Below is a letter to the editor from Marilyn Thompson, Market Communications Director of the APA-The Engineered Wood Association, regarding Greg Havel’s recent Construction Concerns article, “Delaminated Plywood,” followed by Greg’s response.
APA is committed to promoting the proper use of structural wood products to assure the safety of building occupants and firefighters. We understand the concerns raised in the article “Construction Concerns: Delaminated Plywood” that appeared recently on the Fire Engineering Web site. However, some clarification will be beneficial to your readers:
The moisture resistant adhesives used in Exterior and Exposure 1 grade plywood today are thermoset adhesives that do not melt or break down before wood burns. The ignition temperature of thermoset adhesives used in today’s plywood manufacturing is higher than that of wood. It is possible that some interior grade plywood in the 60’s might be manufactured with adhesives that are less moisture resistant. However, the interior grade plywood is no longer accepted today by the consensus-based plywood standard, PS 1-09 Structural Plywood, recognized in the U.S. building code for building construction.
When panels are exposed to extended roof leaks or condensation that may occur with poorly vented attics, there may be risk of panel deterioration from rotting of the wood itself. Also, some earlier generations of fire-retardant-treated plywood had issues with wood strength degradation. This type of strength loss was not due to “delamination”, which is failure of the gluebond as evidenced by a very low percentage of wood fiber pull out (also known as “wood failure”), but rather the strength degradation of wood fibers.
The article provided useful comments for detection of panels that may be failing during firefighting. APA supports and participates in training of firefighters on understanding the design and construction of wood structures using wood products trademarked by APA in order to heighten the safety of building occupants and firefighters.
For more information and APA recommendations for design and construction of wood structures, refer to our website at www.apawood.org.
Greg’s response:
Ms.Thompson doesn’t seem to disagree with the article; but points out that the new plywood standard requires use of thermoset resin glue (takes its shape when heated initially, and does not melt when reheated) rather than the thermoplastic resin glues (soften and melt when reheated) that were often used years ago.
That is nice to know for the future, but doesn’t address the millions of homes and other buildings from decades ago that used plywood manufactured to an earlier, less strict standard. Many of these will be with us for perhaps another 50 years.
The difference between degradation and delamination of plywood proposed by the APA might be too fine a point for most firefighters. When plywood comes apart in layers, it is called “delamination” in Brannigan’s 4th edition (page 143), and in a number of articles in several publications over the years. When a plywood roof deck beneath a firefighter fails because it has come apart in layers, the type of failure (failure of the glue bond, or failure of wood fibers to adhere to each other) will be considerably less important to those involved than a soft landing in a space with no fire present, and an easy way out. While details may not matter to a firefighter in a critical situation, they are great to have when discussing the incident after it is finished.
