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Photograph of  fire retardant identifying markings when newPerformance of FRT Plywood Roofs
History of FRT plywood use, problems, litigation, standards

Properties of FRT Plywood & inspection tips:

This document summarizes the history of the development and use of fire-resistant plywood roof sheathing in the U.S. and explains the issue of structural damage to roof sheathing where fire retardant plywood roof sheathing, or FRT plywood was used on buildings.

Bottom line: FRT-treated material can degrade seriously due simply to high attic temperatures. Special inspection and testing methods are available. The cost to remove and replace FRT plywood roof sheathing was significant. In addition, FRT plywood roofed buildings apparently did not receive the fire-spread resistance that was intended.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Requirements & Specifications for Fire Retardant Plywood Treatment for Roof Sheathing

FRT Plywood success © Daniel Friedman at InspectApedia.comWhat is FRT Plywood or Wood?

Fire-retardant-treated plywood or lumber is pressure-impregnated with chemicals to permanently inhibit combustion. This qualifies it for a lower flame-spread rating (at least as low as gypsum wallboard) and smoke developed index, and reduces its fire-hazard classification. (APA 2009)

FRT or flame resistant plywood is (or was) 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).

The earliest permitted use of FRT plywood in the U.S. was in the 1960s, allowed by building codes applied principally to states east of the Mississippi. The first BOCA approval of FRT plywood for roof sheathing was in 1979. This was an effort to permit omitting of more-costly through-roof parapet walls that were otherwise required to prevent the spread of fire between building units.

Our photo above shows the success of FRT plywood in preventing fire spread from one portion of a flaming building to the next.

[Click to enlarge any image]

Widespread use of FRT plywood began in the U.S. by 1980, and the first FRT failures were observed as soon as 1-3 years following initial construction.

Affecting 250,000 homes or more in the U.S., (Builder 1992), FRT plywood failures, and subsequently also some FRT wood framing (truss) failures were found to be most-rapid and serious where buildings were exposed to higher temperatures and humidity, though temperature is the greater factor in these failures.

FRT Plywood Problem: Structural Deterioration in Environments of Heat & Moisture

In the U.S. including New Jersey where this problem was first brought to our attention, FRT plywood roof sheathing products began to be widely used during the late 1970s and early 1980s (Barbel 1992), mostly in homes east of the Mississippi, and was used in construction up to 1988 (Kryakos 1994) or in a few sources, up to about 1990 (McLain 2017).

FRT deterioration problems were first "discovered" in New Jersey around 1987 and were found throughout the U.S. wherever FRT plywood was used in roof construction.

In the worst cases, roofs made with FRT plywood have required replacement. In these cases the wood had darkened, was very brash and brittle, and crumbled easily.

For the severely degraded roofs brought to our attention, service time has ranged from 3 to 8 years. The publicity generated from the problem has raised serious questions about the causes and extent of wood deterioration.

The magnitude of wood degradation depends on the particular fire retardant formulation used, the temperature levels attained in the roof system, and the presence of moisture.
- (LeVan, USFPS, 1989)

The FRT deterioration problem was widely investigated by experts as you'll see in citations given at the end of this article, and was addressed by ASTM and other standards associations in North America.

FRT plywood was intended to deteriorate in response to a fire at around 400°F by charring to retard flame spread.

But in actual field experience it was found that the deterioration of FRT plywood occurs in response to the heat at temperatures as low as 150°F, commonly-experienced by roof sheathing from sun exposure and attic or roof-cavity temperatures was sufficient to so-weaken the plywood that someone walking on the roof could easily step right through the material. (Peterson 1990)

The effectiveness of the fire-resistant roof sheathing also was called into question.

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.

Watch out: when inspecting a building for the presence of FRT plywood, before walking on the roof, be sure to first inspect the roof sheathing and framing from the attic or under-roof space.

Watch out: If you suspect or confirm that FRT plywood is in place on the roof, do not walk on that roof area as weakened plywood roof sheathing could give way, resulting in roof damage, even possibly a collapse, and severe personal injury.

Premature Degradation of Fire-Retardant-Treated (FRT) Plywood Used in Roof Decks

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.

3 Categories of Fire Retardant Treated Plywood

FRT plywood treatments are divided into three categories:

  1. Exterior grade FRT Plywood
  2. Interior Type A FRT Plywood
  3. Interior Type B FRT Plywood

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.

3 Fire Retardant Chemicals & Their Effects on Plywood & Wood Framing

FRT chemicals that have been used to treat plywood and framing lumber that have been of particular focus include:

All three of these cause an acceleration in wood deterioration or strength loss compared with un-treated controls, but MAP was found to have a significantly greater effect than either of the two organic salt FR treatments. (Winandy 1990)

It is important to note that permanent thermal degradation eventually occurs for both treated and untreated materials exposed at 180°F. Furthermore, it is noteworthy that after thermally induced degradation has initiated (<21 days exposure) and eventually stabilized (>21 days exposure), the raw of strength degradation is similar between treated and untreated materials, even though there arc large differences in strength. (Winandy 1990)

In understanding the effect of these chemicals on wood building products it's worth noting

The results indicate that MAP-treated plywood is lower in bending strength than untreated plywood at all temperature and, as temperature increases, the rate of strength degradation is similar between untreated and MAP-treated plywood.

It was also noted that as relative humidity increased at 170°F, the rate of strength degradation increased.

However, the effect of relative humidity did not appear to be as influential as the effect of the temperature of exposure. (Winandy 1990)

Problems with FRT Plywood Degradation Can Extend to Framing

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.

[The] degradation process is directly associated with environmental conditions of temperature and humidity. The FRT chemicals react with wood during cyclical changes in temperature and humidity causing changes in pH such that the wood becomes brittle. This process is most commonly associated with plywood roof sheathing, where exposure to radiant heat is most significant. (Hodgin 2002).

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.

Watch out: in some buildings roof structures included both FRT-treated plywood decking and also FRT-treated roof-support framing.

At Chesterfield Marlboro Technical College in South Carolina, the collapse of the roof of a building constructed in 1974 was found to be due to deterioration of both FRT plywood roof decking and FRT-treated roof trusses. Derek Hodgin, a P.E., and Andy Lee, a professor of Forest Product Resources at Clemson investigated the deterioration of FRT-treaded wood framing at the college. .

... a recent case study in South Carolina indicates that the effects on southern pine dimension lumber used in roof framing [and treated with fire retardant chemicals] can be equally dramatic. (Hodgin 2002).

Where will Fire Retardant Treated Plywood or Framing be Found?

FRT plywood at fire wall in condominium © Daniel Friedman at InspectApedia.comOn 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 modern 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.

Look for FRT-treated roof sheathing (and possibly framing)

Can an Inspector Determine the Condition & Safety of Existing FRT Roof Decking or Framing?

OPINION: Yes and no.

Yes, a home inspector or building inspector or contractor can certainly observe visual deterioration that is advanced enough that an FRT plywood roof deck is sagging, delaminated, shedding, or visibly damaged, as she might also see roof truss failures if the trusses are FRT also.

No, a more-thorough, professional investigation of the condition of FRT roof sheathing and framing may require probing, mechanical testing, or chemical tests such as a measure of the plywood's pH. (Lebow 1999).

In our OPINION the very least required of an building inspector is the identification of the presence of FRT roof sheathing if there is access to the underside of the roof sheathing such as in an attic, and if visual evidence such as the presence of fire-walls, plywood stamps, variations in the appearance of plywood, and the building age combine to merit both an inspection for and a warning about the actual or potential presence of these materials.

Test protocols for FRT plywood are cited in several of the references given at the bottom of this page. We refer especially to

Winandy et als, THERMAL DEGRADATION OF FIRE-RETARDANT-TREATED PLYWOOD, Development & Evaluation of a Test Protocol [PDF] (2002) cited in more-detail below.

Fire-Retardant-Treated Plywood & Lumber Lawsuits

Early litigation appeared to begin with FRT roof failure cases in New Jersey but spread to other states, as we cite below.

Fire Retardant Treated Plywood Identifying Labels & Brands

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: fire retardant plywood identifying stamp. [Click to enlarge any image]

Fire retardant plywood identifying stamp © Daniel Friedman at InspectApedia.com

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:

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.

FRT Plywood Brands in the U.S.

FRT Plywood Brands & Dates

Up to 1970
Hoover Treated Wood Products Co..
Hoover Universal, Inc.,
Osmose Wood PReserving, Inc.,
After 1970
Circle-M Wood Treating Co.
D-Blaze
Dri-Con
Flame Guard Fire Retardant
Flameproof LHC
Hoover Treated Wood Products, Inc.
Hoover Universal, Inc.
Osmose Wood Preserving, Inc.
Protex

Notes to the table above

How Fire Retardant Treated Plywood Works

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.

Fire-retardant-treated wood is defined in Section 2303.2 of the 2006 IBC as “any wood product which, when impregnated with chemicals by a pressure process or other means during manufacture, shall have, when tested in accordance with ASTM E 84, a listed flame-spread index of 25 or less and show no evidence of significant progressive combustion when the test is continued for an additional 20-minute period.

In addition, the flame front shall not progress more than 10.5 feet (3200 mm) beyond the centerline of the burners at any time during the test.”

The ASTM E 84 test is also called “The Tunnel Test” because the test material is suspended from the lid of a test chamber that is approximately 25 feet long by one foot high and 1-1/2 feet wide.
(APA 2009)

This APA document (cited in detail below and provided as a PDF download) includes details about the flame spread test and the IBC 803 definitions of Class A, B, and C Flame Spread materials.

Current Uses of Fire-Retardant-Treated (FRT) Plywood

Fire resistant treated plywood prevented fire spread © Daniel Friedman at InspectApedia.com

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).

Current Definition of FRT Plywood

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

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.

Factors Reducing the Deterioration of FRT Plywood

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.

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 [PDF] (2009) cited in detail below.

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.

Construction Alternatives Can Eliminate the Need for FRT Plywood

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.

How fire spread is controlled where FRT plywood was or is not used?

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).

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.

Conventional Wood Frame Buildings and FRT Wood use vs Fire Sprinklers etc.

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 use of fire sprinkler systems, fire spacings (set-backs), and fire-resistant-rated walls, floors, and roofs, are required to obtain larger interior spaces.

Fire Retardant Plywood History, Research, Failures

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2019-03-11 - by (mod) -

Dana

To identify a specific plywood product and thus its fire classification you would look for the stamps or markings on the plywood itself. If you can't find that information you're stuck with having to find the original purchase or bill of sale and plywood supplier and to trace that information backwards through the supplier themselves.

On 2019-03-08 by dana

How do I know if my plywood paneling is a class A or B?

On 2018-12-05 - by (mod) -

Fire retardant treated wood as a very long history A.B. dating from before 1900.

In the U.S. including in your state of New Jersey FRT plywood roof sheathing products began to be widely used during the late 1970s and early 1980s. (Barbel 1992)

FRT deterioration problems were first "discovered" in New Jersey around 1987. FRT had been on the market for more than a decade prior to that.

I already have some historical citations at the end of the article above. I'll go through our library and add some additional detail.

You may need to clear your browser cache and wait a few hours to see that added information.

On 2018-12-05 by A Bartolomucci

What year did FRT come to the market, especially in NJ?

On 2017-02-02 by Sam

what is the preferred method of repairing delamination FRT is there a known patching material? to be applied from the inside?

On 2016-07-20 - by (mod) -

Anon: the cost of a single sheet of FRT-treated plywood depends on the thickness required and other properties such as facing type, but you can figure typically around $19. U.S.D. / 1 4x8 foot sheet for quantities under 1000 sheets at the low end to as much as $35.00 for a single 1/2" (15/32") CDX FRT-treated plywood at a building supplier such as Menards.

On 2016-07-20 - by (mod) -

Re-posting from private email, writer kept anonymous:

Typically, how much does a sheet of 4x8 5/8” FRT Plywood cost?

Question:

(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?

Reply:

(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.

Clicking on any of our images will present an enlarged, sharp-focused copy.

You are welcome to use printed copies of this article in a classroom presentation.
Our content use and copyright policies are also available for view online at

inspectapedia.com/Admin/Content_Use_Policy.php

Please also mention to your due diligence staff that we welcome questions (or content suggestions or criticism) and are happy to respond, research, or provide further information as needed.

Reader Question: when were building fire walls first required in homes?

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

Reply:

Firewalls are a modern building safety feature used in multiple occupancy dwellings.
E.g. see historical citations given in the references on this page.

A victorian home built 100 years or more ago will not have used FRT plywood in its original construction.

Watch out: however if the roof was re-decked or re-framed between the 1960s and late 1980s you can't rule out the use of FRT plywood without first inspecting the roof decking and framing.


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