Building Code Requirements & Controlling Fire Hazard
The primary focus of design professionals, building departments, insurance underwriters, and firefighters is on four principals: Prevention, detection, evacuation, and containment, in that order. Prevention and containment are the focus of this presentation as they can be attained through proper design and construction. Detection and evacuation requirements are clearly stated in the building codes and apply to log buildings just as they would for any other type of construction.
For the protection of life and property, building codes define limitations on types of construction based on:
- The intended occupancy or use of the building
- The density and proximity of the building to other buildings
- The availability of fire-fighting services and water sources
- The degree of fire hazard incumbent to the use of the building
- And the combustibility of the materials used in the construction of the building.
Building Code Requirements
Building departments at the local, county, or state level typically require inspection of new construction to insure compliance with the life safety codes they have adopted. To assist these building departments apply uniform standards, several organizations develop and publish these guidelines into “model” building codes. These model codes are supported with training and certification to building departments that adopt them. For residential construction, the ICC (International Code Council) International Residential Code for One- & Two-Family Dwellings (IRC) provides all code requirements in one volume. For commercial construction, the ICC publishes separate model codes that focus on fire (International Fire Code), structure (International Building Code), energy conservation (International Energy Conservation Code), etc. All of these model codes are updated in a rigorous three-year review and approval process. The National Fire Protection Association (NFPA) has also published a set of standards and model codes, the National Electric Code, Life Safety Code, Fire Code, and Building Construction and Safety Code, to name a few.
The current version of the IRC at the time of updating this paper is the 2006 edition. The following provisions of the 2006 IRC may or may not be revised during the 2007-2008 code cycle to result in changes in the 2009 edition.
Flame Spread & Smoke Density
Flame spread and smoke density per Section R315: Wall and ceiling finishes shall have a flame-spread classification of not greater than 200 and a smoke-developed index of not greater than 450 according to ASTM E84 testing. This does not apply to trim, doors, windows, or materials that are less than 1/28-inch in thickness cemented to the surface if these materials have a flame-spread characteristic no greater than paper of this thickness cemented to a noncombustible backing. The ratings are grouped in three Classes:
- Class A has a flame spread rating of 0 to 25 and is usually specified for escape routes in buildings with large occupancy expectations.
- Class B ratings of 25 to 75 apply to rooms over 1500 square feet in area and escape routes in buildings with moderate size occupancy loads.
- Class C ratings, 75 to 200, apply elsewhere in low to moderate fire hazard conditions.
In the United States the standard used to evaluate flame-spread characteristics of materials is ASTM E-84, Standard Test Method for Surface Burning Characteristics of Building Materials. The test procedure exposes candidate materials in a horizontal, rectangular tunnel typically 17” to 22" wide by 12" in height and 25 feet long. The tunnel is equipped with two gas burners at one end that direct a flame onto the surface of the test material under a controlled air flow. Flame spreads along the surface of the material as the test progresses. The distance of flame travel and the rate at which the flame front advances during a 10-minute exposure determines the calculated flame spread index. To provide standard conditions for each test, the tunnel is calibrated to an index of 0 for noncombustible materials and 100 for 23/32" red oak flooring. Indices for tested materials can range from 0 to over 1000.
Based on ASTM E-84 testing, flame spread indexes for softwoods range from 60 to over 150. The results typically meet Class C code requirements. A sample of test results follows, as published in Table 17-1 of the Wood Handbook, published by the Forest Products Laboratory, US Forest Service. The most complete and current list for wood products is in AF&PA’s Flame Spread Performance of Wood Products - DCA 1 (download a pdf copy at http://www.awc.org/Codes/dcaindex.html#FirePubs).
| Species (a) | Flame Spread Index (b) | Smoke Spread Index (c) | Source (d) |
| Yellow-cedar (Pacific Coast Yellow Cedar) | 78 | 90 | CWC |
| Baldcypress (Cypress) | 145-150 | ------- | UL |
| Douglas-fir | 70-100 | ------- | UL |
| Fir-Pacific Silver | 69 | 58 | CWC |
| Hemlock, western (West Coast) | 60-75 | ------- | UL |
| Pine, eastern white (eastern white, northern white) | 85, 120-215 (d) | ------- | UL |
| Pine, lodgepole | 93 | 210 | CWC |
| Pine, pondersoa | 105-230 (d) | ------- | UL |
| Pine, red | 142 | 229 | CWC |
| Pine, southern (southern) | 130-195 | ------- | UL |
| Pine, western white | 75 (e) | ------- | UL |
| Redcedar, western | 70 | 213 | HPVA |
| Redwood | 70 | ------- | UL |
| Spruce, eastern (northern, white) | 65 | ------- | UL/CWC |
| Spruce, Sitka (western, Sitka) | 100, 74 | -------, 74 | UL/CWC |
| Hardwoods | |||
| Birch, yellow | 105-110 | ------- | UL |
| Cottonwood | 115 | ------- | UL |
| Maple (maple flooring) | 104 | ------- | CWC |
| Oak (red, white) | 100 | 100 | UL |
| Sweetgum (gum, red) | 140-155 | ------- | UL |
| Walnut | 130-140 | ------- | UL |
| Yellow-poplar (poplar) | 170-185 | ------- | UL |
(a) In cases where the name given in teh source did not conform to the official nomenclature of the Forest Service, the probable offical nomenclature name is given and the name given by the source is given in parentheses.
(b) Data are as reported in the literature (dashes where data does not exist). Changes in the ASTM E84 test method have occurred over the years. However, data indicate that the changes have not significantly changed earlier data reported in this table. The change in the calculation procedure has usually resulted in slightly lower flame spread results for untreated wood. Smoke developed index is not know to exceed 450, the limiting value often cited in the building codes.
(c) CWC, Canadian Wood Council (CWC 1996); HPVA, Hardwood Plywood Manufacturers Association (Tests) (now Hardwood Plywood & Veneer Association); UL, Underwriters Laboratories, Inc. (Wood-fire hazard classification Card Data Services, Serial No. UL 527, 1971).
(d) Footnote of UL: In 18 tests of ponderosa pine, three had values over 200 and the average of all tests is 154.
(e) Footnote of UL: Due to wide variations in the different species of the pine family and local connotations of their popular names, exact identification of the types of pine tested was not possible. The effects of differing climatic and soil conditions on the burning characteristics of given species have not been determined.
Fireblocking
Fireblocking per Section R602.8: “Fireblocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space.” For example, fireblocking is required in cavities of framed walls (at the ceiling and floor level) and at 10-foot intervals both horizontal and vertical (including floor and roof framing). Also, at openings around vents, pipes, ducts, cables and wires at ceiling and floor level, an approved material is required to seal the opening to resist the free passage of flame and products of combustion.
Fireblocking is not an issue in most log structures. Where logs stack directly on one another, the walls are an assembly of solid wood and do not have cavities through which flames can spread. For log wall technologies that incorporate a space between the logs, it is quite common to provide solid wood bearing blocks on regular intervals (not exceeding 10 feet (3048 mm) to comply with R602.8). Many second floor assemblies are constructed using beam and deck assemblies, again without concealed cavities conducive to flame spread. Similarly, roof systems are often beam and deck construction.
Containment
Dwelling unit separation per Section R317: Dwelling units in two-family dwellings shall be separated from each other by wall and/or floor assemblies of not less than 1-hour fire-resistive rating when tested in accordance with ASTM E119. EXCEPTION: A fire resistance rating of 1/2-hour shall be permitted in buildings equipped throughout with an automatic sprinkler system installed in accordance with NFPA 13R.
ASTM E-119, Fire Tests of Building Constructions and Materials, rates by time the ability of an assembly to maintain structural integrity under full design load, prevent flame penetration, and limit temperature increase on the side opposite the burn. Testing a large wall assembly includes applying a design load from above and a direct flame using furnaces. For a one-hour fire rating, the furnace is stopped after a specified amount of time and a 30-pound water stream is applied. If no flame or water penetrates the assembly and the temperature opposite the burn remains at a surface temperature less than 250oF, the assembly is proven to be a one-hour fire-rated wall.
Dwelling unit separation is just one application of the concept of containment. Even within a single structure, differing use may require an assembly to separate one space from another. In dwellings, the wall between the living area and the garage is typically required to be a one-hour fire rated assembly. The conventional response is to apply 5/8" Type "X" gypsum board on common walls/ceilings and to use a 20-minute fire-rated door (wood solid core) with automatic closing device.
Other typical fire code issues reviewed by building officials include
- Enclosed accessible spaces under stairs are required to have walls and soffit protected on the enclosed side with 1/2" gypsum board.
- Maintain 6-5/8" minimum distance from inside of flue to combustible materials with a minimum 4" solid masonry.
- Wood stove, chimney, heat shield, and hearth to be installed per appliance manufacturer's specifications and local code. Maintain minimum clearances to wall surfaces.
Controlling Fire Hazard
Preventing fire within or outside of the structure is the ultimate key to minimize the risk of damage from fire. The factors below should be considered regardless of your selected source of log building materials.
Suppression Systems
The requirements for fire resistive construction may be relaxed when an approved system of detection, alarm, and suppression is installed. Fire suppression is the application of a smothering, non-combustible substance (e.g., water, foam, dirt) that isolates the fuel from the oxygen required for combustion and/or cools the fuel source below its ignition point. The suppression system is most commonly a method of distributing water over a large area or onto specific areas. In today’s high tech world, large rooms full of computer equipment are often protected with chemical or inert gas systems that will not damage the room’s valuable contents.
The decision to invest in a suppression system needs to be balanced with the hazard assessment of the building type and the building site. In rural areas where forest fire potential is high, the choice may be to install non-combustible roofing or use a sprinkler system to protect combustible shingle roof coverings. Interior sprinkler systems are more commonly used in buildings that will have higher occupant loads such as hotels, assembly rooms, lodges, etc.
There are many decision points in selecting a fire suppression system, but the critical elements on which to focus are the system’s reliability and its response time. The response time of bringing the water to the fire is compounded by the response time of the detection and alarm system. As stated in the Forward sidebar, the presence of an operating smoke detector is the single most important factor in surviving a residential fire.
Environmental Conditions
The ambiance and character of a log building tend to lead the owner to building it in a more rural setting than more common types of construction. The extent of property loss from wildfires in rural environments since the 1980’s spawned the development of ICC’s International Wildland-Urban Interface Code (IWUIC).
Building Density
Log home subdivisions probably provide the greatest density of log buildings in a specific parcel of land, but the distance from one building to another reduces the danger from a neighboring structure fire. More often, the danger from a structure fire is from outbuildings and garages located near the log building on the same lot. The distance of the outbuilding to the log home, its intended use, storage of hazardous materials, and type of construction are the primary considerations. The key is to assess the potential hazard and provide an appropriate distance between the log home and the outbuilding.
Defensible Space
Chapter 6 of the IWUIC defines the nature of the surrounding space in terms of the fuel load. This Code is primarily intended for protection “from wildland fire exposures, exposures from adjacent structures and to mitigate structure fires from spreading to wildland fuels.”
Controlling the available fuel source around the building can decrease the hazard of fire. The Code defines three fuel modification distances that are measured along the grade from the outermost projection of the building (eave overhang, exterior deck, etc.). In a moderate hazard, defensible space means that combustible materials and vegetation must be modified or removed for a distance of 30 feet from the projected perimeter of the building (50 feet for high hazard, 100 feet for extreme hazard). The requirement for modification is measured horizontally between crowns of the trees to any other combustible fuel or potential ignition source.
- A minimum distance of 10 feet to overhead electrical wires, chimney outlets, etc.
- Keep limbs pruned to a height 6-feet above grade
- Regularly remove dead limbs and litter
- Limit storage of firewood or other combustible material to 20-feet from the projected building perimeter and 15-feet from trees (measured horizontally to the crown of the tree).
- Use spark arresters for any fuel-burning appliance to minimize the risk of fire spread.
Ready Response
There are several possible responses to the detection of a fire. The most common method of extinguishing burning solids is to use water. This is because of its available supply, ease of application, low cost, and effectiveness in the cooling, smothering, and dilution of a fire. Thus, an available water supply with proximity to the structure becomes a worthy consideration.
Chapter 4 of the IWUIC describes appropriate man-made or natural water supply. The critical elements are the replenishment of the water supply and access to it. For one-and two-family dwellings, the minimum duration of supply is 30 minutes when pumped at a rate of 1,000 gallons per minute (less than 3600 sq. ft.) to 1,500 gallons per minute (over 3600 sq. ft.). Other occupancy types are to have an available water supply of 1,500 gallons per minute for up to two hours.
When the source of the fire is occupancy-related, it may involve other than solid fuels as the source of the problem. Electrical fires, for example, are best extinguished directly by carbon dioxide, dry chemical or halogenated agents or controlled after the power is shut off using water. Hazardous/flammable liquids and gases can also be extinguished directly with appropriate fire extinguishers. Automatic shutoff valves tied to detection systems are recommended.
Building Design and Material Selection
As previously mentioned, building codes call for early detection (smoke, heat rise and flame) and the presence of clear exit routes. Code-writing agencies have responded to the life-saving success of smoke detectors by requiring residences to have at least one direct-wired and one battery-powered smoke detector supplied per dwelling, located near the entrance to each bedroom or group of bedrooms.
Beyond the specific code requirements, there are important preventive steps that can be taken to reduce the risk of fire damage. Many of these recommendations were developed following the study of widespread fire damage in residential areas. Possibly the most significant of these studies was following the damaging fires in the hills surrounding Oakland, California in the fall of 1991.
Windows & Wall Openings
It is not practical to limit the amount or number of glazed openings in buildings, especially when there isn’t any apparent danger from neighboring structures. Yet, open windows allow structure fires to spread.
When internal gases explode during flashover, windows are commonly blown out allowing flame and combustible gases to exit the building and damage the exterior of the building. The size, number, and location of the windows can determine the extent of spread or damage. In fact, the window geometry can determine the nature of the flame plume that projects from the window as outside oxygen combines with the escaping gases. The flame plume tends to extend out horizontally when the window is tall and narrow but becomes vertical from short, wide windows.
The effective fire block can provide the moments needed for evacuation. Fire spread to upper floor windows can be limited by fire resistive construction above the lower window. This construction may be:
A balcony or eyebrow
A 36” high wall below the upper floor window
An interesting finding of residential fires shows that the thermal performance of the glazing in windows combined with the type of interior window treatments has a critical impact on the passage of fire through an opening. This is due to two factors: 1. The higher thermal performance of the glazing allows the opening to resist transfer of heat to the other side. 2. These windows usually have more than one layer of glazing, thus the glass exposed to the fire may be sacrificed while the opposite panel remains intact. Since the ability of the glass to resist breakage is significant for protection, tempered glass is also a good option.
The longest resistance to the high temperature of a fire is a closed window with high-performance (energy efficient) glazing and non-combustible window treatments. As the temperature on the non-fire side of the glazing increases, the distance of combustible material from the window becomes more critical. For better protection of openings, it is recommended that tempered or multiple glazing layers be used for exterior windows and skylights.
Overhangs & Gutters
The edge of the roof produces interesting issues in the study of fire behavior. First, the overhang is a focal point for fire resistive construction because it can trap flames, heat and smoke. Where fire hazard is moderate or higher, the IWUIC requires noncombustible materials for the construction of gutters and downspouts.
- For a Class A roof, two-inch thick (nominal) fascia boards are required to protect the edge of the roof assembly for 1-1/2 hour rated assembly, with a one-hour rated soffit construction.
- In Class B roof assemblies, the eave must be protected with a minimum of 3/4” thick material, and rafter tails (except for heavy timbers) cannot be exposed.
“For roof coverings where the profile allows a space between the roof covering and the roof decking, the space at the eave ends shall be fire stopped to preclude entry of flames or embers.”
Roof Coverings
Roof coverings are rated for fire resistive construction as Class A, B, or C. Materials are tested according to ASTM standard test methods (primarily E108) to measure how long they resist ignition, stay in position, and whether or not burning pieces can be broken off and blown away. In the National Roofing Contractors Association overview of the International Building Code, requirements for roof coverings are as follows:
Examples of typical Class C roofing are asphalt composition shingles, treated wood shingles, or cold-applied built-up roofing.
