A majority of home builders and remodelers have made the switch to
oriented strand board (OSB) as a substitute for plywood roof
sheathing. Initially, the decision was driven by the low
introductory price of OSB, but the honeymoon is over. As of this
writing, a sheet of OSB sells for a few dollars less than plywood.
Nevertheless, the marketplace conversion to OSB continues.
OSB's acceptance has been slow in coming. Early on, OSB got a bad
rap because of its association with waferboard, a sheathing
material with an amorphous blend of wood flakes that became
available commercially in 1963. The first true OSB with layers of
aligned fibers was introduced by the Elmendorf Manufacturing Co. of
Claremont, N.H., in 1982. Among builders, its initial reception was
chilly. Response warmed gradually as OSB established a reliable
track record, and now OSB dominates the national market for
residential sheathing, accounting for 60% or more of the structural
sheathing sold nationwide. While many builders have become
comfortable using OSB, new suspicions surfaced when code advisers
in Florida's Dade and Broward counties charged that OSB fails as
structural roof sheathing in wet, windy exposures, such as those
caused by Hurricane Andrew's rampage across southern Florida in
CONCERNS AFTER ANDREW
The damage from Hurricane Andrew was formidable and has only
recently been surpassed by the quadruple whammy of 2004's
record-breaking storms. In response to public outcry, the Florida
government moved into action, creating an advisory committee to
investigate the nature of the structural damage and make
recommendations that would limit future reoccurrences. Jose
Mitrani, a civil engineer and professor at Florida International
University in Miami, was involved from the beginning on the task
force charged with investigating the failures. He does not think
that OSB performs adequately as structural roof sheathing. "During
that experiment we call Andrew, OSB did not behave as an equal to
plywood," claims Mitrani. "I saw shattered OSB all over the place
and I did not see a lot of plywood."
Mitrani later served as vice chair of the task force subcommittee
investigating the performance of OSB, and reviewed mountains of
reports submitted by consulting engineers and industry members. APA
— the Engineered Wood Association, a trade association
representing the OSB and plywood manufacturers, provided test data
demonstrating how OSB performs when it's wet and when it's dry.
"The data was convincing," Mitrani says. "But when we plotted the
results our way, the behavior of OSB was simply not as good as
plywood." Mitrani is convinced that dry OSB behaves as well as or
even better than plywood, but when wet, it's another story. He
claims OSB swells when wet and causes nails to pull through the
panels easily. APA rejects Mitrani's condemnation.
Ed Keith, senior engineer with APA, was there from the beginning
too, as part of Florida's official damage assessment team. He
claims, "The problems with roof sheathing were related to the fact
that builders were not building correctly, and the inspectors were
not inspecting adequately." Keith insists that APA's quality
assurance testing of OSB and plywood is reliable, ensuring that
builders get a durable roof sheathing. Panel manufacturers must
submit to random testing to earn an APA stamp. APA-stamped OSB and
plywood are tested for bending strength, stiffness, shear strength,
and concentrated load performance. APA measures fastener holding
capability and tests glue bonds with accelerated weathering wet/dry
cycle tests to assure durability. The APA tests outlined in PS2/PRP
108 are product performance standards, and they predict that OSB
and plywood perform as equals for their intended end use. This begs
the question: Why did Mitrani and others see so much failed OSB
during inspections? Proponents of OSB say it's simple: Most homes
were built with OSB (rather than plywood), so there was more of it
around. And the OSB that failed did so because of poor workmanship
and faulty installation.
"When I did inspections after Andrew, I saw panels with literally
only four nails in the corners and roof framing members with no
holes in them, indicating that the sheathing had never been nailed
off or that sheathing nails had completely missed the roof framing
targets," says Keith. He claims the overall consensus among experts
who have looked at roof-sheathing reports is that failures
resulting from Hurricane Andrew were related to "workmanship and
improper fastening," not the selection of OSB.
John Pistorino, a principal of Miami-based Pistorino and Alam
Consulting Engineers, considers OSB to be a viable product.
Pistorino has been a special consultant for the South Florida
Building Code since 1974. His company was a key participant in the
production of the 1993 Federal Emergency Management Administration
(FEMA) report "Building Performance: Hurricane Andrew in Florida,"
and Pistorino took every picture in the report. "I saw more of the
hurricane damage than most people because of the position I was
in," says Pistorino. The tests he conducted convinced him that OSB
provides the structural capacity required for roof sheathing, and
that OSB is not much more susceptible to water than plywood.
"Immersing OSB in water for a long period of time certainly shows
that OSB swells more than plywood," Pistorino explains. "But I am
not sure this is what you see as a normal condition in the field."
He believes that OSB roof sheathing is more prone to rot and
weakening when a roof is allowed to fall into disrepair, but
considers this a maintenance issue. Pistorino found that improper
fastening was the cause of most roof-sheathing failures during
Andrew, echoing the opinions expressed in the majority of reports.
He advocated for the banning of staples that is now part of the
current South Florida Building Code. Tighter nailing schedules and
ring-shank nails are now required. Florida currently has a
statewide building code that allows the use of OSB and plywood as
roof sheathing — except in Dade and Broward counties.
Recently, I served on a National Research Council panel where we
determined that there were literally thousands of code
interpretations enforced in this country. Even states with a
statewide code have communities with different interpretations and
requirements for a given application, so it's hard to say anything
absolute about "the code." However, there has been a move to create
a uniform code through the development of the International Code
Congress (ICC). The ICC permits the use of OSB roof sheathing
interchangeably with plywood in its 2003 International Residential
Code (IRC), which has been adopted statewide in 26 states
nationwide, including nine states along the eastern seaboard and
Section R803.2 of the IRC regulates "wood structural panel
sheathing" based on standards used to qualify panel performance,
not the material used to make the panel. Of all the building codes
that I am familiar with, it appears that Dade and Broward counties
stand alone in their ban of OSB as acceptable roof sheathing. The
reason: OSB fails the large missile impact test, in which a 9-pound
2x4 is fired at a panel from a cannon at a speed of 34 mph. The bar
is set at the level of resistance provided by 19/32-inch-thick
plywood, which is prescribed by Dade and Broward counties. OSB must
be 30% thicker than plywood to pass this test. Some experts think
the established test limits are arbitrary; they suggest it might
make more sense to fire a 25-pound roof tile at 50 mph and see what
happens to a panel.
Nearly all structural-use plywood and OSB share the same Exposure 1
durability classification. This classification deems a panel
suitable for temporary exposure to the weather, which a panel
typically would face during construction. OSB and plywood also
share the same set of performance standards and span ratings.
Recommendations for installing both materials, including
prescriptions for blocking, fastening, and the use of H-clips, are
identical. Independent research conducted by Professor Poo Chow, a
researcher at the University of Illinois, and others demonstrates
that nail withdrawal and pull-through for OSB is as good as or
better than plywood. However, there are real differences,
particularly in the responses of each type of material to
When nailed properly, OSB resists wind uplift as well as
plywood, but behaves differently on impact. While these test panels
show similar results to the impact of a 9-pound 2x4, the
splintering plywood (lower right) will hold a 2x4 during pressure
cycles better than the OSB (left).
The wet test. Plywood gets saturated much
faster than OSB. Plywood can get saturated when exposed to a couple
of days of rain, while it will take more than a week to saturate
OSB. However, plywood dries quickly, and OSB does not. Some
consider this the Achilles' heel of OSB.
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 can telegraph through thin roof
coverings such as asphalt roof shingles. Manufacturers have reduced
the likelihood of "ghost lines" by coating OSB panel edges to
reduce water absorption, but the phenomenon is still common.
Plywood does not have this problem; it swells uniformly, less
dramatically, and reversibly.
Compounding this weakness is the fact that OSB is often made from
aspen and poplar, neither of which are rot-resistant woods. Because
OSB holds water, prolonged exposure to roof leaks or excessive
humidity and condensation in unvented attics can cause OSB roof
panels to degrade faster than plywood panels. So OSB, in its
current state of development, is more sensitive to moist
conditions, while plywood is more forgiving.
Stiff and strong. Wood is a widely
variable material, but OSB makes up for this variability. With
about 50 strands of fiber across its thickness as opposed to the
4-veneer thickness of plywood, OSB's range of variability is much
narrower. This means all sheets are consistently stiff and strong.
While plywood may be slightly stronger on average, OSB is more
uniform, with less difference from sheet to sheet. You never have a
"soft spot" in an OSB panel because two knotholes overlap.
Likewise, you don't have to worry as you do with plywood about
knotholes at the edge of an OSB panel when it is nailed in
THE BOTTOM LINE
Hurricane Andrew redefined the way we think about risk and damage.
The dramatic increase in development and human population along our
coastlines has upped the ante when it comes to structural damage
control. And because it's been estimated that more than 80% of the
losses caused by Andrew were related to roof failures and
associated water damage, roof construction clearly deserves special
attention. Andrew proved that improperly fastened roof sheathing is
easily ripped from framing supports. And when wind-borne
projectiles crash into walls, windows, and roofs, significant
damage results. Roof punctures may not immediately level a home,
but they do allow rain to penetrate. When interiors are wet, the
structures are weakened as rain-soaked ceilings collapse, removing
the reinforcement provided by ceiling joists.
In the final analysis, there's no doubt that both OSB and plywood
roof sheathings work fine when installed and maintained correctly.
But there's also no escaping the fact that OSB simply is not as
resistant to impact as plywood. Is OSB's impact resistance strong
enough? Many experts think it is. But when pressed, few will give
up the advantage plywood provides in high-velocity hurricane zones
just to save a few dollars per sheet. ~
Paul Fisette is director of Building Materials and Wood
Technology at the University of Massachusetts, Amherst.
ROOF FASTENING ZONES FOR WIND
Zones shown indicate areas of the roof with different fastening
requirements and should not be confused with ASCE 7 pressure
(a) Edge spacing also applies over roof framing at gable-end
(b) Use 8d ring-shank nails in this zone if mean roof height is
greater than 25 feet.
Higher wind pressures at eaves, corners, and gable ends calls
for tighter fastening schedules at the roof edges. Nail spacing for
"High Wind Uplift" zones should be considered for all houses on the
hurricane-prone coastlines of the Atlantic and Gulf States. Nailing
for Intermediate Wind Uplift is appropriate for inland regions with
basic wind speeds above 80 mph.