Understanding and distinguishing between construction types is fundamental to building codes. Yet, how many times have you heard buildings being described as “masonry” or “concrete” or “steel” or “wood” construction without any of the descriptors used by building codes? All these terms are fine for laypeople to use to get a sense of what a building looks like. However, understanding the building classification and why it's identified in that way is crucial for budgetary reasons in new construction and adaptability in existing construction. The purpose of the building code is to identify the level of potential fire resistance inherent in the construction so that it can be evaluated consistently for the purposes of safe occupancy.
Construction types are fundamentally about knowing two terms and their opposites: combustible vs. noncombustible and protected vs. unprotected.
Every building must be classified as one of five regulated construction types, and each type of construction deals with, among other things, the allowance for combustible materials, or the requirement to use non-combustible materials. Because one of the primary aspects of safe building design is accounting for how a fire will affect the building, it only makes sense that the code regulates the use of combustible versus non-combustible elements in a building.
Combustible
Combustible means that the material can ignite and burn at a temperature exceeding 1,382°F (750°C) or if it fails the ASTM E136 combustibility test. Examples of combustible materials are wood, composition board and plastics. Combustible materials are very common in building construction. Contrary to some beliefs, it is impossible to make a combustible material non-combustible by applying some sort of after-market chemical, application or treatment. Fire treatment is not equivalent to fire resistance.
Noncombustible
Noncombustible means that the material will not ignite, burn, or release flammable vapors when exposed to fire or heat at a temperature of 1,382°F (750°C) or if it passes the ASTM E136 combustibility (pass/fail) test. Examples of non-combustible materials include steel, concrete, masonry, glass, and some insulating materials. For example, fiberglass itself is noncombustible, but the resins used can be combustible. Both rockwool and mineral wool insulation, however, are noncombustible.
Limited Combustible
There is also a term called limited combustible. To qualify, construction materials need to be able to produce a heat value less than 3,500 BTU/lb. when tested in accordance with NFPA 259 (Standard Test Method for Potential Heat of Building Materials). This term is exclusive to the NFPA construction types found in NFPA 220, Standard on Types of Building Construction, used in both NFPA 101 – Life Safety Code and NFPA 5000 Building Construction and Safety Code and not the ICC construction types used by most building departments.Protected & Unprotected Construction
Protected construction adds fire resistant materials to what is being assembled as a building. Materials used as components of protected construction include:
· Gypsum Board: Fire-rated drywall used on walls and ceilings.
· Plaster: Applied over lath to provide a dense thermal barrier.
· Spray-Applied Fireproofing: Cementitious or mineral fiber mixtures sprayed onto structural steel.
· Intumescent Coatings: Special paints that swell and insulate steel when heated.
Unprotected construction is the absence of such fire-resistant materials added to a structural element such as wood, steel, masonry, concrete, etc. for walls, floors, beams, columns and roofs.
Once these elements are known, and the duration of fire-resistance is verified by fire test or other means, a specific construction type can be attached to the building.
What happens when more than one construction type is associated with a building?
It is classified as mixed construction. The International Building Code (IBC) generally requires a single classification, but allows multiple types if separated by fire walls, or via specific assemblies like a concrete podium under wood frame construction, allowing a wood frame building to increase in additional height. Otherwise, the entire structure defaults to the stricter or more combustible rating, therefore further restricting its size and occupancy capabilities.Mixed Construction Safety Impacts
· Stricter Classification: The building often takes the more restrictive or less fire-resistant construction classification for safety compliance.
· Height and Area Limits: Allowable square footage and number of stories are restricted by the lower or more combustible type used in the mix.
· Fire Separation: Rated fire walls, horizontal assemblies, or specific hourly separation ratings are required between the different types to prevent fire spread.
Common Exception Examples
· Podium Buildings: A Type I concrete ground floor or parking garage with multiple Type V wood-frame residential stories built on top.
· Roof Exceptions: Non-combustible commercial structures featuring small sections of wooden roof decking that fall below allowable percentage thresholds.
· Renovations: Older buildings may be expanded or retrofitted with newer, differing structural materials over time without affecting the construction type classification.
If a building construction type can be decreased, can it be increased as well?
Theoretically, yes. In fact, this idea was the basis for the addition of Types IV-A, IV-B and IV-C mass timber / cross-laminated timber construction to the list of construction types in 2021.
· Type IV-A: Maximum 18 stories; requires complete noncombustible protection on all timber elements for maximum fire rating.
· Type IV-B: Maximum 12 stories; allows a limited amount of exposed interior mass timber surfaces.
· Type IV-C: Maximum 9 stories; permits the most exposed timber surface area while relying on inherent 2-hour timber fire resistance and protected concealed spaces.
Types IV-A, IV-B, and IV-C were added to the International Building Code (IBC) to safely permit tall mass timber and cross-laminated timber (CLT) buildings. Extensive fire testing and research by the Ad Hoc Committee on Tall Wood Buildings (AHC-TWB) proved that encapsulated mass timber maintains structural integrity and meets high-rise fire safety demands as well as create building structures of sustainable material.
The AHC-TWB determined fire testing was necessary to validate and verify that the performance level of passive fire protection intended by the IBC was retained. Large-scale fire tests were developed to simulate characteristics of the three new construction types using cross-laminated timber (CLT) and glued laminated timber.
Where can you find an example of a building using a mass timber construction type? Just ninety minutes from Chicago (where wood construction has been banned from downtown since 1872) you’ll find Ascent MKE, a mass timber hybrid high-rise apartment building in Milwaukee, Wisconsin, completed in 2022. The 284-foot, 25-story high-rise is the world's tallest mass timber structure, edging out Norway's Mjøstårnet. It features 259 luxury apartments, retail space, an elevated pool with operable window walls, and a sky-deck. A federal grant assisted with the testing needed to prove mass timber's ability to perform as well as traditional building materials like concrete and steel to meet U.S. building codes.
Of course, numerous fire protection measures are additionally taken to ensure construction safety and building code compliance, from incorporating fire-resistant building materials such as gypsum board (fire-rated drywall), fire-rated doors, fire-rated insulation, and sprinkler systems. However, the overall fire resistance of a building must always be identified correctly for its intended occupancy, whether the building is new or existing, to verify its code compliance and its future potential use.
Getting your construction type right from the start protects your budget and your timeline. Burnham's code consulting team confirms construction type classification early in design, before it becomes a costly correction.
Have a construction type question on your current project? Reach out to Burnham's code consulting team to talk it through.
