huangyhg

osb use
with the current state of affairs/pricing with plywood, i would appreciate any personal experiences using osb as a substitute for commercial projects.
the only side effect that i have read about is that osb has a sudden failure mode at the limit. i thought that might be less of a concern with a bit of overdesign.
one client claims that it is more stable with respect to delaminating and warpage than plywood, but that is just one opinion.
any and all opinions will be appreciated.
apa rated sheathing whether plywood or osb, should perform the same mechanically (bending, shear, etc.) if the apa rating is the same. that being said, some contractors in my area prefer plywood for roof sheathing and have no preference for wall sheathing. the few incidents of poor performing osb, that i have read about, have been attributed to improper installation (spacing between sheets).
only thing i heard about osb is it tends to shatter more when edge nailing is very close.
as an engineer, it makes no difference to me whether osb or plywood is used, as long as the material used is apa-rated properly.
lots of problems with osb....more to follow!
ron, i'm glad you said something. without having any supporting data or studies, i was afraid to chime in.
our policy is no osb allowed, no exceptions. my boss calls it "wood sponge board". but we don't do new design, remediation only, primarily in the sf bay area. we also do failure analysis and litigation work. we have plenty of anecdotal evidence that osb does not stand up as well to high humidity and exposure to moisture. it also seems to be a bit better at providing a growing medium and food source for mold. and the truth is, it is a rare project that building materials are handled exactly as the manufacturer states in their literature, which is usually substantially different from what the sales rep will tell you.
as far as cost, it's variable, but plywood generally runs about $3 more per sheet than osb.
i'd love to know if there is reliable 3rd party test data comparing plywood and osb under less than ideal conditions. so far i've not run across it.
cass,
your boss is confusing osb with particle board. osb isn't all that bad. as far as mold goes, i found it amusing that it is an issue. after all, every wood stud, joist, beam, and truss is mold food.
nope, the boss may be over-reacting (he has a tendancy to do that), but he's not confused about the material.
like i said, i have plenty of anecdotal evidence that does not cast osb in a particularly good light. for our particular projects and clientele, three bucks a sheet is no big deal. but i would not go so far as to tell any of our contractors to voluntarily substitute plywood for osb on a competatively bid project.
i have done several failure investigations on the use of osb, for roof sheathing and for wall sheathing. it has good structural strength under dry conditions, but will deteriorate under wet conditions, and in a different failure mode than plywood. osb swells at the edges (i have measured the swelling from the edges completely across sheets in 1-inch increments, then plotted the results. looked like a parabola) and plywood plies start to ripple as they fail.
osb will swell at its edges even with small amounts of moisture increase. this happens primarily on roof sheathing. the edges swell, the board increases in length and volume, and "picture framing" results. you see waviness in shingle roofs from its dimensional instability.
if you subject osb to the same testing done for plywood for durability (boil specimens in water), you get a significantly different result (osb will sometimes fall apart..not always, but not predictable either).
the concept of osb is great....the process used to keep it competitive with plywood makes it less durable and less predictable.
i have never heard of a class action suit involving plywood. there are numerous class action suits involving osb and osb-like materials.
i won't specify it, even though it achieves apa span ratings and the like.
ron, plywood also expands (as in becomes 48.2" x 96.2")when wet and buckles when there wasn't enough joint space. also, i have seen plenty of "ripple" on roofs, no matter what osb or plywood.
if your drawings specify apa-rated shtg, you are legally not at fault. lawyers retain unethical "expert" engineers, same as ambulance-chasing lawyers retain unethical "medical expert" doctors. they invent reasons for malfeasance, and osb is just a handy victim. the judge doesn't know any better. plywood has a nasty habit of delaminating when rained on before the roofing, and i have heard builders say that's why they prefer osb for roofs, and plywood for heavily nailed shearwalls.
falseprecision...i agree that specifying apa rated sheathing helps you comply with the standard of care and puts you on better legal footing.
given leaking and saturation, both will deteriorate and in different modes. in general, they are not rated for continuous wet service, only intermittent exposure.
i have tested both osb and plywood. while each has some specific advantages, in my opinion, plywood is the overall better choice.
i agree with you ron, both products are viable but in the long run plywood is the best choice.
comparative product data on plywood and osb can be found at canply's website. there are engineering values for both plywood and osb on axial compression, planar shear, bending and tension and thickness swell.
jb

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