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mgbinspect

OSB Wall Sheathing

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I'm standing on the ground this morning watching siding guys pull off vinyl siding to determine the cause of two buckles in a long expanse of end wall on a home that is a year and a half old. We were all convinced it was bowed studs.To our surprise, the buckles were in the OSB - between studs. Apparently, the sheathing was too tight (no space between sheets). That was a surprise. This wall get direct sun all day long.

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Contractors will argue about this, but I always put 1/8" gaps between sheathing panels in both directions (and don't use OSB). If the stuff you were looking at was getting wet, so much the worse...

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Seems like I remember gapping directions stamped on sheets of OSB. One common bulge source I've seen is from carpenters bracing a gable end of a trussed roof where studs are turned flatways.

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I love OSB. In insurance work, it makes it possible to put in a 16 x 16 patch in roof sheathing, when a roof gets punctured. You can't do that with plywood. OSB is RIGID.

Now particle board is trash, but OSB - a champ!

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I love OSB. In insurance work, it makes it possible to put in a 16 x 16 patch in roof sheathing, when a roof gets punctured. You cn't do that with plywood. OSB is RIGID.

Now particle board is trash, but OSB - champ!

Granted that plywood is polarized...rated in the long dimension only...but why couldn't it be used in patching a roof deck if it's oriented correctly?

Marc

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I love OSB. In insurance work, it makes it possible to put in a 16 x 16 patch in roof sheathing, when a roof gets punctured. You cn't do that with plywood. OSB is RIGID.

Now particle board is trash, but OSB - champ!

Granted that plywood is polarized...rated in the long dimension only...but why couldn't it be used in patching a roof deck if it's oriented correctly?

Marc

It was my call. When you put your weight on a 16 x 16 plywood patch, it feels like a trampoline top compared to OSB, which won't give a bit.

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I can see pros and cons for OSB. We are seeing it more and more because good quality plywood is getting harder to produce.

In recent years, the filler plys have been made from inferior grades of wood, Hemlock and maybe even cottonwood, and the outer sheets are sapwood with a lot of patches where the knots were trimmed out.

Good quality plywood was produced back in the day, from old growth Douglas Fir. When logs are scaled, the highest grade peelers are the butt logs from old growth DF with no pitch pockets or rot and no offset to the heart.

Now we have to use a smaller second growth log, which will have a lot of knots in it and a wider grain.

The con with OSB isn't so much the strength factor. It is the fact that it absorbs water thru all those short fibers, unless it is sealed with a coating of some kind.

The correct OSB for exterior use will have the edges painted according to the grade and the stamp will be turned to face inwards.

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Builders here are beginning to use it as wall sheathing on the entire house, which I think is great. It sure beats fiber board sheathings, in my book. It seems that the OSB out these days tolerates a soaking pretty well.

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I sheathed my neighbors garage with OSB and used galv. screws as spacers for lack of anything else handy which left about a 3/16" gap. The gap has since closed off completely.

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Granted that plywood is polarized...rated in the long dimension only...but why couldn't it be used in patching a roof deck if it's oriented correctly?

Of course, The O in OSB stands for oriented. Since the strands are oriented to one axis, the OSB has the same issues as plywood if one installs either 90 degrees out from its intended orientation. Additionally, the fibers on the bottom side of OSB are longer than the fibers on the top to better resist the tensile forces acting on the face of the sheet so OSB also has a top and a bottom.

Marc, I know you knew this stuff, but in case anyone misunderstood the intent of your question as presented, I figured I'd take a minute to elaborate.

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I've used lots of OSB and it's fine, but it's not better than plywood. For sheathing, half-inch five-ply plywood (which is the only kind I'll use) outperforms half-inch OSB in every way.

For flooring, I absolutely hate OSB because it doesn't hold nails worth a damn. When you try to secure underlayment to it, the nails always work loose and screws just spin. Installing traditional hardwood flooring over OSB is a mess; the nails never bite into it properly.

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What about the wide thread screws? I've gotten it to hold those screws OK.

If it's glued, it seems as good as plywood for sheathing. I could be wrong, but it seems to make a pretty strong box.

I'd never put hardwood flooring over it, though. The nails don't hold.

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What about the wide thread screws? I've gotten it to hold those screws OK.

If it's glued, it seems as good as plywood for sheathing. I could be wrong, but it seems to make a pretty strong box.

I'd never put hardwood flooring over it, though. The nails don't hold.

I couldn't even get the wide-thread screws to hold. However, around here, we have to assume the the OSB has been thoroughly soaked for weeks. When It's fresh it holds the screws ok, but when it's been soaked, it just won't hold them.

It works fine for sheathing. I just think that 5-py plywood works better.

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You will see other issues that contributed to the condition but this is some OSB I found last month.

Click to Enlarge
tn_201161205036_2011%2005%2013%20RRO%206423.jpg

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and here is what it looked like on the ground after I removed it with my bare hand.

Click to Enlarge
tn_201161205310_2011%2005%2013%20RRO%206424.jpg

89.38 KB

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Like any engineered material, it has to be used in a design specific manner. Which, of course, rarely happens.

How often do folks see Exposure 1 material being used? How often do you see Exposure 2 for sheathing?

Honestly, I hardly ever see the stuff. I've built with it, but I don't see it all that often on other than for reroofing over skip sheathing.

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How often do folks see Exposure 1 material being used? How often do you see Exposure 2 for sheathing?

All I've ever seen is exposure 1.

I just did a framing inspection on a house that, by my estimate, saw 11 inches of rain while the deck was open to weather. the Advantech deck was completely unaffected.

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I've used lots of OSB and it's fine, but it's not better than plywood.

My personal experience with it is pretty limited. It came along just as I was really getting out of the disaster restoration game. But, a 16 x 16 patch with 1/2 plywood roofing sheathing felt like the whole affair would fold in and your foot would go into the attic. But, that same patch with OSB - you could dance on it. It must be all about that orientation Chad described. The difference between the two options was pretty amazing.

How often do folks see Exposure 1 material being used? How often do you see Exposure 2 for sheathing?

All I've ever seen is exposure 1.

I just did a framing inspection on a house that, by my estimate, saw 11 inches of rain while the deck was open to weather. the Advantech deck was completely unaffected.

I had to call an engineer in one time a while back, because the first floor sheathing of a home under construction had been wet for so long. A hurricane had gone through, which stopped the job for quite a few days. I actually figured that the material was OK. It certainly felt like it was, except some minor swelling of the top wood strands. But, the buyer was so wary, that the only way he'd be satisfied was to hear it acceptable from an engineer. The engineer blessed it. It does handle moisture surprisingly well.

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I built a treehouse with Advantech. Sucker's about 10 years old, lottsa exposure, and it's fine. The stuff is amazing.

Cramer talked about a project in Florida that got hammered and stripped by a hurricane, and the Advantech floor decking sat in full exposure Florida weather for over a year before they got it closed in again.

The only thing they had to do was sand down a few seams where the stuff had swelled slightly.

So, when there's talk of OSB, I think the discussion has to include the exposure rating of the material. The Exposure 1 stuff seems pretty tough.

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I guess the only thing you have to be careful of is allowing for some expansion, then. The wall in question was about thirty feet by two and a half stories plus a gable - no openings whatsoever. There were only two buckles (each about 48" tall between two studs), but they were pretty pronounced.

When I was first called back, I felt the home owner was being really hyper sensitive about the appearance of his home. I had to study the crap out of the home to actually notice the buckles, but the home buyer assured me that when the sun was just right, they stood out quite well. Once the siding was stripped away, they looked like two blisters.

I know I'll be considerably more aware of spacing between sheets of OSB wall sheathing, when performing phased new construction inspections.

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Granted that plywood is polarized...rated in the long dimension only...but why couldn't it be used in patching a roof deck if it's oriented correctly?

Of course, The O in OSB stands for oriented. Since the strands are oriented to one axis, the OSB has the same issues as plywood if one installs either 90 degrees out from its intended orientation. Additionally, the fibers on the bottom side of OSB are longer than the fibers on the top to better resist the tensile forces acting on the face of the sheet so OSB also has a top and a bottom.

Marc, I know you knew this stuff, but in case anyone misunderstood the intent of your question as presented, I figured I'd take a minute to elaborate.

I was aware of the fibers being oriented but not of any practice or custom of installing it in a certain orientation nor of any industry rating that was specified for one orientation only. At least I don't recall.

Thanks.

Marc

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I think OSB got a bad name from the stuff that was produced back in the 1990's and also during the EIFS enlightenment period of 1998 to 2003. The stuff made in the 1990's did have major problems with water and exposure to the elements. It had somthing to do with a change in the resin formulation.

Now OSB is widely used as wall sheathing, sub flooring and roof decking in my area. I seldom see plywood in use on new home construction.

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Like any engineered material, it has to be used in a design specific manner. Which, of course, rarely happens.

How often do folks see Exposure 1 material being used? How often do you see Exposure 2 for sheathing?

Honestly, I hardly ever see the stuff. I've built with it, but I don't see it all that often on other than for reroofing over skip sheathing.

My photos are of exposure 1 rated OSB installed in 2004.

It seems that once the water is introduced to the OSB the lignan breaks down very quickly. The adhesives seem to have very little resistance to moisture.

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I built a treehouse with Advantech. Sucker's about 10 years old, lottsa exposure, and it's fine. The stuff is amazing.

Cramer talked about a project in Florida that got hammered and stripped by a hurricane, and the Advantech floor decking sat in full exposure Florida weather for over a year before they got it closed in again.

The only thing they had to do was sand down a few seams where the stuff had swelled slightly.

So, when there's talk of OSB, I think the discussion has to include the exposure rating of the material. The Exposure 1 stuff seems pretty tough.

We took a scrap of Advantech floor decking and placed it in a bucket full of water for two weeks. We removed it and let it dry. The piece seemed unaffected.

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My photos are of exposure 1 rated OSB installed in 2004.

It seems that once the water is introduced to the OSB the lignan breaks down very quickly. The adhesives seem to have very little resistance to moisture.

Joe L. has the same thing to say about it. During manufacturing, all the natural decay resisting materials are extracted so the "plastic" can bond with the wood better. Once water gets in, it's over.

That said, I've never seen Exposure 1 go down in the manner described.

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