Properties of FRT Plywood & inspection tips:
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).
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 vertical through-roof firewalls on multiple-dwelling buildings, the advent of FRT-plywood permitted omission of those more costly fire-walls that had to be built up extending through roof design.
The omission of that vertical through-roof extension simplified building roof construction thus reducing building cost as well as re-roofing costs.
On entry into the attic space of a multiple-dwelling structure such as attached town homes or condominiums, look first for the fire-wall that should have been built between the abutting units. In proper moden construction that firewall should extend from the foundation up through the occupied space and up through the attic space to at least the roof surface.
In that design, 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. You can see that design in the photo below. The concrete block wall on the left of the photo is the fire-wall, and you can see that the plywood roof sheathing adjacent to the fire-wall looks different (like plywood) than the remaining roof sheathing (that is OSB in the right portion of the photo).
In more recent construction as well as in retrofit jobs intended to improve building fire safety you may see a double layer of fire-resistant drywall installed for as much as four feet against the roof sheathing on either side of the party wall or fire-wall between adjacent living units.
But on older buildings where FRT plywood was used, attic heat and age was found to lead to deteriorated roof decks even where no actual fire had ever occurred.
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.
Alternative products to using FRT plywood as roof sheathing for the roof section covering the area of abutment of multiple living units have included
Where these products and designs were used FRT plywood may have not been required.
The following is adapted with permission from an original Fire-Retardant-Treated (FRT) Plywood article in Professional Roofing 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.
Below left: fire retardant plywood identifying stamp. [Click to enlarge any image]
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, adaptation, edits and additions by Daniel Friedman.
Fire retardant treated FRT plywood, while it is still a combustible material, has been chemically treated to provide a lower flame-spread rate than un-treated plywood used in building construction.
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).
In the Uniform Building Code, Fire-Retardant-Treated Wood is defined as
any wood product impregnated with chemicals by a pressure process or other means during manufacture, and which, when tested in accordance with UBC Standard 8-1 for a period of 30 minutes, shall have a flame spread of not over 25 and show no evidence of progressive combustion
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:
A Fire-Retardant-Treated backboard will be designated with a fire-rated stamp “branded” or “stamped” along the edge or center of the plywood—“UL FR-S Plywood 1780 R-7003.” Marine-grade plywood does NOT qualify even though it is saline “treated”—as it will have a different UL number.
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.
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.
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.
My house was built with no wall separation ( firewall) in the attic! When did the Victorians start to use them? The sash windows upstairs has two bars up and down making six panes of glass in all but four downstairs! Chimney pots are yellow with one bar top and bottom. I would be much grateful for any help dating the house! - A.C. 6/29/14
Firewalls are a modern building safety feature used in multiple occupancy dwellings.
E.g. see historical citations in
Window panes: long history of number of panes (larger = newer) but varies widely by geographic area and of course pane size varies also by sash dimensions.
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(Oct 20, 2012) Bill said:
I have a townhouse that still has FRT plywood. Is it safe? The th was built in 1985
(Dec 24, 2012) Jeremy said:
I was told that a newer TH with OSB roof sheathing and no FRS plywood was OK because a sprinkler system is installed in the house. Sprinklers are not in the attic. Is this ok?
(Apr 24, 2014) Eric Bates said:
Trying to educate my real estate due diligence staff on problematic fire retardant plywood. Need attic pictures showing the use and deterioration of the product. Can you direct me toward some pictures?
(Apr 24, 2014) (mod) said:
Eric, I've added another photo of the identifying stamp found on FRT plywood as it might be visible in an attic. Other photos of where the FRT plywood is likely to appear are shown in the article above.
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