Noncombustible construction is the International Building Code’s term for buildings whose structural elements are made of materials that will not ignite, burn, or release flammable gases when exposed to fire. The IBC groups these buildings into two of its five construction types, Type I and Type II, and both require noncombustible materials throughout the structural frame, bearing walls, floors, and roof.1International Code Council. 2021 International Building Code – Chapter 6 Types of Construction What separates the two types, and the subtypes within each, is how long those elements must keep standing in a fire. That hourly rating drives nearly everything else: how tall the building can go, how large each floor can be, and what the structural materials have to do before they can be used.
Noncombustibility Is Not a Fire-Resistance Rating
These two ideas get mixed up constantly, and the distinction matters. A material is noncombustible if it does not burn. A fire-resistance rating, measured in hours, describes how long a full assembly holds up under a standardized fire exposure.
Steel is the clearest illustration. It is noncombustible by definition, because it does not ignite. But an unprotected steel beam loses roughly half its load-bearing capacity at around 1,000°F, a temperature a building fire reaches within minutes. So a steel column can be entirely noncombustible and still fail to meet a two-hour or three-hour rating without applied fireproofing. One test asks whether the material burns. The other asks how long the assembly survives. Both have to be satisfied.
Type I: Fire-Resistive Construction
Type I is the most stringent classification in the IBC and the reason skyscrapers, large hospitals, and major airport terminals exist in the forms they do. Every structural element must be noncombustible, and the hourly ratings in Table 601 are the highest in the code. Type I splits into two subtypes.1International Code Council. 2021 International Building Code – Chapter 6 Types of Construction
Type I-A
- Primary structural frame: 3 hours
- Bearing walls, exterior and interior: 3 hours
- Floor construction and secondary members: 2 hours
- Roof construction and secondary members: 1½ hours
Type I-B
- Primary structural frame: 2 hours
- Bearing walls, exterior and interior: 2 hours
- Floor construction and secondary members: 2 hours
- Roof construction and secondary members: 1 hour
The IBC allows a one-hour reduction for primary frame members and bearing walls when they support only a roof and no floor loads.1International Code Council. 2021 International Building Code – Chapter 6 Types of Construction
The payoff for meeting these ratings is scale. Type I buildings have no tabular height or area limits under the IBC. That is why virtually every high-rise above 75 feet is Type I construction; no other classification allows the dimensions.2International Code Council. 2024 International Building Code – Chapter 5 General Building Heights and Areas
Type II: Noncombustible Construction
Type II uses the same noncombustible materials as Type I but carries lighter fire-resistance demands. It is the common choice for mid-sized commercial projects where three-hour ratings are not justified by the building’s height or occupancy.
Type II-A
Every element in Table 601 carries a one-hour rating: primary structural frame, bearing walls, floors, and roof. Schools, mid-sized retail buildings, and offices that stay below Type I thresholds commonly use this subtype.
Type II-B
Every element carries a zero-hour rating.1International Code Council. 2021 International Building Code – Chapter 6 Types of Construction The materials themselves will not burn, but the assemblies carry no required rated period of structural integrity in a fire. Warehouses, strip malls, and single-story commercial buildings often use Type II-B because it keeps the noncombustible designation while cutting material and labor costs.
Height and Area Limits
Unlike Type I, Type II is capped by occupancy. Without sprinklers, Type II-A tops out at 65 feet and generally three to five stories depending on use; Type II-B tops out at 55 feet and generally two to four stories.3International Code Council. 2021 International Building Code – Chapter 5 General Building Heights and Areas Adding a code-compliant automatic sprinkler system throughout the building raises Type II-A to 85 feet and Type II-B to 75 feet. Trying to squeeze additional stories out of a Type II building without upgrading the sprinkler system or the construction type leads to permit denials.
What the Code Still Allows to Be Combustible
A frequent misunderstanding is that every component of a Type I or Type II building has to be noncombustible. It does not. IBC Section 603 lists specific items that may be combustible even in noncombustible construction, and knowing them prevents both over-specification and installation errors.4UpCodes. Section 603 Combustible Material in Types I and II Construction Among the significant allowances:
- Fire-retardant-treated wood in nonbearing partitions with a required rating of two hours or less, in nonbearing exterior walls where fire-resistance-rated construction is not required, and in roof framing and decking.
- Thermal and acoustical insulation with a flame spread index of 25 or less, except foam plastics, which fall under the separate rules of Chapter 26.
- Interior wall, ceiling, and floor finishes installed under their respective code sections.
- Doors, door frames, window frames, and decorative trim.
- Combustible roof coverings with an A, B, or C fire classification.
- Intumescent and sprayed fire-resistive coatings, which are themselves combustible but are expressly permitted when tested and installed per the code’s fire-resistance requirements.
The structural skeleton has to be noncombustible. The code has always expected that interior finishes, insulation, and certain framing elements will not be.
Materials That Qualify as Noncombustible
Materials that pass the noncombustibility test share one trait: they do not ignite, burn, or release flammable gases at temperatures that would destroy most organic materials. A few dominate commercial practice.
Structural Steel
Steel does not burn, so it is noncombustible. The catch is heat vulnerability. Because unprotected steel loses roughly half its load-bearing capacity at around 1,000°F, bare steel columns and beams rarely satisfy Table 601 ratings on their own. Intumescent coatings and sprayed fire-resistive materials are the two usual fireproofing options, and the code explicitly permits them under Section 603 even though they are themselves combustible.
Reinforced Concrete
Concrete is noncombustible, and its mass slows heat transfer through walls and floor slabs better than nearly any other structural material. A reinforced concrete assembly can often reach a two-hour or three-hour rating from slab thickness alone, without added fireproofing. The concrete cover over embedded rebar is what protects the steel from reaching temperatures that degrade its strength, which is why the code specifies minimum cover depths for different ratings.
Masonry
Brick, concrete block, and similar masonry units are manufactured at temperatures far above anything a building fire produces, so they do not decompose or release flammable gases under fire exposure. Masonry walls perform well in rated assemblies and are common in bearing-wall construction for schools, warehouses, and low- to mid-rise commercial projects.
Gypsum Board
Standard gypsum board contains about 21 percent chemically combined water by weight. That water has to be driven off before the board can degrade in a fire, which buys time. For rated assemblies, Type X and Type C gypsum boards are specified rather than standard panels. Type X is the baseline for most fire-rated walls; Type C adds reinforcing fibers and performs better, and is often required for floor and ceiling assemblies because a single layer can achieve ratings that would otherwise need multiple layers.
Glass
Glass in noncombustible assemblies must meet tempered or wired standards to resist shattering from thermal shock. Fire-rated glazing products are tested to maintain integrity for a rated period, and some are designed to block radiant heat as well. Standard window glass is the weak point in an otherwise noncombustible wall, which is why the code limits the size and placement of glazed openings in rated walls.
How a Material Earns the Label
Whether a material counts as noncombustible is not a judgment call. Under the IBC, qualifying materials are tested under ASTM E136, or under ASTM E2652 using the same acceptance criteria.5International Code Council. 2021 International Building Code – Chapter 7 Fire and Smoke Protection Features One practical exception: a noncombustible structural base may carry a surface layer up to 1/8 inch thick, provided that layer has a flame spread index no greater than 50.
ASTM E136
The test places four identical specimens of a material into a vertical tube furnace set at 750°C (1,382°F) for at least 30 minutes. To pass, at least three of the four specimens must meet all of the following:6ASTM International. ASTM E136-22 Standard Test Method for Assessing Combustibility of Materials Using a Vertical Tube Furnace at 750 Degrees C
- Temperature rise: Neither the surface nor the internal temperature of the specimen may exceed 30°C above the furnace temperature.
- Weight loss: The specimen may not lose more than 50 percent of its original weight. If it does, the temperature criterion tightens: the specimen’s temperature may not exceed the furnace temperature at all.
- Flaming: No flaming from the specimen after the first 30 seconds. If weight loss exceeds 50 percent, no flaming is permitted at any point.
These criteria are deliberately strict. A material that chars, smolders, or sheds mass at this rate contributes thermal energy to a fire even if it never shows a visible flame. Results from an E136 submission are treated as definitive by building officials during permit review.
ASTM E119
ASTM E119 answers a different question. Where E136 asks whether a material burns, E119 asks how long an assembled wall, floor, column, or roof survives a standardized fire exposure. The test measures heat transmission through the assembly, passage of hot gases, and whether load-bearing elements maintain structural capacity under load.7ASTM International. ASTM E119-20 Standard Test Methods for Fire Tests of Building Construction and Materials The hourly ratings in Table 601 come from E119. A two-hour wall assembly held all three benchmarks for two hours under test conditions.
Both tests have to be satisfied to use a material and its assembly in Type I or Type II construction. E136 determines whether the material is eligible at all. E119 determines whether the finished assembly carries the hours Table 601 requires for that element and construction type.