Combustible, Non-combustible or Limited Combustible Materials…That is the Question

By Jon Nisja

Fire codes, building codes, and fire protection standards (such as NFPA 13) use words and terminology that often sound like the same thing. For instance, terms such as noncombustible, fire resistant, fire retardant, flame retardant, and fireproof sound like they mean pretty much the same thing — but they don’t. The same goes for combustible materials.

Fire protection contractors and authorities having jurisdiction (AHJs) often read these terms but don’t always understand the differences. Some of these terms apply to structural materials while others only apply to interior furnishings, decorations, or trim. Here the terms will be identified, and their differences will be discussed.

The “Combustibles”

There are three terms used to describe the combustibility of materials used for building construction, furnishings, finishes, decorations, and furniture: combustible, noncombustible, and limited combustible.

• Combustible means that the material can ignite and burn. Examples of combustible materials are wood, paper, plastics, fabrics, etc. Combustible materials are very common in building construction and also for furnishings, trim, decorations, and furniture. Layperson’s explanation: the material burns. Contrary to some claims, it is impossible to make a combustible material non-combustible by applying some sort of after-market chemical, application, treatment, or other pixie dust. (NFPA 1, 2021 edition, Section 3.3.59).
• Noncombustible means that the material will not ignite, burn, or release flammable vapors when exposed to fire or heat. Examples of non-combustible materials include steel, concrete, masonry, glass, and some insulating materials (fiberglass itself is noncombustible but the resins used can be combustible, rockwool insulation is noncombustible). Layperson’s explanation: the material doesn’t burn. (NFPA 13, 2022 edition, Section 4.9.1).
• Limited combustible means that the material is combustible, but it has a limited heat release rate when it burns (3,500 Btu/lb. – less than half of the heat released from most burning wood products). While gypsum wallboard is considered by most building codes to be noncombustible, it does have a thick paper backing that is combustible. While a few other products are technically limited combustible (some paper-backed insulation materials), the term is generally intended to apply to gypsum wallboard. NFPA 13 often allows sprinkler protection to be reduced or exempted in spaces having noncombustible or limited combustible construction (see NFPA 13, 2022 edition, Section 9.2.1 for concealed spaces). Layperson’s explanation: the material burns but not very well. (NFPA 13, 2022 edition, Section 4.9.2).

Terms Related to Building Construction

The following terms relate to the building or structure itself and the materials used to construct the building:

• Fire resistant or fire resistance-rated refers to the fire ratings of the building’s floors, wall, and ceilings. Fire resistant or fire resistance-rated walls are intended to contain a fire inside that compartment and prevent it from spreading for a period of time (expressed in minutes or hours). Examples would include a two-hour fire resistance-rated wall or a 20-minute fire-rated door. Fire sprinkler protection often allows the fire resistance rating to be reduced.
• Fire retardant or fire retardant treated (FRT) refers to chemicals, coatings, and treatments used to make combustible building materials resistant to charring and decomposition when exposed to fire. Examples include “fire retardant plywood” or “fire retardant-treated lumber.” Fire retardant-treated lumber can only be accomplished in a factory setting; there are no after-market products that can give lumber a “fire retardant-treated” listing. The addition of fire-retardant treatments does not make an item noncombustible but may reduce the need for fire sprinklers in certain concealed spaces. (NFPA 13, 2022 edition, Section 9.2.1.13).
• Flame spread rating (or flame spread index) refers to how fire spreads across the surface of a material. It is used to provide a Class A, B, or C flame spread rating on materials used on walls or ceilings. Class A flame spread ratings are the best. There are chemicals that can be applied that will reduce the flame spread rating of a material. Most non-combustible materials have a Class A flame spread rating allowing them to be used for walls and ceilings in a building. Fire sprinkler protection often allows the flame spread rating to be reduced by one class.
• Fireproof is an old, outdated term that was intended to denote that something would not burn. Unfortunately, history has shown us that many “fireproof” buildings burned so this term has fallen out of favor. The building construction materials themselves are rarely the items that are first ignited; it is the building’s contents that are initially ignited in most fires. The building’s contents are almost always combustible, which means they are capable of burning.

Terms Related to Decorations, Furnishings and Trim

• Flame resistant, fire retardant, flame retardant or ignition resistant means to apply chemicals or treatments that make combustible decorations, textiles and films resistant to charring and decomposition. These treatments are commonly used for clothing, curtains, drapes, upholstered furniture and fabrics. For synthetic materials (such as plastics, foams and certain clothing and textile products) the flame resistance is added during the manufacturing process (often using a halogenated element such as Chlorine, Fluorine or Bromine). For natural materials, such as wood, paper, cotton or wool, chemicals can be applied in the field that will increase their resistance to flames and ignition. Caution must be exercised since many fabric materials are now synthetic based (Nylon, Rayon, Polyester, etc.) or are a blend of natural and synthetic materials. These after-market flame resistance products will not work when applied to synthetic materials as they work through absorption into the material and many synthetics are not capable of absorbing liquid materials. The fire code often allows larger amounts of combustible decorations when fire sprinklers are present.

The fire protection codes and standards use terminology that often sounds similar and can easily be confused with analogous terms. Sprinkler designers, contractors, and AHJs need to be aware of the differences in these terms. Some of the terminology applies to the materials used to construct a building while other terms only apply to the furnishings and decorations in the building.

In some cases, sprinkler protection can be waived or modified where noncombustible or limited combustible construction is present. People often think that fire retardant-treatments will make a material noncombustible or limited combustible; they won’t. These are two completely different things.

In other cases (especially in combustible concealed spaces) NFPA 13 allows the omission of sprinklers where limited combustible materials are present or fire retardant-treated lumber is used. The proper materials should be used when exempting sprinkler protection in a space.

DO YOU HAVE A QUESTION ABOUT Combustible Materials? THE NFSA CAN HELP YOU.

The NFSA’s Expert of the Day program is a vital resource for professionals seeking further knowledge about the fire sprinkler industry. This service provides comprehensive answers to any codes and standards inquiries, making it an invaluable tool for enhancing your understanding in this technical field.  NFSA members have unlimited access to the association’s Expert of the Day service.

This service provides members with a codes interpretation in less than one business day! For more information on our Expert of the Day service, or to become an NFSA member, visit our membership page today!