Within sprinkler system design, the correct interpretation of hazard classifications is critical. NFPA 13 makes a clear distinction between storage and non-storage occupancies, and understanding this distinction is fundamental to achieving a safe, compliant, and cost-effective sprinkler design.
This article explains how hazard classifications for non-storage occupancies are determined under NFPA 13, why this process can be challenging in practice, and which common mistakes should be avoided.
Two fundamentally different protection concepts
Sprinkler regulations recognize two fundamentally different protection concepts: protection for storage and protection for non-storage. When referring to non-storage, the focus is on occupancy classification, essentially, how a space is used during normal operations.
Warehouses with extensive racking systems and stacked goods are considered storage occupancies, while offices, hospitals, retail spaces, and similar environments are typically classified as non-storage occupancies.
Why the distinction matters
This distinction is crucial. All non-storage occupancies are addressed in Chapter 19 of NFPA 13. Once a space meets the criteria for storage, the requirements of Chapter 19 no longer apply, and the sprinkler system must be designed in accordance with the applicable storage chapters instead.
When does a space become storage?
A non-storage occupancy may include spaces such as offices, hospitals, greengrocers, or post offices. Within NFPA standards, storage is defined primarily by the presence of stacked goods and the height of those stacks.
As a general guideline, storage heights exceeding approximately:
- 3.7 meters for goods without plastics, or
- 1.5 meters for goods containing plastics
are considered storage. These limits are indicative and depend on commodity classification and storage configuration, but they serve as a clear threshold: once exceeded, the space falls outside Chapter 19 and into the storage provisions.
Hazard classifications under NFPA 13
NFPA 13 defines three primary hazard classifications for non-storage occupancies:
- Light Hazard
- Ordinary Hazard
- Extra Hazard
Ordinary Hazard is subdivided into Group 1 and Group 2, and Extra Hazard is subdivided into Group 1 and Group 2.
Typical examples of Light Hazard occupancies include offices, hotel guest rooms, and patient areas in healthcare facilities. At the other end of the spectrum, production facilities, such as factories manufacturing plastic components and using significant quantities of hydraulic oils, are commonly classified as Extra Hazard Group 2.
Factors influencing classification
The hazard classification of a space is determined by evaluating several factors, including:
- the expected rate of fire growth, and
- the continuity of combustibles.
Continuity of combustibles refers to both the quantity and distribution of combustible materials within the space and their potential contribution to fire spread.
Basis in full-scale fire tests
The hazard classifications in NFPA 13 are not theoretical constructs; they are based on extensive full-scale fire testing. For example, an office environment may be recreated, a wastebasket fire initiated, and observations made regarding fire development, heat release, and sprinkler response. Based on the results of such tests, appropriate hazard classifications are assigned.
The office example in practice
An important consequence of this testing-based approach is that all offices are classified as Light Hazard occupancies. However, it is essential to consider what an “office” represents in practical terms.
In the United States, office layouts commonly include cubicles and a relatively high density of combustible furnishings and paper storage. In many other regions, including parts of Europe, offices may have a lower overall fire load.
Limits of project-specific interpretation
Despite these differences, spaces that function as offices are still classified as Light Hazard under NFPA 13. This illustrates an important principle: NFPA standards are developed around typical American practices and standardized test scenarios. A lower fire load in a specific project does not automatically justify a lower hazard classification.
Risk of underestimating the hazard
Determining the correct hazard classification is one of the most challenging aspects of sprinkler system design for non-storage occupancies. If a classification is underestimated, the resulting sprinkler system may be incapable of controlling or suppressing a fire under design conditions.
Overclassification as a common reaction
A common response to this uncertainty is to select a higher hazard classification “to be on the safe side.” While well-intentioned, this approach leads to inefficient and unnecessarily expensive designs.
As the hazard classification increases, so do:
- the required discharge density,
- the size of the fire pump,
- the hydraulic design area, and
- the volume of the water storage tank.
For example, an Extra Hazard classification may require a discharge density of approximately 16.3 liters per minute per square meter over a design area of 230 square meters, while a Light Hazard classification requires only about 4.1 liters per minute per square meter over 140 square meters. Selecting an unnecessarily high classification therefore has a direct and significant impact on system cost and infrastructure.
Common mistakes in practice
One of the most common errors in sprinkler system design is failing to subdivide a building into distinct occupancy types. NFPA examples frequently identify hospitals as Light Hazard occupancies, which can lead designers to apply a single Light Hazard classification to the entire building.
In reality, this classification typically applies only to patient rooms, corridors, and similar areas. Other spaces within the same building—such as parking garages, workshops, or loading areas—may fall under Ordinary Hazard Group 2.
A single building can therefore contain multiple hazard classifications, each of which must be correctly identified and incorporated into the overall sprinkler design.
Consequences of oversimplification
Treating a building as a single hazard classification oversimplifies the requirements and increases the risk of both underprotection and unnecessary overdesign.
Key takeaways for designers
Correctly determining hazard classifications for non-storage occupancies is one of the most critical, and complex, steps in sprinkler system design under NFPA 13. A clear understanding of the distinction between storage and non-storage occupancies, awareness of how hazard classifications are established through testing, and careful evaluation of each individual space within a building are essential.
Hazard classification should always be approached as a space-by-space engineering decision, rather than a building-wide assumption. By applying these principles, designers can achieve sprinkler systems that are both effective in fire control and efficient from a technical and economic standpoint.
