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Redwood Lumber
Fire Hazard Classification

There are numerous similar instances over the years in which redwood's fire resistance has aided in stopping fires or in which redwood has been specified because of its fire-resistive property. Fire walls of crib construction have given excellent service in warehouses and commercial buildings. Curtain walls and fire stops of redwood have stopped fires in many structures, including warehouses and bridges.

Redwood does not ignite easily and it burns slowly because it contains little or no volatile resins or oils to aid in combustion. Once flame has covered the surface, a layer of char is developed which adheres to the unburned wood beneath. This retards further combustion, and in many fires, this layer of charred wood has prevented the fire from doing more than superficial damage to the structure.

Fire Hazard Classification

Fire hazard classification rates three buring characteristics of materials--flame spread, fuel contributed, and smoke development--on an arbitrary scale in which asbestos cement board is rated 0 and nominal 1-inch untreated red oak lumber is rated 100. The lower the rating, the more flame resistant is the material.

Poperty damage and loss of life by fire are closely related to how fast the flame spreads, how much fuel is added to the fire by the various materials of which a building is contructed, and the quantity of smoke and fumes produced.

Flame spread. Of the three ratings, flame spread is the most important and the most commonly used. All four model building codes, the Uniform Building Code (UBC), Basic Building Code (BOCA), National Building Code (NBC) and Southern Standard Building Code (SSBC) require certain minimum flame spread ratings for interior finish materials.

The Uniform Building Code lists three flame-spread classes:

Flame Spread Classification (UBC Table 42-A)
Flame Spread Classififcation
(Tunnel Test)

The BOCA flame-spread Classes are similar to those of the UBC, while NBC and SSBC have only two flamespread groups: 0-75 and 76-200.

Ratings of Redwood Lumber

In April 1971, Underwriters' Laboratories conducted their standard fire hazard classification tests on nominal 1- by 6-inch and nominal 3/8- by 6-inch untreated redwood lumber including both heartwood and sapwood.

Fire hazard classification tests were conducted in accordance with Underwriters' Laboaraties, Inc., "Standard Test Method for Fire Hazard Classification of Building Materials" (UL723), and ASTM Standard E82-81A, "Standard Method of Test for Surface Burning Characteristics of Buidling Materials."

The moisture content of the lumber samples tested ranged from 5.9 to 8.2 percent--well within the requirements of the standard.

The results of the fire hazard classification tests are shown in Table II. The flame-spread rating of 65 for 1-inch redwood lumber qualifies this material as Class II under the UBC.

In 1981, tests were conducted at the University of California Fire Test Laboratory in Berkeley to determine whether redwood is suitable for rooftop decks and walking surfaces by qualifiying as a Class B system as tested under U.L. 790. As a result of these tests, the International Conference of Building Officials and Bureau of Buidling Inspection of the City and County of San Francisco have determined that panels made from Certified Kiln Dried redwood heartwood meet the requirements of the Uniform Building Code for use as walking surfaces over existing fire retardant roofs. ICBO has issued Research Report #4101 which provides details for the construction of rooftop decks in accordance with the study. The San Francisco BBI has issued General Approval 706 C67 in regard to this matter.

Redwood Lumber Fire Hazard Classification
Flame Spread Fuel Contributed Smoke Development
1" nominal redwood lumber*
65 60 75-115
3/8" nominal redwood lumber
102 85 95-100

* The net surfaced size tested was 3/4 inch. Lumber surfaced to 11/16 inch will also qualify for a flame-spread rating of 75 and less (Class II UBC) by extrapolation between the values obtained from the nominal 1-ich and nominal 3/8-inch lumber tests.

At present, less importance is given to ratings of fuel and contributed and smoke development by the majority of building codes, although some--e.g., the San Francisco city code--require certain minimums. Redwood's ratings for these factors are included in the table above.

The results of these tests permit the use of redwood in a wider range of interior uses than heretofore. The UBC classification allows the use of redwood without fire retardant treatment or coating for many interior applications where code restrictions otherwise rule out untreated wood products.

As a Class II material, 1-inch redwood lumber can be used for interior walls and ceiling finish any place in all types of buildings under the UBC except for enclosed vertical exitways and a few other minor-use areas requiring Class I materials. Under the NBC an SSBC, with very few exceptions, 1-inch redwood lumber can be used for interior walls and ceiling finish anywhere.

The flame-spread classification of 3/8-inch redwood lumber, while not qualifying as Class II, is comparably low for products of this thickness.

A test of the magnitued necesary to include all possible patterns is not only impractical, but unnecessary. Based up on test results, all widths, patterns, surfaces, and textures in these thickneesses qualify for similar ratings.

For comparative purposes, some flame-spread rates of other material are shown in the table below.

Flame-Spread Rate of Other Material
(1-inch nominal)
Material Flame Spread
Fire retardant treated lumber
Redwood 65
Red oak lumber
Cypress lumber
Ponderosa pine lumber

Reference Cited

Uniform Building Code. International Conference of Building Officials, Whittier, California, 1982.

Report on Fire Hazard Classification of Redwood Lumber. Underwriters' Laboratories, Inc., Subject 723. Assignment 71SC509. March 16, 1971.

C60 Wood--Fire Hazard Classification. Card Data Service. Underwriters' Laboratories, Inc., 1971.


In the recent great fire of San Francisco, that began April 18th, 1906, we succeeded in finally stoppintg it in nearly all directions where the unburned buildings were almost entirely of frame construction and if the exterior finish of these buildings had not been of redwood lumber, I am satisfied that the area of the burned district would have been greatly extended.

P. H. Shaughnessy,
Chief Engineer of the San Francisco Fire Department during the 1906 Earthquake and Fire.

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