Fire-retardant treated wood building structures
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The present invention provides a fire-resistant building comprising exterior surfaces and interior support members, wherein the exterior surfaces and interior support members are formed from fire-retardant treated wood.

Mader, Henry J. (Pittsburgh, PA, US)
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1. A fire-resistant building comprising exterior surfaces and interior support members, wherein the exterior surfaces and interior support members are formed from fire-retardant treated wood.

2. The fire resistant building according to claim 1, wherein the exterior surfaces are selected from the group consisting of walls, studs, and roof sheathing.

3. The fire resistant building according to claim 1, wherein the interior support members are selected from the group consisting of walls, studs, floor joists, flooring, roof trusses and roof sheathing.

4. The fire resistant building according to claim 1, wherein the building is a house.

5. The fire resistant building according to claim 1, wherein all of the exterior surfaces and interior support members are formed from fire-retardant treated wood.

6. The fire resistant building according to claim 1, wherein the fire-retardant treated wood is pressure treated.

7. The fire resistant building according to claim 1, wherein the building further comprises window shutters formed from fire-retardant pressure-treated wood.

8. The fire resistant building according to claim 1, wherein the exterior surfaces and interior support members of the building formed from fire-retardant treated wood are capable of resisting degradation or structural failure due to fire for a time period of at least about 5 minutes at a temperature of about 450° F. (232° C.) or more



This application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 60/788,990, filed Apr. 4, 2006.


1. Field of the Invention

The present invention relates to fire-resistant building structures prepared from fire-retardant treated wood, such as houses.

2. Description of the Prior Art

Currently, model building codes require that the exterior walls of buildings constructed in the “urban-wildland interface” be of non-combustible materials or to be of one-hour “fire-resistance rated construction”. Heavy timber or log wall construction is also permitted because of its inherent fire resistance.

We have all seen news clips of brush/forest fires setting fire to dwellings, which increase the fire load of a forest fire and add to the propagation of the fire. With more and more people moving to the country or, more accurately, to remote areas, we are hearing about and seeing more residential fire losses from woodland and brush fires. While the California brush fires seem to get the most attention in the news, Florida, Arizona, Colorado and Montana in recent years have had their share of fires and it is heartbreaking to see occupants fleeing their homes, or worse, returning to the ashes of their former homes.

The urban wildland interface codes promulgated by the International Code Council or other standards writing groups are beginning to address this problem. There is a need for building structures, such as houses, which are capable of resisting woodland and brush fires.


The present invention provides a fire-resistant treated building comprising exterior surfaces and interior support members, wherein the exterior surfaces and interior support members such as walls, studs, floor joists, flooring, roof trusses and roof sheathing are formed from fire-retardant treated wood.


FIG. 1 is a perspective view of a fire-resistant treated house according to the present invention; and

FIG. 2 is a perspective view of the interior of a fire-resistant treated house according to the present invention showing interior support structures such as studs, walls, flooring and joists.


The present invention provides a home in which most or all structural members of the home are constructed of approved, commercially available fire-retardant treated wood, for example using conventional frame construction. The exterior wood cladding is pressure treated with exterior type fire retardant treated wood (FRTW), since it would be exposed to the weather.

Referring now to FIG. 1, the exterior surfaces or structural members 10 would include most or all roof trusses 12, roof sheathing 14, walls 16, corner posts 18 and optional door and window covers or panels (shown in phantom as 19). As shown in FIG. 2, the interior structural members 20 would include most or all studs 22, the floor joists 24, flooring 26. roof trusses and roof sheathing. Structural lumber not exposed to the weather could be of an approved interior-type FRTW.

In other embodiments, plywood panels of fire-retardant treated wood large enough to cover windows, doors or other openings would be prepared and available to cover such openings in advance of an approaching forest/brush fire to prevent the fire from penetrating such openings and igniting the interior building contents. These panels would prevent the entry of fire through windows, in much the same manner that shutters protect openings from wind and water in hurricane prone areas.

The roof of the house can be formed from FRTW or other fire-resistant materials such as steel, tin or concrete. The walls of the house can be formed from FRTW, as discussed below, or other fire-resistant materials such as masonry.

Fire-retardant treated wood has undergone many fire tests but is not considered to be a “non-combustible” material by current definitions in model building codes. However, wood, properly pressure treated with fire retardant chemicals, can perform a function as well as materials currently classified as non-combustible. The use of such materials to protect dwellings from fire is based on the fact that while forest fires are quite hot when contacting a building, the duration of exposure by the flame front is quite short, usually lasting only six minutes. As used herein, “fire resistant” means that the exterior surfaces and interior support members of the building are formed from fire-retardant treated wood capable of resisting degradation (other than external charring) or structural failure due to fire for a time period of at least about 5 minutes at a temperature of about 450° F. (232° C.) or more.

Rather than igniting and contributing to the fire, the FRTW house of the present invention may char upon exposure to the flame front but would not appreciably contribute to further combustion. When the forest fire flame front passes, the exposed structure/house of the present invention would not continue to burn. The char could be removed and the exterior refinished, but the structure would remain intact.

Compared to untreated wood, FRTW is more resistant to ignition and more resistant to flame propagation when tested in either a horizontal or vertical configuration.1 The usual test for FRTW is ASTM E84 standard test method for surface burning characteristics of building materials. Other terms have been used to describe its fire performance such as “reduced rate of heat release” and “enhanced charring properties”, allowing it to maintain strength by limiting the reduction of cross section of members. 1 Letter from Underwriters Laboratories Inc. Mr. Jack Bono to Mr. Wallace Norum, Bldg. Code Engr. AWPA, Nov. 26, 1961.

A non-limiting example of FRTW products suitable for fabricating a fire-resistant house according to the present invention include DRICON® fire retardant wood products which are commercially available from Arch Wood Protection, Inc. of Smyrna, Ga. DRICON® fire retardant treated wood products are pressure impregnated with Dricon FR fire retardant chemical to provide fire resistance. Other suitable fire retardant treatments for wood are disclosed in U.S. Pat. No. 6,652,633 (assigned to Arch Wood protection, Inc.), incorporated by reference herein. An FR-S surface burning characteristic classification has been assigned to all softwood species of Dricon FRT wood. Dricon FRT wood has a flamespread and smoke developed index of 25 or less when tested in the E-84 tunnel test and shows no evidence of progressive combustion in 30 minutes. The tunnel test compares surface burning characteristics of tested materials to those of asbestos cement board and untreated red oak lumber. A rating of 0 is assigned to asbestos cement board and a rating of 100 is assigned to untreated red oak flooring. Flame spread ratings of various species of untreated lumber range from 60 to 230. A rating of 25 qualifies for Class 1/Class A requirements. The American Wood-Preservers' Association standards C20 and C27 and Appendix H of the AWPA Use Category System differentiate between “low-hygroscopic” Type A products and other more hygroscopic Type B products. Type A products must remain at or below the fiber saturation point of wood (28% moisture content) when conditioned at 92% relative humidity and 80° F. Furthermore, Type A products are differentiated by their intended application (i.e., Type A high temperature (HT) and Type A low temperature (LT). Dricon FRT wood is listed as an interior Type A (HT) product by AWPA. See www.dricon.com website.

For example, one inch thickness FRTW plywood panels could be used to cover doors and windows. The exterior siding could be nominal FRTW board and batten, tongue and groove, shiplap or plywood siding of suitable wood species.

A properly designed FRTW building according to the present invention can withstand a fire in the Urban Wildland Interface as described in Appendix G of the IUWIC. This concept is described in Appendix G of the IUWIC in the sections dealing with “Enhanced exterior fire protection”, and/or “Shelter in place”.2 As the code text states; “There are obvious limitations to this alternative. First and foremost is the means of adequately evaluating the proposed fire resistance of any given assembly. Testing techniques to determine fire resistance for such objects as drywall and other forms of construction may not be applicable to exterior application. Nonetheless, code officials should determine the utility of a specific fire resistance proposal by extrapolating conservatively”. 2 International Urban/Wildland Interface Code, 2003, 2003 International Code Council (ICC).

While the present inventor is not suggesting that this proposal be a code requirement, one skilled in the art would expect that a successful test of an entire FRTW house in the “simulated laboratory ‘Forest Fire’ environment” would give code officials the information that would help them determine the utility of this specific fire resistance proposal. Also, this information might be useful to a prospective homeowner who could consider this option as one means for complying with the code.

In the past, interior fire-retardant treatments were hygroscopic and were, in varying degrees, corrosive to metal connectors. Depending on relative humidity conditions, such a building might not burn down, but if enough fasteners failed through corrosion, the building in time would just fall down. Also, the exterior type FRTW had not yet been invented, precluding a satisfactory product for weather-exposed uses. Through development of an exterior type FRTW, the non-corrosive, non-hygroscopic properties were developed. These same non-corrosive, non-hygroscopic properties are available in a new second-generation interior FRTW.

The present invention can also minimize the possibility of electrical fires starting a fire within the structure. Fires started in untreated frame buildings have set fire to surrounding woodlands, initiating forest fires which, in turn, destroyed other houses.

This present invention can also minimize the possibility of fires during construction of dwellings.

A large-scale fire test at a suitable testing laboratory could simulate a forest fire exposure on such a house proving the performance results.

Successful performance in such a test would give a homeowner the option of choosing this method of home construction, should be acceptable as a method for complying with the Urban-Wildland Interface Code, and should receive favorable consideration by the property insurance industry.

There are also some definite advantages to such a dwelling which may be of interest to a prospective owner and perhaps to the property insurance industry.

1. Possible reduction in builders risk insurance. Structures have burnt and been destroyed while under construction. More recently, two dozen homes were either damaged or destroyed in December 2004 in the Washington D.C. suburb of Hunters Brook, Md. Regrettably, arson was the reported cause of these fires.3 3 Washington Post; Jun. 6, 7, 2005; Staff writers Joshua Partlow and Eric Rick.

2. Reduction of electrical fires. Electrical fires igniting wood framing can be a cause of structure fires. This could be minimized if not outright prevented with the all fire retardant-treated wood house. Similarly, and regardless of the origin of a structure fire, this in itself could ignite the surrounding woodlands starting a fire that the IUWIC has been written to address. Radio reports indicated that the July 2005 fires near Wenatchee, Wash., were started by such a structure fire.

3. Reduction in probability of termite attack. Some of the currently marketed interior fire retardant treatments contain boric acid levels high enough to permit them to be registered as a pesticide. At the level used in the treated wood, the boric acid is not injurious to humans but is lethal to termites. This would minimize the need for the exterminator in areas prone to termite attack.

The FRTW house according to the present invention can be tested in a facility such as Factory Mutual Global's (FM Global) fire test facility in which the house can be built and tested with a simulated “forest fire”.

At the recent 2005 ICC Spring code change sessions in Cincinnati, Ohio, there were several proposed changes to the International Urban-Wildland Interface Code in which FRTW was denied for use on exterior walls. (These changes dealt with Section 504.5 and 505.4 of the IUWIC 2003 Code). The primary reason for rejection was the lack of presentation of substantiating fire test data other than the ASTM E84 test referenced in the IBC and IRC.

Other than ASTM E84 testing, most people probably are not aware of some of the “Diversified” tests that FRTW has been subjected to in order to gain the acceptances, which currently appear in the International Building Residential codes.

Fires in the past, occurring either in buildings classified as “non-combustible” or in untreated wood, have over the years prompted the testing of FRTW products to reduce combustibility hazards or to show that FRTW could perform well compared to construction materials classified as “non-combustible”.

In 1953 a fire occurred at the General Motors Hydra-Matic plant in Livonia, Mich. A quote in an insurance company publication stated the following: “before 3:40 p.m. on Aug. 12, 1953, nobody even suspected that a fire like this could happen . . . . It was non-combustible building with non-combustible machinery using non-combustible materials, and the end product was non-combustible. Yet the building burned, resulting in a 28 million-dollar loss”.4 4 The Spectator: October 1953; a (a publication of the Factory Insurance Association.)

This event prompted the joint effort of a large scale fire test in 1963 known as the “White House test” in which a 20-ft wide×100-ft long FRTW roof deck was subjected to a standard time temperature curve ignition source.5 The details of the test are described in a UL report dated Jun. 12, 1964. The FRTW roof deck was granted a Class 1 rating, now classified as a FM Class 1 Deck, or a NM 501 deck using the UL classification. 5 Report on Roof Deck Construction: Jun. 12, 1964; Underwriters Laboratories Inc.

This led to the increase in exposure for FRTW from the standard 10 minute ASTM E84 test to a 30 minute test, enabling it to be differentiated from other building materials that obtain only a “flame spread rating” for surface burning characteristics in the 10 minute test.6 As a result of the “White House” test, the 30-minute ASTM E84 test is now referenced for FRTW in all the current model building codes. 6 Wood Preserving News; May 1964, The Case for Fire Retardant Treated Wood, R. C. Stange.

Interestingly, the American Wood Preservers Institute (AWPI), the American Plywood Association, and The National Lumber Manufacturers Association jointly sponsored the project in 1963. It is gratifying to know that various segments of the wood industry could cooperate to open a market for wood, which was not available to them prior to this cooperative venture.

A major United States aluminum producer has used and may still use FRTW Class 1 decks for the roofs of their aluminum ingot plants. Steel Class 1 decks are not practical in such a corrosive environment, and for many years, the Class 1 FRTW deck has remained the logical solution in this application. Perhaps the current permitted use of FRTW plywood on either side of party walls in apartment and condominium construction for the elimination of parapet walls is based, in part, on the successful performance in the “White House” test program.

Thirty to forty years ago, fires known as the Bel Aire, Calif. fires prompted the development in 1965 of the first exterior fire retardant treatment for red cedar shakes and shingles to achieve a Class C fire rating for roof covering materials.7 The Bel Aire fires called attention to the hazards of burning and flying brands which would precede an actual fire front igniting the roofs of other homes well in advance of the major fire. The burning brand test is one of four separate tests required to obtain a Class C wood shingle or shake roof covering rating. 7 Effectiveness of Fire-Retardant Treatments for Shingles after 10 years of outdoor weathering: Susan L. Levan, Carleton A. Holms, USDA Research Paper FPL 474 1986.

This was the first use of an exterior fire retardant treatment for wood (Non-Com Exterior) and was developed by the Koppers Company, a Fortune 500 Corporation and a major factor in the wood preservation business until acquired in a take-over venture in 1988. Many thousands of squares of this type roofing are still in service, and other companies still produce Class C FRT shakes and shingles.

Two impressive fire tests were conducted in the early to mid 1970's, and because of the successful performance of each of these tests, a new market, quite by chance, developed for the exterior FRTW for the electric utility industry. One test was conducted by Koppers Company and the other by the AWPI. Both showed favorable fire performance of FRTW for specific applications. As mentioned earlier, the idea to perform fire test developmental work is sometimes started because of fire involving applications using an untreated wood product.

A major fire in the wood cooling tower at an electric utility near Conomaugh, Pa., was the example which prompted the Koppers Company to evaluate the use of the same exterior FR treatment for cedar shakes and shingles for the pressure treatment of cooling tower lumber, plywood and fill material.

Fire testing of a full sized end section of a typical cooling tower cell was done at Underwriters Laboratories as were some smaller scale fire tests comparing untreated wood fill to “Non-Com Exterior” treated fill. “Non-Com Exterior” was the trade name of Koppers exterior fire retardant treatment at the time.

The second test, which was sponsored by the AWPI, is known as the Full Scale Wall corner burnout test at Factory Mutual Research (now FM Global) in which ½″ FRT plywood panels were tested. This test is designated as FM Procedure 4880 and in the International Building Code is designated as an acceptable “Diversified” test, successful passage of which permits the use of foam plastic without the required thermal barrier.8 Photos of the test: one just after ignition, and the second after maximum burning of the 750 lb. wood crib show that the flames fire did not progress to the 50 ′×38′ extremities of the test facility. These results compare favorably with results obtained under similar conditions with gypsum wallboard. 8 Factory Mutual Research, Norwood Mass; July 1975, Full scale building corner test of FRT plywood panels; American Wood Preservers Institute.

As a result of this research, the insurance underwriting syndicates for nuclear power plants agreed to issue an insurance rate credit to U.S electric utilities willing to treat all of their construction lumber with the exterior fire retardant treatment. Duke Power was the first U.S. utility to use exterior FRTW at their nuclear plant construction site near Charlotte, N.C. While the rate credit was an incentive, the real motivator for the electric utility was the enormous cost the utility would have to pay for buying power from another utility for each day they were late generating their own power because of a possible fire-caused construction delay.

As a result of the fire performance of FRTW in the full-scale wall corner burnout test, building codes recognized this significance resulting in exterior cladding uses. In the early to mid 1980's, Exterior Insulation Finish Systems (EIFS) were being evaluated for fire performance on the exterior of buildings. Probably the most publicized of these test programs was the multi-story exterior fire test developed at Southwest Research Institute in San Antonio, Tex.

At this time the National Research Council of Canada had developed a similar test protocol intended to evaluate the fire performance of combustible components used for cladding on the exterior of noncombustible buildings. The test exposed the cladding assembly to an ignition source (exterior fire plume) originating from a post-flashover fire within the building that resulted in severe exposing and attaching to the exterior wall finish.

Koppers Co. was producing a phenolic foam plastic insulation that needed to be evaluated for its fire performance, similar to the work of the treated wood products division, which was already producing and marketing an exterior FRTW. Fortunately, personnel with Koppers had been working closely with personnel from the Canadian Wood Council (CWC) on ASTM fire standards and other common wood industry association initiatives. Through these contacts, Koppers became aware of CWC's interest in conducting similar research in Canada on “combustible” cladding systems, which presented an excellent opportunity for mutual collaboration to expand market opportunities.

The NRC test apparatus at the Almonte Fire Laboratory, which is located just outside of Ottawa, Ontario, involves a test specimen containing exterior fire retardant treated plywood as the exterior finish on the wall. This cooperative effort resulted in the development of a Canadian national fire test standard by the Underwriters Laboratories of Canada (ULC), CAN/ULC-S 134-Standard Method of Fire Test of Exterior Wall Assemblies. This standard is now referenced in the National Building Code of Canada (NBCC) to identify combustible cladding assemblies permitted to be used on noncombustible buildings.

More information on combinations of materials tested in the research and development of this protocol can be obtained from the Canadian Wood Council or the Fire Research Group at NRC.

A shanty was built with FRT and plywood sheathing at the Koppers product development and pressure treating plant in Orville, Ohio. At this location, the Non-Com Exterior product for Class C shakes and shingles was developed, and commercially treated along with lumber and plywood. The exterior cladding and roof were of Non-Com Exterior treated Class C western red cedar shakes, all materials readily available at a plant that pressure treats these products. To add a little “class”, the interior finish was untreated Luan plywood. It was further revealed that the origin of the fire, which occurred at night, was not known nor were the furnishings and contents of the foreman's shanty. New windows, a door and the charred shakes were replaced on the remaining FRT structure.

The IUWIC now requires that “exterior walls of buildings or structures shall be constructed with materials approved for a minimum of 1 hour fire-resistance-rated construction on the exterior side or constructed with approved noncombustible construction”.

The first two paragraphs of Appendix G are entitled “IDENTIFICATION OF THE PROBLEM” and “STRUCTURAL SURVIVABILITY”. In this text it is mentioned that the actual exposure of a building to the flame front by the perimeter of the fire was usually less than six minutes. It also discusses factors like “ashes . . . and . . . heavier embers . . . in front of the fire . . . intrusion of flame front . . . radiant heat flux . . . exposing the structure to . . . various levels of radiant heat . . . ”.

It appears a large-scale fire test conducted with an appropriately designed fire could respond to many of the issues addressed in Appendix G while possibly opening new markets for FRTW. I can only urge the industry to conduct the test and provide one possible answer to an ever-growing problem of fires consuming homes in the urban wildland interface.

While the present invention has been described in conjunction with the specific embodiments set forth above, many alternatives, modifications and other variations thereof will be apparent to those of ordinary skill in the art. All such alternatives, modifications and variations are intended to fall within the spirit and scope of the present invention.