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This document summarizes the issue of structural damage to roof sheathing where fire retardant plywood roof sheathing, or FRT plywood was used on buildings. The material can degrade seriously due simply to high attic temperatures. Special inspection and testing methods are available.
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FRT or flame resistant plywood is required by building codes for certain structures such as on either side of the fire wall between building units in multiple-living unit structures (apartments, condos, townhouses). Alternative products have included masonry walls that penetrate the roof and fire-resistant drywall laminated on the under-side of the roof sheathing on either side of the wall.
Our photo (left) shows the success of FRT plywood in preventing fire spread from one portion of a flaming building to the next. [Click any image to see an enlarged, detailed version]
As a substitute for through-roof firewalls on multiple-dwelling buildings, the advent of FRT-plywood permitted omission of the more costly fire-wall extending through roof design and simplified building construction. Typically FRT plywood roof decking was used for four feet on either side of the firewall between building sections, and the firewall terminated just below the roof decking.
Apparently the fire resistive treatment, intended to lead to a "surface charring" of the plywood to slow flame spread, also led to surface oxidation and deterioration. Often structural repairs will be required.
While FRT plywood seemed as if it was going to be a terrific product, it appears that high attic temperatures in some buildings caused early deterioration of the material.
In some cases the plywood became so soft that someone walking on the roof could simply step right through it. The material, as it was formulated in its problematic form, is no longer used in new construction but may still be found on some buildings.
Original Fire-Retardant-Treated (FRT) Plywood article in Professional Roofing was by Tom Bollnow, Professional Roofing, May 1999 p.62.
Q: Have there been any significant efforts made to eliminate premature degradation of fire-retardant-treated (FRT) plywood used as roof decks?
A. During the late 1980s, there was an outbreak of structural roof deck failures directly related to degradation of FRT plywood used as roof sheathing. Because the potential for FRT ply- wood degradation still exists, roofing professionals should be knowledgeable about FRT plywood properties so the likelihood of degradation occurring can be reduced.
FRT plywood is produced by pressure treating plywood with fire retardant chemicals. During the mid 1980s, the search for lower hygroscopic (i.e., less moisture-absorbing) chemical compounds to treat plywood resulted in a change from ammonium sulfates that cause fastener corrosion to ammonium phosphate salts. Ammonium phosphate salts with additional treatments using buffers, such as Borax, and organic and less acidic chemicals were developed to decrease fastener corrosion and raise the threshold temperatures of fire-retardant materials.
FRT plywood's structural strength changes from 10 percent to 20 percent after an initial pressure treatment procedure. The drying process follows the pressure treatment procedure and is critical to achieving maximum product performance. Problems result if the kiln drying process is accelerated. Air drying causes fewer problems, but it is more time-consuming. Products should be marked "KDAT" if kiln dried after treatment or "ADAT" if air dried after treatment.
FRT plywood treatments are divided into three categories: exterior, interior Type A and interior Type B. A roof deck typically will be interior Type A because it is not exposed directly to outside elements. Type B treatments can cause excessive moisture to accumulate in wood, allowing chemicals to react with steel fasteners and connectors.
Building code authorities, such as the Building Officials and Code Administrators (BOCA) International Inc., have specific requirements for treatment processes and labeling. For example, plywood must be manufactured according to American Wood Preservers Association (AWPA) standards, and the treatment process must be evaluated by BOCA Evaluation Services, National Evaluation Services or an AWPA-approved, independent agency.
In addition, each plywood piece must be labeled properly with its performance rating and design-strength adjustment values. FRT plywood must be used according to manufacturers' recommendations. It must be kept dry and used strictly within the parameters of design-load values.
Open flames' elevated temperatures activate fire-retardant chemicals that produce. low-level acids (i.e., acid hydrolysis) in FRT plywood. The acids lower the temperature at which thermal degradation occurs, increase the amount of surface char and reduce the production of flammable volatiles (i.e., by-product gases that contribute to flame spread). The results are a reduction of the flame spread across a surface and capacity to support combustion. When a flame is removed from FRT plywood's surface, the plywood will char but not burst into flames.
Chemicals that produce low-level acids causing fire-retardant effects also cause premature FRT plywood degradation at lower temperatures. Untreated plywood experiences no major problems at temperatures up to 200 F (93 Q. Roofing professionals should note that achieving fire retardancy at the expense of structural integrity is not desirable.
Acid hydrolysis and degradation can occur at lower elevated temperatures of about 130 F (54 C to 180 F (82 Q. Temperatures at the interface surface between a roof covering and deck can reach 200 F (93 X with 150 F (66 C commonly found. As a result, degradation can occur at temperatures that are below open flame temperatures.
Roofing professionals should note that there are construction alternatives available that can eliminate the use of FRT plywood. But local codes (e.g., fire, building) first must be referenced to be sure the alternative construction is in compliance. These options include fully sprinkled interior systems; noncombustible decks; %-inch- (16-mm-) thick water- and fire-resistant gypsum board beneath untreated plywood; and fire walls that extend through a roof system on a multi tenant building (e.g., an apartment complex).
If FRT plywood is installed new or encountered during a recover situation, the use of light-colored shingles, a radiant-reflecting roof covering (e.g., white single-ply) or improved ventilation may diminish potential degradation. These materials may lower temperatures at a roof deck's surface. Roofing professionals should use caution and precise documentation when confronted with FRT plywood roof decks to avoid repercussions if failures occur.
-- Original Source: Page 62 Professional Roofing May 1999. Photograph, edits and additions by D. Friedman.
Our second photo of a contained spread of fire (left) illustrates the object of using this fire resistant roof sheathing.
The plywood industry states that the flame-spread rate of FRT plywood is at least as low as gypsum wallboard (although without specifying which fire-rated wallboard was used for comparison).
Current Definition of FRT Plywood
In the Uniform Building Code, Fire-Retardant-Treated Wood is defined as
Identification of and Current Applications of Modern FRT Plywood
In a "how to" article on equipment room fire safety design discussing FRT plywood backer boards for electrical panels, thanks to engineer Ronald Belleza de los Santos, datacom provides this FRT identification detail:
Fire resistant or fireproof buildings and FRT wood use
According to the APA, and in accordance with the International Building Code (IBC), noncombustible buildings Types I and II (usually built of steel and concrete), allow fire-retardant-treated plywood and heavy timber construction in limited uses.
In buildings type IIIA and IV (less fire resistant than Types I and II), interior walls, floors, and roofs may be built of conventional, untreated wood. Non-combustible exterior walls (required for building types IIIA and IV) are required however. The IBC permits FRT wood for these exterior walls as a design option.
Conventional Wood Frame Buildings and FRT wood use
IBC building type V (conventional wood frame buildings) have the lowest fire resistance and are the least costly to construct. Type V buildings may be constructed using conventional un-treated wood throughout the structure. However the sue of fire sprinkler systems, fire spacings (set-backs), and fire-resistant-rated walls, floors, and roofs, are required to obtain larger interior spaces.
American Plywood Association Information about Fire Retardant Treated FRT Plywood
For a detailed, industry-provided and current description of Fire Retardant Treated FRT Plywood, see Fire-Retardant-Treated (FRT) Plywood, American Plywood Association (APA), representing the engineered wood industry, publication NO. K320, January 2009.
This document includes the types of construction where FRT plywood is used, specifies the proper type of fasteners used for FRT plywood, describes the burn-through resistance and design capacities of FRT wood, provides the FRT plywood treating process and test standards, outlines code-approved applications for FRT Plywood, and explains how to identify fire-retardant treated plywood. -- thanks to Arlene Puentes for assistance with this material. Also see WOOD ROOF COATINGS & FIRE RATINGS and for other roofing topics see ROOFING INSPECTION & REPAIR.
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