Flammable vs. Combustible:
Understanding the Distinction in Fire Safety

In fire safety, distinguishing between flammable and combustible materials is critical. This knowledge informs proper storage, handling, and risk assessment, which impact safety, compliance, and insurance coverage. It also plays a key role in determining a property fire risk.

Defining Flammable and Combustible Materials

Flammable and combustible materials refer to substances that can ignite and burn. The key difference lies in their flash point—the temperature at which they release enough vapor to ignite.

• Flammable liquids have a flash point below 100°F (37.8°C) and include substances like gasoline and acetone.
• Combustible liquids have a flash point at or above 100°F and include diesel fuel and motor oil.

Note that regulatory agencies such as the NFPA and OSHA use these terms specifically for liquids. If discussing flammable solids or gases, different classifications apply.

Safety Implications and Risk Assessment

Improper storage of these materials can lead to devastating fires. According to the National Fire Protection Association (NFPA), more than 51,000 structure fires involving flammable or combustible materials are reported annually in the U.S. alone.

“Recognizing the fire risks associated with various materials is essential. It's not just about compliance; it's about safeguarding lives and property,” said Tammy Schwartz, Guidewire’s Senior Director of Analytics.

To reduce risk, it’s essential to implement clear safety protocols, including:

• Proper labeling and hazard communication
• Storage in fire-resistant, approved containers
• Keeping materials away from ignition sources

Storage requirements vary significantly between flammable and combustible liquids, as outlined in NFPA 30. Flammable substances, due to their low flash points, demand more stringent controls—explosion-proof containment, fire-rated storage cabinets, and greater clearance from ignition sources. Combustible liquids may be stored under slightly more flexible conditions but still require proper ventilation and defined separation from heat and flame.

When Improper Material Classification Leads to Disaster: The 2015 Rocky Fire

A dramatic example is the 2015 Rocky Fire in Northern California. The fire began in a small outbuilding packed with flammable liquids and powered by a malfunctioning gas water heater—part of an illegal cannabis operation. The result: over 69,000 acres burned, 96 structures destroyed, and 13,000 residents evacuated.

According to the National Interagency Fire Center, firefighting costs reached $46.1 million—$57.9 million in today’s dollars. Improper classification and storage turn manageable risks into large-scale environmental and financial disasters.

As seen in other cases like the 2018 Camp Fire where PG&E plead guilty to involuntary manslaughter, liability can extend to criminal charges. Regulatory Standards and Compliance

Flammable and combustible materials aren’t just everyday risks. They’re regulated by some of the most important fire safety standards in the country. The key to both preventing incidents and ensuring a property meets legal and insurance requirements begins with understanding these frameworks:

• NFPA 30 covers the classification, storage, and handling of flammable and combustible liquids.
• NFPA 1140 focuses on wildland fire protection, including defensible space around homes and using fire-resistant building materials.
• OSHA regulations mandate workplace safety measures related to hazardous materials.

Compliance not only prevents fire incidents but also influences resilience and insurance liability in the event of a loss.

Wildfire Zones and the Role of Flammable vs. Combustible Materials

In wildfire-prone areas, especially in the wildland-urban interface (WUI), the difference between flammable and combustible materials is amplified by environmental exposure.

Flammable items like gasoline or paint thinner, when stored outside or in unsealed sheds, can be ignited by wind-driven embers. Combustibles such as wood fencing or mulch, while slower to ignite, contribute to sustained fire spread. This layered threat explains why NFPA 1140 emphasizes defensible space and non-combustible building upgrades.

For example, in California:

• Vegetation must be cleared at least five feet from homes
• Firewood and other combustibles must be stored in containers that are ember-resistant and non-combustible
• Vents, roofs, and siding should be ember-resistant and non-combustible

These strategies reduce ignition risk and form the basis for defensible space requirements that are validated through expensive insurance inspections.

Insurance and Fire Risk Assessment for Flammable and Combustible Materials

Insurance companies carefully evaluate fire risk when underwriting policies. Key factors include:

• Whether flammable and combustible materials are correctly stored
• If local and national fire codes (NFPA, OSHA) are followed
• The presence of proactive risk mitigation efforts

“Insurance assessments are increasingly data-driven. Demonstrating proactive risk management can positively influence coverage options,” says Schwartz. Many insurers now rely on HazardHub risk data to assess site-specific exposure.

Leveraging Technology for Risk Mitigation

Fire prevention is evolving beyond traditional inspections. Today, insurers and property owners use data-backed technology tools like HazardHub to measure risk in real time.

• IoT Sensors and Smart Monitoring: Continuously track heat, gas, and vapor levels, sending alerts if danger is detected.
• Real-Time Alerts and Mobile Integration: Notify property owners and emergency services immediately when danger is imminent
• Wildfire Zone and Defensible Space Analysis: Especially in California and wildfire-prone areas, tools help verify compliance with NFPA 1140 defensible space standards.

Schwartz also reiterates that “Technology doesn’t replace fire prevention best practices. Instead, it enhances them through a continuous feedback loop,  identifying overlooked risks and providing essential data to inform risk mitigation strategies.”

By combining technology with proven safety practices, insurers and homeowners can shift from a repair and replace mentality to predict and prevent. Transitioning from reactive to proactive  reduces risk, protects property, and saves lives.

How to Strengthen Your Fire Risk Assessment Strategy

Knowing the difference between flammable and combustible liquids, as well as storing them safely, can reduce your risk profile and protect your home or business.

Following this checklist can help protect you and your property from preventable damage.

• Review NFPA and OSHA compliance
• Use non-combustible materials in wildfire zones
• Store hazardous materials in fire-rated, labeled containers
• Use risk scoring tools like HazardHub to assess your property

Take the Guidewire HazardHub Quiz today to evaluate your risk and learn how small changes can offer lasting protection.