Wood works: Flame spread burning questions
By Ang Schramm, Columbia Forest Products

Is your plywood fire rated? What is the fire rating (classification) of your plywood?

These seemingly redundant but frequently asked questions often generate a certain amount of confusion. It is important that before we begin this discussion we understand the difference between the terms "fire rated" and "fire retardant treated." All wood products have a de facto fire rating. As we will see, generally speaking, the Flame Spread Index (SDI) rating is Class C for every day, ordinary untreated wood. Solid or composite wood panel products may also be available treated in some manner with fire retardant compounds. Such panels are considered "fire retardant treated," usually to a Class A Flame Spread Rating.

The questions above sound like they are asking the same thing, but the asker of the first probably wants to know if the product is fire retardant treated (FR or FRT) to a Class A Flame Spread Index. The asker of the second probably just wants to know the rating of our untreated panels.

Why Do We Need to Know the Flame Spread, Anyway?

Wood is a combustible material. It will burn. If it did not burn, we might not be here. It is the first fuel source ever used by man. The most common use of wood in the world today is arguably for fuel to generate heat for cooking and warmth in developing regions. Even wood that has been chemically treated to retard or slow the rate of burn will still burn...just much slower, and wood thus treated will cease to burn much more quickly once the flame source is removed or burns out. Wood is also a versatile building material, providing both structural integrity and aesthetic beauty and warmth (no pun intended). In fact, wood is probably the most common building material in use today. Combine that information with the previously acknowledged fact that wood will burn, and it becomes important to know a little more. In that regard, many building codes and local regulations require a declaration in some form as to the flame spread of wood products used in projects in their jurisdiction.

The rating systems 

The most commonly accepted flame spread rating system in use today is set forth by the National Fire Protection Association, Life Safety Code, NFPA 101. Other model codes are offered by the Basic National Building Code (BOCA), the Standard Building Code (SBC), and the Uniform Building Code (UBC). The NFPA classifies flame spread in the following manner:

Class A 0 to 25 Flame Spread 0 to 450 Smoke Developed

Class B 26 to 75 Flame Spread 0 to 450 Smoke Developed

Class C 76 to 200 Flame Spread 0 to 450 Smoke Developed

SBC also uses Classes A, B, and C, while BOCA and UBC use Classes I, II, and III, all with the same corresponding values. The SBCCI, UBC, and BOCA combined resources in the mid 1990s to form the International Code Council (ICC).

These values are determined using the test methodology established by ASTM International, formerly known as the American Society for Testing Materials (ASTM). This test, designated ASTM E-84, also called the Steiner Tunnel Test, stipulates the equipment design and conditions under which any product shall be tested, whether wood, aluminum, or gypsum board, to name a very few. The values thus produced generally represent a comparison to arbitrary flame spread values for asbestos cement board at zero (0), and red oak flooring at 100. They are determined by exposing the test specimens to a live flame source and plotting the burn over time.

On rare occasion, a questioner may request the Smoke Developed Index (SDI) for a given product in addition to the Flame Spread. This is a relative term referring to the measure by photo cell of the concentration of smoke developed from tested specimens. It is based off the same arbitrary values established for flame spread, with asbestos cement board at 0 and red oak flooring at 100.

The American Forest & Paper Association, Inc. has conducted numerous tests on a variety of wood products, all in accordance with ASTM E-84. The findings from these tests are published in the Design for Code Acceptance, "Flame Spread Performance of Wood Products." Of these 40 tests (not including numerous tests also conducted on solid wood), only one specimen developed a flame spread of 200. Among the remaining 39, the highest flame spread value was 173, with an overall range reported among those of 75 to 173.
The report also lists test results for a total of 29 panel products of various configurations including paper and vinyl overlaid particle board, MDF, hardboard, and hardwood plywood, as well as factory finished hardwood plywood and MDO Douglas Fir plywood. The values generated from these tests range from 99-191, with most under 160.

Of all wood products included in the test, only solid Northern White Pine with a range of values from 120 to 215 and solid Ponderosa Pine with a range from 105 to 230 produced results outside the upper limit for Class C.

Numerous tests we have had conducted by an independent testing agency in accordance with ASTM E-84 have all produced similar results. In fact, the oldest test result I have in my files was conducted by the HPVA (then HPMA) in September of 1981. It was ¼" Veneer Core natural birch, and the actual result from that test was a Flame Spread Value of 165 and a Smoke Density Factor of 50.

The above referenced Design for Code Acceptance thus concludes, "as can be seen from these examples, most tested wood products have a flame spread index less than 200, making them acceptable under current building codes for a wide range of interior finish uses." As the published values also show, no panel product exceeded a flame spread value of 200. From this data, it is reasonable to conclude that most untreated wood products manufactured by Columbia Forest Products meet the requirements for Class "C."

Certification Programs are available, and the cost and ancillary recordkeeping can be justified for a long term program with large volumes and contractual commitments. However, these just aren't employed as a contingency against those infrequent requests for a one-time shipment of a small volume of plywood. If some form of independent certification is required, the Hardwood Plywood and Veneer Association offers the prescribed ASTM E-84 Tunnel Test for $575 plus the cost of the panels. But...even if we submit the exact product covered by the request, because the test is destructive in nature you could not use the actual tested product. Any certification based on any actual test would have to state essentially what I have written above, and that is that products "similar" to the ones tested were found to meet Class C.

Class "A"

Panels with a Class A, 0-25 Flame Spread are obtainable on request. These are produced utilizing a substrate that consists of components that have been treated with one of the many fire retardant compounds, including various phosphates, borates, or sulphates. As of this writing, only particle board core and medium density fiber board core are available with a Class A rating, and these are subject to quantity restrictions. These engineered cores are manufactured with particles or fibers that have been treated prior to manufacture. They are quite stable, having proven themselves as acceptable for use as substrates for high quality architectural millwork and panel systems. The Fire Code Summary published by the Architectural Woodwork Institute points out that the application of an untreated wood veneer face 1/28-inch thick or thinner does not alter that rating for the finished panel.

There have been a few attempts over time to treat veneer core with fire retardant treatments, all requiring a "blank" to be manufactured with a water proof glue so that they could be pressure treated with the fire retardant compounds in liquid suspension. Past treatments usually left the core with a very high moisture content which often led to instability, warp, core telegraphing, and checking, and as of this writing none are currently in use.

There are also some effective on-site treatments that may be applied to finished surfaces. I would highly recommend getting as much information as possible from the treatment manufacturer before attempting this method. This is best done by skilled factory trained personnel to ensure the desired end result. One such company with a sound reputation for their experience and knowledge regarding their product quality and code requirements is Firefree Coatings, Inc. According to their website, www.firefree.com, they are "a leading developer of fire resistant technologies that protect human lives and property from fire. The company is the only provider of fully tested fire retardant and fireproofing intumescent coating products uniquely designed to provide a high level of fire protection at a reasonable cost."

On the structural panel side of the industry, there were numerous costly lawsuits floating around in the very early 1990s as a result of rapid deterioration of the cellulose (the actual wood) in these products in the field attributed directly to fire retardant treatments. The compounds that contributed to the deterioration have been discontinued or modified so that today these products are expected to perform up to expectations where fire resistance is important to structural integrity but aesthetics is not a consideration. Columbia Forest Products


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