Understanding Deflection in Metal Building Roof and Wall Systems

When we think about structural performance in metal buildings, we often focus on strength: can it hold the load? But there’s another critical factor—deflection. Deflection refers to how much a structural component bends or sags under weight or stress. And while it might not cause immediate failure, excessive deflection can lead to water leaks, cracked walls, misaligned doors, and even uncomfortable vibrations.
 

In this article, we’ll break down:

• What deflection is
• Why it matters in roof and wall systems
• How much is too much
• And how Butler’s systems are engineered to keep it under control

What Is Deflection?

Deflection is the movement or bending of a structural element—like a roof purlin or wall girt—under load. Think of it like a diving board: when you stand on it, it bends. That’s deflection.
 

In metal buildings, deflection is often measured as a fraction of the span length, written as something like L/240 or L/360.
Here’s what that means:

• "L" stands for the length of the span (usually in inches or feet).
• The number after the slash is the deflection limit—how much the member is allowed to bend under load.

For example:

• L/240 means the maximum allowable deflection is the span divided by 240.
  So, if a purlin spans 20 feet (240 inches), its deflection must not exceed 1 inch (240 ÷ 240).
• L/360 is stricter—the same 20-foot span could only deflect up to 0.67 inches.

These limits are set to ensure that the structure doesn’t sag, bow, or create problems like leaks, cracked finishes, or stuck doors and windows.

Roof purlins often use limits like L/180 or L/240, while
Wall girts may use L/240 to L/360 to avoid visible deformation or damage to cladding.

Why Does Deflection Matter?

Even small amounts of deflection can cause big problems:

• Roof Panels: Too much sag can stretch seams or compromise standing seams, leading to leaks.
• Wall Panels: Deflection can bow the panels outward or inward, pulling trim out of alignment or stressing fasteners.
• Insulation Systems: Sagging can crush insulation and break vapor barriers, reducing energy performance.
• Interior Finishes: Misaligned girts can crack drywall or shift mounted equipment like lights or sprinklers.

In the case of roof systems like MR-24®, excessive deflection can disrupt the panel’s double-lock seam and floating clip system, which are both critical to weather performance.

What Are the Industry Standards for Deflection?

According to the MBMA Roof Framing Guide and structural design authorities, the most common deflection limits are:

✅ Note: “L” stands for the length of the span; “H” is height. So, if your wall height is 20 feet, an H/400 limit means no more than 0.6" of lateral drift during wind events.

Some environments—like offices or high-end retail—may demand even stricter tolerances due to aesthetic or comfort needs.

How Butler Systems Help Control Deflection

Butler’s roof and wall systems are engineered from the start to manage deflection—and maintain performance over decades.

✅ Factory-Punched Purlins & Girts

These eliminate guesswork and ensure that all structural members align perfectly, avoiding twist or stress concentrations that worsen deflection.

✅ MR-24® Roof Panels

The MR-24 system uses a 360° Pittsburgh double-lock seam and floating clips that allow up to 2.5 inches of thermal movement. This reduces the buildup of stress caused by deflection during seasonal temperature swings.

✅ Cross-Fluted Panels

Some Butler panels (like MR-24) use cross-fluting, which increases rigidity and resists uplift, reducing mid-span sag.

✅ ThermaLiner™ & TBS™ Systems

These insulation systems are installed above the purlin—keeping the vapor barrier intact and preventing deformation caused by internal building pressure or heavy insulation compression.

How to Design Against Deflection

1. Know Your Loads
   Use proper live, snow, and wind loads. Don’t rely on minimal design assumptions—especially if your building is in a snow-prone or high-wind zone.
2. Use Pre-Engineered Framing
   Butler’s Widespan™ and Landmark™ 2000 frames are engineered to keep drift and sag within safe, functional limits—even across large spans.
3. Don’t Skip Stiffness Checks
   Use systems with cross bracing, flange braces, or continuous bridging. Deflection doesn’t just affect one panel—it affects the entire system alignment.
4. Verify with Testing
   Butler components are tested using real-world simulations like the Guarded Hot Box for thermal integrity and structural behavior.

Final Takeaway

Deflection might seem like a small issue—but in metal buildings, it’s important. If left unchecked, it can cause leaks, poor energy performance, broken finishes, and even long-term structural damage.

The good news? Systems from Butler are built to keep it in check.

If you're replacing panels, girts, or purlins—or re-roofing entirely—contact us at Butler MFG Parts. We’ll help you get the exact components you need to ensure your building stays straight, dry, and efficient for decades to come.