Most mold issues are more prevalent with OSB. Most good builders know the difference and prefer the plywood.

Once OSB is cut, the unprotected areas are very prone to water infiltration. The manufacturer recommends that these cut ends be sealed - I have never seen a builder seal the cuts!

Plywood on the other hand will perform much better. Most contractors who use OSB on the roof do so as it is cheaper in cost, not much, but a little bit here and there, it all starts to add up.

Just some added thoughts!

 

I respectfully cannot agree Doug, and the research supports the fact that there is no significant difference between the performance for OSB and plywood, even as far as roof sheathing goes.

It merely comes down to personal preference.

I use exclusively OSB for roof decks, floors and walls, but it took me years to overcome my prejudices against OSBs....After doing extensive research as well as experience in the field, I conclude no significant differences at all, though there are pros and cons to each.

Plywoods delaminate and are actually less strong over the same spans than OSB. Plywoods have less sheer strengths compared to OSB. Plywoods are far costlier than OSB. OSB are a better use of the resource.

OSBs, on the other hand, are less prone to swelling form casual exposure to water, but more prone to retain swelling if saturated. And must be edge treated in the field after cutting. And it's true that many do not edge treat OSB.

But few edge treat treated lumber either after every cut, despite the product manufacturer's and building code's insistance that it be.

Bad practice does not a bad material make.

I've had this debate dozens of times and have concluded that there is no truth the the superiority of plywoods at all.

Plywoods and OSB are similar engineered wood products. One not ultimately better than the other.

Might as well argue floor trusses or or I-joists. Bith work and work well.

And the ultimate use is one over the other is solely a matter of personal choice and economics....like flavor of ice cream or frozen yogurt and it's cost.

Nothing more.

Feel free to disagree...and let the reader decide.

 

Manhaten42:
You do have good points, and it is obvious that you are OSB sold for your work. However, There are a couple of things that have been over looked in your studies.

In many large cities in this country, OSB cannot be used on any re-roof jobs, or any new construction. Here is the problem. When OSB gets wet, it gets soft and loses its strenght. It can get wet for many reasons. Dampness from inside from the temperature differences. A hole in the shingels, holes in the roof vents, etc. You know the routine. Well back in the early 90's, there were several fireman who were on the roofs of houses, putting out fires, and the OSB sub roffing gave way and they fell through the roofs. In cities where this happened, new fire codes and building codes were changed to allow plywood only. In fact 5/8" plywood is the code in many areas.

I am not sure where you live, and obviously OSB is ok there. But in this forum it is the policy to give only the best information available, especially where and when we do not know where the person who asked the question lives. Such is the case here. Ann asked which was best for her particular situation. Since she had plywood on her roof, then plywood is what should go back on. Doug was correct in his answer. He designs homes, he knows. I build homes all over the US and I know also.
We respect your answers for your particular area, but one must be careful about telling someone else who lives somewhere else in the country how to do their building. OSB might not be allowed by here code. We know plywood is allowed. Have a good day.

 

Local codes aside, because local codes can make up any rules they wish so long as they exceed model code minimums....

All major national codes recognize along with the wood manufacturers themselves that there is absolutely NO DIFFERENCES between the performance of OSB vs plywood on roofs, and Jack the Contactor's information regarding OSB becoming wet and failing applies the same way to plywood or sawn lumber sheathing for that matter. Left to get wet long enough, ANY wood sheathing will fail, and OSB fails and no faster rate than does plywood. I've actually personally fallen through more plywood and sawn lumber roof decks that have rotted than OSB roofs if my experience amounts to anything.

If we are going to give the 'best' information let's at least make it 'correct' information and not just something from our biases.

I've done my own 'tests' relating to the strength of OSB when wet, and have left sheets outside exposed to the rain for several years, then spanned the sheets between saw horses and stood and jumped on the OSB in response to a challenge to my former beliefs that plywood was 'superior'. I'm over 300lbs and the OSB DID NOT GIVE and I did not fall through...This was but one of the tests that sold me when I incorrectly believed as do Doug and Jack that OSB is inferior to plywood. It simply isn't.

OSB today isn't your father's OSB. OSB performs just as well or better than plywood for roofs.

And there simply isn't anything in the literature to support the claims that plywood is superior.

And for the record I am also a homebuilder and have built all over the US as well am a nationally certifed building code official. I'm well versed in the research and unfortuantley it does NOT support your (plural) claims about plywood being superior to OSB for roofs. The support for those claims just doesn't exist from independent 3rd party testing labs.

To avoid being 'warned' about being argumentative, I'll cease from this discussion, but I do not agree with either Jack or Doug's assessment here.

The real problem here is not OSB, but rather one of perception and resistance to change.

There are still people who believe sawn lumber joists, girders, and rafters are still the only way to go and will have nothing to do with glulams, LVLs, I joists or any other type of engineered lumber.

The same resistance to change occured in the last century when those who built strictly with sawn lumber sheathing first started to encounter plywood. "Real' builders wouldn't even think of using plywood, yet plywood proved the better product because it was more economical and a better use of the materials just as OSB is proving the better product because of economics and material use today.

The real issue is not with OSB being inferior, the issue is with builders, engineers, architects and designers and even homeowners getting stuck in their ways of doing things and mistakenly thinking their personal preferences equate to product superiority when they do not.

Plywood, OSB or sawn lumber for your roof sheathing will work equally well and each will have its pluses and minuses.

Doesn't matter to me what the owner uses here.

But it does matter when the information provided isn't based on the facts.

I'll continue to use OSB because it has proven to me to be the best product for the price for all sheathing applications....including roofs.

And continue to resist the claims that plywood is superior for roof decks...because it simply is not true.

All respect due to everyone.

God Speed in your decisions.

 

Charlotte Ann:
Even though this thread has gotten off track, the real answer to your question is still plywood. Have a good day.

 

OSB is the dominant material used for roofs in this wet state of Washington. Go figure.

 

Mike:

I know, I have built over 1000 homes in Washington State. Have a dry day.

 

OSB is the dominant material being used because it is less expensive. Same thing is true in my area. But if I were building for myself, I would use plywood as it is less susceptable to damage if it occasionally becomes wet.

In my experience working on existing homes, plywood holds up better than OSB does to occasional wetting. This would be in contrast to the earlier statement, "OSBs, on the other hand, are less prone to swelling". I think I'll perform a simple experiment in my kitchen and let you know how it turns out: Take a scrap of OSB and a scrap of plywood. Put them in a glass of water for a while. Then take them out and see the difference, observing which absorbed the most water and which has swollen more. Sure, if you left them both in water for years, they would both rot, but I think the point our esteemed members are making regarding the benefits of plywood over OSB is well taken.

 

Not true.

The research has already been done and the truth is quite the opposite.

It is an established fact that plywood absorbs water MORE READILY than OSB.

The difference between the two, however, is that when they are both saturated for long periods of time, plywood is more apt to shrink back to it's original size while OSB will remain permanently swollen.

For short term water exposures, OSB is LESS likely to develop water damage issues than plywood. And casual exposure of OSB to water even during construction shows no ill effects on the product.

"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. "

Source: http://www.umass.edu/bmatwt/publications/articles/osb_vs_plywood.html

 

Manhatten42:
It is quite obvious that you are an OSB man. You probably own alot of stock in it. But what you have negelected to understand, is that there are others that do not share you obvious line of thinking, and it seems that all of the builders and remodelers who like plywood are wrong in your opinion. Thats ok. Quit trying to change their minds, or jumping in saying that they are wrong. We all understand that you have read alot of studies. The difference is we use it everyday, and know how it acts. Now you have bad mouthed me, and thats ok, I'm a big boy, but you picked on Doug, and now Sleeper. So the next time you respond to this thread I will become aggravated and its not a good thing to mess with me. Have yourself a good day.

 

Bottom line, if the roofing is installed correctly either one will last for a long time. Install the roofing incorrectly and you will have problems with either. You all have a great day and let's all agree to disagree at times.