Steel vs. Wood: How High Heat Changes Your Home’s Structural Integrity

The fire is out, but the battle for your home’s structural integrity is just beginning. Beyond the visible charring and smoke damage, an invisible war has been waged on the very bones of your house: its structural framework of wood and steel.

As structural engineers, we often see a critical misunderstanding: that if a beam isn’t burnt to a crisp, it must be fine. The truth is, high heat fundamentally alters the properties of both wood and steel, often compromising their load-bearing capacity in ways that aren’t immediately obvious.

Understanding these changes is crucial for ensuring a safe and lasting repair.

When Wood Fails: More Than Just Visible Char

Wood is a common and reliable building material, but it has specific weaknesses when exposed to fire. It’s not just about what you can see; it’s about what happens inside the wood.

• Charring is a Shield (to a point): When wood burns, it forms an insulating layer of char. This char can actually protect the inner, unburnt wood from immediate degradation. However, this protection comes at a cost – the outer layers are permanently lost.

• Loss of Cross-Sectional Area: Every millimeter of char reduces the effective size of the wood member (like a beam or stud). If a 2×10 floor joist is charred by an inch on its bottom, it essentially becomes a 2×8 in terms of strength, drastically reducing its ability to support weight.

• Engineered Wood Products (EWPs): Materials like I-joists and glulam beams rely on adhesives. High heat can melt these glues, causing the layers to delaminate or the “web” of the I-joist to separate from the “flanges,” leading to catastrophic failure even without significant charring. This often happens at temperatures far below the ignition point of solid timber.

• Hidden Damage & Drying Out: Beyond visible char, sustained heat can “bake” the moisture out of wood, making it brittle and prone to cracking or splitting later. Fire suppression efforts can also introduce water damage, leading to rot or mold if not properly dried and assessed.

When Steel Stretches: Creep, Warping, and Yield Strength

Many assume steel is impervious to fire, but intense heat is steel’s silent enemy. While it won’t burn, it will deform and lose strength.

• Reduced Yield Strength: Steel’s strength comes from its “yield strength”—the point at which it begins to deform permanently. As steel heats up, its yield strength dramatically decreases. At around 1,100°F (593°C), steel loses about 50% of its strength. At 1,800°F (982°C), it’s only about 10% as strong as it was cold.

• Warping and “Creep”: Unlike wood, steel will visibly warp or sag under its own weight or applied loads when hot. This permanent deformation is called “creep.” A warped beam is a structurally compromised beam; it can’t be simply “straightened” without a full engineering assessment and often replacement.

• Connection Failure: The bolted or welded connections that hold steel members together are also vulnerable. Heat can weaken welds or cause bolts to stretch and fail, leading to localized collapse even if the main beams appear intact.

• Concrete Spalling: Steel is often encased in concrete (e.g., in foundations or columns). Extreme heat can cause the outer layers of concrete to “spall” (flake or burst off), exposing the reinforcing steel (rebar) to direct heat, which then weakens.

The Critical Need for a Professional Structural Assessment

Whether your home is primarily wood-framed, steel-reinforced, or a combination of both, a post-fire inspection by a qualified structural engineer is non-negotiable. We analyze:

• Temperature Exposure: Estimating the heat intensity and duration.

• Material Degradation: Quantifying the loss of strength and integrity in affected members.

• Load Path Analysis: Determining if critical load paths have been compromised, potentially leading to progressive collapse.

• Code Compliance: Ensuring all proposed repairs meet current building codes and safety standards.

Don’t guess when it comes to the safety of your home. The unseen damage from high heat can have devastating long-term consequences.