Read Round One of the OSB vs. Plywood discussion from the May issue of Pushing the Envelope.

One of the most tested fire performance properties of construction materials is flame spread, which describes the material’s surface burning characteristics. The best known test method for developing a flame spread rating (‘FSR’) or flame spread index (‘FSI’) is ASTM E-84.1 The flame spread rating is expressed as a number on a continuous scale where inorganic reinforced cement board is 0 and red oak is 100.

Ignition temperature of a material may be tested with ASTM E-13542; however, little published test results are available for direct comparison of OSB and
plywood performance.

The most commonly used flame spread classifications are:

  • Class I (or A), with an FSR of 0 – 25 (e.g., brick, tile or gypsum sheathing)
  • Class II (or B), with an FSR of 26 – 75
  • Class III (or C), with an FSR of 76 – 200

Many solid wood products have a Class II rating; however, there is a wide FSR range for different wood species. For example, the Western Wood Products Association publishes a 45 FSR for Western Larch softwood lumber, 115 FSR for Ponderosa Pine softwood lumber and 170-185 FSR for Yellow Poplar hardwood.

Engineered wood products such as plywood and OSB must meet a Class III classification. FSR values published by the American Wood Council4 include:

  • 1/2” Douglas Fir plywood: 130-150
  • 3/8” Southern Pine plywood: 100-105
  • 1/2" OSB: 74-172

Note that the wide FSR range for OSB is partially associated with the various wood species used to manufacture the product. Many species are used to manufacture
plywood and OSB panels; however, the most commonly used species are:

  • OSB – southern pine, lodgepole pine and ‘aspen/poplar’. The raw material for the original waferboard product, which was made from square wafers, was aspen. As this industry expanded and OSB became the predominant product manufactured, other species, such as Southern Pine, white birch, red maple, sweetgum, and yellow-poplar were found to be suitable raw materials as well. Small amounts of some other hardwoods can also be used for OSB.
  • Plywood – Douglas fir, western larch and hem-fir (‘western plywood’ manufactured west of the Rocky Mountains) and southern pine (‘southern pine plywood’ manufactured in southern states). Plywood can be manufactured from over 70 species of wood. …These species are divided according to species strength and stiffness into five groups. … Group 1 species are the strongest and stiffest. Canadian softwood plywood allows 13 species for face veneer and 20 species for the inner plies.

While comparative evaluation of flame spread performance of specific OSB and plywood products easily can be carried out if product-specific FSR values are provided by the manufacturers, such information is not readily available on the Internet. However, review of published literature from various sources indicates it is reasonable to assume that commonly available OSB and plywood sheathing have roughly comparable flame spread performance; but the performance of plywood tends to be somewhat superior.

For the purposes of standard hot roofing applications, such differences, if any, in FSR performance between typically available OSB and plywood sheathing would not pose a significant risk; however, considering the wide range of potential FSR values for either product (due to differing species, densities and manufacturing processes), it would be prudent of the Roofer to request written FSR test results from the manufacturers. Such published FSR values may lead the Roofer to specify a particular brand of OSB (or plywood) sheathing or to favor a particular product in higher risk installations.

In addition to flame spread rating conducted per testing protocols established by ASTM E-84, a valuable source of comparative testing is ongoing work carried out by the Forest Products Laboratory of the University of California, Berkeley.

The UCFPL testing includes the following reports:

The test protocol for the UCFPL roof test involved placement of a burning "Class A" brand on the test roof deck and exposing it to a 12 mph wind source per testing protocols established by ASTM E-10810. The test was terminated when glowing combustion or smoldering stopped, or when the test deck failed due to flame penetration to the underside of the roof deck. The testing indicated somewhat superior performance by plywood sheathing in comparison with OSB:

We used oriented strandboard (OSB) in our test roof decks because it has become the predominant roof deck sheathing material in new construction. However, when we found that some "Class A" rated roof assemblies failed, we repeated the tests with plywood sheathing instead of OSB. The fact that the roof assemblies with plywood passed the test shows that the choice of sheathing material, though not specified by some roofing manufacturers, may be critical to the fire performance of the roof.

The test protocol for the UCFPL wall testing addressed ‘sheathing only’ configurations and typical siding over sheathing construction. The siding materials, applied over OSB or plywood sheathing with 15# felt, were exposed to a propane burner flame for 10 minutes to simulate a wildfire event. The test ended when flame-through occurred or when there was no evidence of sustained combustion and temperatures measured by IR were declining. Again, the testing (‘sheathing only’ and siding over sheathing) indicated somewhat superior performance by plywood sheathing in comparison with OSB:

  • Sheathing-only tests - Flame-through occurred at the following flame exposure times (min:sec): OSB with joint (12:15), plywood with joint (15:20), OSB without joint (16:30) and plywood without joint (20:15). Thus the (unavoidable) butt joints between panels are vulnerable, and OSB appears to be more vulnerable than plywood.

Note that the results published by UCFPL do not identify comparative data such as species, type or manufacturer for the tested plywood and OSB sheathing. The results are not necessarily representative for similar products; however, the testing provides further support for the position that the fire-resistive performance of typically available plywood sheathing tends to be somewhat superior to comparable OSB sheathing.

In summary, while plywood sheathing appears to offer somewhat superior fire-resistive performance when compared to OSB, such differences would not pose a significant risk during typical hot roofing applications carried out in compliance with industry standards.

September 2007
Issue Nine, Volume Two

This article by Colin Murphy and Lonnie Haughton.

Colin Murphy
Murphy is a founder and managing partner of Trinity | ERD.

Lonnie Haughton
Haughton is a construction codes and standards consultant with Richard Avelar & Associates.

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