Pressure Treated Wood: Fire Resistant? The SHOCKING Truth!

Unveiling the Truth About Pressure Treated Wood and Fire

Wood structures, while aesthetically pleasing and environmentally sustainable, are not immune to the devastating effects of fire. In fact, according to the National Fire Protection Association (NFPA), wood and other solid materials were the first items ignited in an estimated 54,500 structure fires in 2021 alone. This underscores the critical importance of understanding the fire-related properties of building materials, including those marketed with seemingly protective qualities.

One such material is pressure treated wood, a staple in outdoor construction projects across the globe. From decks and fences to landscaping timbers, pressure treated wood is valued for its resistance to rot, insects, and fungal decay.

But a crucial question arises: Is pressure treated wood truly fire resistant?

The answer, unequivocally, is no.

Despite the common misconception, pressure treatment does not render wood fireproof or even significantly fire resistant. It primarily targets biological threats, not thermal ones. This article aims to debunk this widespread myth and explore the reality of pressure treated wood’s behavior in fire scenarios.

Article Scope and Objectives

We will delve into what pressure treated wood actually is, the science behind its treatment, and why it’s often mistaken for a fire-resistant material. We will examine the factors influencing its performance in a fire and navigate the relevant building codes and safety regulations.

Finally, we’ll consider real-world scenarios where relying solely on pressure treated wood can be dangerous and discuss alternative approaches to fire safety in wood construction.

What Exactly Is Pressure Treated Wood?

Pressure treated wood is a type of lumber that has been infused with chemical preservatives under high pressure to protect it from decay and insect infestation. This process significantly extends the lifespan of the wood, especially in outdoor applications where it is exposed to moisture and soil contact.

The Pressure Treatment Process Explained

The pressure treatment process typically involves placing the wood in a large, cylindrical steel retort. The retort is then sealed, and a vacuum is applied to remove air and moisture from the wood cells. Next, the treatment chemicals are pumped into the retort under high pressure, forcing them deep into the wood’s cellular structure. This ensures that the preservative is thoroughly distributed throughout the wood, providing long-lasting protection. Finally, the pressure is released, the excess solution is drained, and the wood is allowed to dry.

This process fundamentally alters the wood’s composition, making it far less appealing and accessible to organisms that would otherwise cause it to rot or be consumed.

Common Preservative Chemicals Used

Several types of chemicals are used in the pressure treatment process, each with its own strengths and weaknesses. Here are some of the most common:

• Borate: Borate-based preservatives are effective against a wide range of wood-destroying insects and fungi. They are often used in interior applications or in areas with moderate moisture exposure. Borates are known for their low toxicity to humans and animals.

• Copper Azole: Copper azole preservatives combine copper, a known fungicide, with azole compounds, which further enhance the wood’s resistance to fungal decay and insect attack.

  It’s a popular choice for decks, fences, and other outdoor structures.
  Copper azole is considered a more environmentally friendly alternative to some older preservatives.

• Alkaline Copper Quaternary (ACQ): ACQ preservatives use copper as the primary fungicide and quaternary ammonium compounds (quats) as insecticides. This combination offers broad-spectrum protection against decay and insect damage.

  Like copper azole, ACQ is widely used in residential and commercial construction.

  It’s favored for its effectiveness and relatively low environmental impact.

The Core Purpose: Protection Against Decay, Not Fire

It is crucial to understand that the primary purpose of pressure treatment is to protect wood from biological threats. The chemicals used are designed to prevent rot, fungal decay, and insect infestation. They are not formulated to provide significant fire resistance. While some treatments might slightly slow down the initial spread of flames, this is merely a secondary effect and not the intended purpose of the treatment. To reiterate: The focus is on longevity in the face of organic threats, not fire.

The Fire Resistance Myth: Separating Fact from Fiction

While pressure treated wood undergoes a chemical infusion process, it’s crucial to understand what these chemicals don’t do. The preservatives used, like borates and copper azole, are primarily designed to combat biological threats, not thermal ones. This distinction is where the fire resistance myth originates and needs to be dispelled.

Pressure Treated vs. Fire-Retardant-Treated Wood: Understanding the Critical Difference

The terms "pressure treated" and "fire-retardant-treated" (FRT) wood are often mistakenly conflated, leading to dangerous assumptions. Pressure treated wood resists rot and insects; FRT wood is specifically treated to slow down combustion.

FRT wood undergoes a similar pressure treatment process, but the chemicals used are fundamentally different. These chemicals are designed to react when exposed to heat, creating a char layer on the wood’s surface.

This char layer acts as an insulator, slowing the rate at which the wood heats up and delaying ignition. It also reduces the amount of flammable gases released during combustion.

Does Pressure Treatment Offer Any Limited Fire Protection?

While pressure treatment’s main purpose is not fire resistance, there might be a subtle, secondary effect on combustion. Some studies suggest that the chemical preservatives used in pressure treatment may slightly delay ignition or slow the initial spread of flames.

However, this effect is minimal and inconsistent. It is nowhere near the level of protection offered by fire-retardant treatments.

Relying on pressure treatment for any meaningful fire protection is a dangerous gamble. The amount of protection offered is too insignificant to provide a meaningful safety margin in a fire scenario.

Limitations: Pressure Treated Wood Will Still Burn

It’s essential to emphasize that pressure treated wood will burn. While it might take slightly longer to ignite under certain conditions, it is still combustible. Once ignited, it will contribute to the spread of fire just like untreated wood.

The preservatives present in pressure treated wood don’t fundamentally alter the wood’s flammability. They merely provide resistance against biological degradation.

Flame Spread and its Relevance to Structural Integrity

Flame spread is a crucial factor in determining a building material’s fire performance. It refers to how quickly flames propagate across the surface of a material.

A lower flame spread rating indicates that a material is less likely to contribute to the rapid spread of fire, giving occupants more time to escape and allowing firefighters to respond more effectively.

Pressure treated wood, without additional fire-retardant treatment, does not significantly improve flame spread characteristics. In a fire, flames can spread rapidly across the surface of pressure treated wood, compromising structural integrity and endangering lives.

While understanding the differences between pressure-treated and fire-retardant-treated wood is crucial, it’s equally important to recognize that even seemingly small factors can dramatically alter how pressure-treated wood behaves in a fire. These factors, often external to the wood itself, dictate the speed and intensity of combustion.

Factors Influencing Fire Performance of Pressure Treated Wood

The fire performance of any wood product, including pressure-treated varieties, isn’t a fixed property. It’s a dynamic process influenced by a complex interplay of external conditions. Understanding these conditions is key to appreciating the limitations of pressure treatment and ensuring proper fire safety measures.

The Decisive Role of Fire Intensity

The intensity of a fire is perhaps the most obvious, yet critical, factor. A small, contained flame will have a vastly different impact than a raging inferno. Higher intensity fires generate more heat.

This increased heat accelerates the pyrolysis process, breaking down the wood into flammable gases at a faster rate. Pressure-treated wood is still susceptible to this process. A sufficiently intense fire will overwhelm any marginal fire-delaying properties that the treatment might offer.

Even though, at low flame, the chemicals in pressure-treated wood might slightly delay ignition. But as soon as the fire gets very strong and the heat gets high, there won’t be any difference.

Oxygen: Fueling the Flames

Oxygen is a fundamental component of combustion. Without a sufficient supply of oxygen, a fire cannot sustain itself. The availability of oxygen directly impacts the rate at which wood burns.

In open-air environments, wood has ample access to oxygen, facilitating rapid and complete combustion. Conversely, limiting oxygen supply, through techniques like smothering or using fire-resistant coatings, can significantly slow or even extinguish a fire.

Poorly ventilated areas increase the risk of accumulating flammable gases, potentially leading to rapid fire spread or even explosions.

Ignition Point: Where Wood Meets Flame

The ignition point is the minimum temperature at which a substance will spontaneously ignite in the presence of oxygen. While pressure treatment may subtly affect the chemical composition of the wood, it does not drastically alter its ignition point.

Pressure-treated wood will still ignite at temperatures comparable to untreated wood. The primary difference, if any, might be a slight delay in reaching that ignition point under very specific low-intensity conditions.

However, once the ignition temperature is reached, the wood will burn, regardless of the pressure treatment.

Practical Fire Management near Pressure Treated Wood

Given the limited fire protection offered by pressure-treated wood, proactive fire management is paramount. Here are some basic precautions:

• Maintain clearance: Keep combustible materials away from pressure-treated structures like decks and fences. Clear away dry leaves, brush, and other potential fuel sources.

• Supervise open flames: Never leave open flames, such as grills or fire pits, unattended near pressure-treated wood structures.

• Use appropriate fire extinguishers: Ensure that you have readily accessible fire extinguishers rated for Class A fires (ordinary combustibles) and know how to use them.

• Regular inspection: Periodically inspect pressure-treated wood structures for signs of excessive dryness, cracking, or decay, as these conditions can increase their flammability.

Even with a grasp of how fire intensity and oxygen availability affect combustion, selecting the appropriate materials and construction methods for any project also demands an understanding of the regulatory landscape that governs fire safety. This is where building codes and fire safety regulations come into play.

Navigating Building Codes and Fire Safety Regulations

Building codes are a cornerstone of modern construction, designed to ensure the safety and well-being of occupants and the public. They establish minimum requirements for structural integrity, fire resistance, and other critical aspects of building design and construction.

Understanding the Code Landscape

These codes are not static documents; they are constantly evolving to reflect new technologies, materials, and research findings.

Wood construction is specifically addressed in building codes, recognizing both its inherent advantages and the potential fire risks. These regulations dictate requirements for fire-resistant assemblies, flame spread limitations, and the use of fire-rated materials in specific applications.

The Role of ASTM Standards

Among the most crucial resources referenced within building codes are the standards developed by ASTM International (formerly the American Society for Testing and Materials). These standards provide standardized testing methods and performance criteria for a wide range of materials and products.

ASTM E84: Surface Burning Characteristics

One particularly relevant standard is ASTM E84, "Standard Test Method for Surface Burning Characteristics of Building Materials." This test measures the flame spread and smoke development characteristics of materials when exposed to fire. The results are expressed as a flame spread index (FSI) and a smoke-developed index (SDI).

These indices are used to classify materials and determine their suitability for use in various building applications. Materials with higher FSI and SDI values are generally considered to pose a greater fire risk.

Pressure-treated wood, while not specifically designed to achieve a low FSI, may exhibit some marginal improvement compared to untreated wood due to the presence of chemical treatments. However, it’s crucial to remember that pressure treatment alone does not guarantee compliance with fire safety regulations.

Other Relevant ASTM Standards

Other ASTM standards relevant to fire safety in wood construction include those that address fire-resistant coatings, fire-stopping materials, and the fire performance of structural assemblies.

The International Building Code (IBC)

The International Building Code (IBC) is a widely adopted model building code that provides comprehensive regulations for various aspects of building design and construction, including fire safety.

The IBC references ASTM standards and provides specific requirements for fire-resistant construction based on occupancy type, building height, and other factors. It addresses issues such as:

• Fire-resistance ratings for walls, floors, and roofs
• Requirements for fire-rated doors and windows
• Sprinkler system requirements
• Means of egress

Compliance with the IBC is essential for obtaining building permits and ensuring the safety of occupants.

The Influence of Insurance Companies

Insurance companies also play a significant, albeit indirect, role in promoting fire-safe construction practices. Insurers have a vested interest in minimizing fire losses, and they often offer incentives or discounts for buildings that incorporate fire-resistant features.

These incentives can encourage builders and owners to go above and beyond the minimum requirements of building codes, implementing additional fire safety measures such as:

• Installing sprinkler systems
• Using fire-resistant materials
• Implementing fire detection and alarm systems

Furthermore, insurance companies may conduct their own risk assessments and require specific fire safety measures as a condition of coverage. This indirect influence further reinforces the importance of prioritizing fire safety in all construction projects.

Even with a grasp of how fire intensity and oxygen availability affect combustion, selecting the appropriate materials and construction methods for any project also demands an understanding of the regulatory landscape that governs fire safety. This is where building codes and fire safety regulations come into play.

Real-World Scenarios: When Pressure Treated Wood Isn’t Enough

While pressure treated wood offers invaluable protection against rot, insects, and fungal decay, it is crucial to recognize the limits of its fire performance. There are numerous real-world situations where relying solely on pressure treated wood for fire safety is simply insufficient, potentially leading to catastrophic consequences.

The Illusion of Fire Resistance

A common misconception is that because pressure treated wood resists decay, it inherently possesses significant fire resistance. This is simply untrue. While the chemicals used in pressure treatment may offer a marginal slowing of combustion, they do not render the wood non-combustible or adequately delay fire spread under real-world conditions.

Situations Demanding More Than Pressure Treatment

• Proximity to Ignition Sources: In areas where ignition sources are prevalent, such as around outdoor fire pits, grills, or smokers, pressure treated wood alone provides inadequate protection. Sparks, embers, or direct flames can easily ignite the wood, leading to a rapidly escalating fire.

• Structural Supports: Critical structural supports, particularly in larger buildings or those with high occupancy, require superior fire resistance than pressure treated wood can offer. The rapid failure of these supports in a fire can lead to collapse and significant loss of life.

• Confined Spaces: In confined spaces, such as decks close to buildings or fences along property lines, a fire involving pressure treated wood can quickly spread to adjacent structures.

• High Wind Areas: High winds accelerate the spread of fire, negating any limited fire-retardant benefit of pressure treatment. Embers can be carried long distances, igniting other combustible materials.

A Combined Approach to Fire Safety

The key to mitigating fire risks associated with wood construction lies in a combined approach. Pressure treated wood should be strategically used in conjunction with other fire-resistant materials and construction methods to create a more robust defense against fire.

Practical Applications and Material Combinations

• Decks: Consider using non-combustible decking materials, such as composite decking with a Class A fire rating, especially near potential ignition sources like grills or fire pits.

  Alternatively, install a fire-resistant underlayment beneath the deck surface, such as a mineral wool barrier, to protect the underlying joists and beams.

• Fences: When constructing fences near property lines or structures, integrate sections of non-combustible fencing materials, such as masonry or metal, to act as firebreaks.

  Maintain a clear zone around the fence, free of combustible vegetation and debris, to minimize the risk of fire spread.

• Structural Supports: Protect structural supports with fire-resistant cladding, such as gypsum board or spray-applied fire-resistive materials (SFRM).

  Consider using larger dimension lumber for structural members to increase their resistance to fire-induced charring and structural failure.

The Importance of Fire Blocking and Draft Stopping

In addition to material selection, employing effective fire blocking and draft stopping techniques is crucial. These methods restrict the movement of fire and smoke within concealed spaces, slowing its spread and providing valuable time for occupants to escape.

Properly installed fire blocking can prevent flames from rapidly spreading through wall cavities and floor joist bays. Draft stopping creates barriers in attics and crawl spaces to limit the horizontal movement of fire and smoke.

Remember, relying solely on pressure treated wood for fire safety is a dangerous gamble. A comprehensive approach, incorporating fire-resistant materials, proper construction techniques, and adherence to building codes, is essential to protecting lives and property.

So, after digging into all the details, hopefully you’ve got a clearer picture about whether or not is pressure treated wood fire resistant. It’s definitely not a simple yes or no answer, right? Stay safe out there, and thanks for reading!