A Comprehensive Guide to Choosing Purlin Sizes

What Is a Metal Building Purlin?

Purlins are one of the most fundamental components of any building project, no matter the scale. A purlin is a horizontal structural beam that supports the roof decking in a building, spanning between your main structural frames and running perpendicular to your rafters or beams. Their primary purpose is to transfer and redistribute roof loads to the main supporting elements of your structure.

Understanding Purlin Materials: Steel vs. Wood

While steel purlins are generally more common than their wooden counterparts, particularly for commercial and industrial applications, knowing the difference between the two can be useful depending on the scope of your project. 

Steel Purlins (Recommended for Metal Buildings)

Metal building purlins are typically cold-formed from galvanized steel, and are often coated with zinc for added protection. Steel purlins are the standard choice for metal building applications and offer a few notable advantages:

• Superior strength: Steel purlins offer a higher load-carrying capacity than wood.
• Weather resistance: Galvanized coating provides excellent rust protection and can endure high humidity and temperature fluctuations without becoming distorted.
• Dimensional stability: Steel purlins don’t warp, twist, or shrink like wood over time and exposure to the elements.
• Fire resistance: Their non-combustible material improves building fire safety.
• Longevity: Steel purlins are designed to last the lifespan of your building.
• Precision manufacturing: Their consistent dimensions ensure proper fit without natural fluctuations or flaws in organic grain. 

The durable properties of steel purlins definitively complement the superior strength and protection that metal roofing provides. 

Wood Purlins

Wood purlins are typically used with cement sheeting applications. Their strength lies in their capacity to ensure the room beneath remains breathable, which is particularly useful for storing organic material (such as grain or livestock). However, they do pose significant limitations when compared to steel, including:

• Moisture vulnerability: Wood purlins can rot when exposed to humidity.
• Dimensional instability: They pose the risk of warping or shrinking over time.
• Maintenance requirements: Wood purlins, as opposed to metal purlins, need regular inspection and replacement.
• Limited strength: Wood cannot match steel’s load-carrying capacity.

In general, for metal building applications, steel purlins are almost always the better choice, especially in our humid Southern climate.

Types of Metal Building Purlins

Metal building purlins are manufactured in two main varieties: 

Z‑Purlins

Z-purlins are aptly named and easily identifiable because they are shaped like a “Z,” ideal for overlapping at supports. This type of purlin is able to support a superior load capacity and is thus more ideal for longer spans and heavy-duty applications. Their complex cross-sectional shape and their continuous span capability with overlapping connections makes them especially useful in commercial and industrial settings. 

C‑Purlins 

C-purlins are the most common type of metal building purlin, shaped like a “C” with parallel flanges. These purlins for metal roofs are commonly used for roof and wall support, and floor joists. Their advantages include fast and easy installation procedures (with no side drilling), excellent straightness, and the ease at which bridging can be added for lateral stability. Their cost effectiveness and simple connection makes them perfect for single-span roof systems, residential, and light commercial projects. 

Purlin Sizes & Span Guide

The most common purlin sizes and their typical spanning capabilities include the following:

C Purlin Sizes

• C4 (4-inch depth): Spans up to 12 feet – Light residential applications
• C6 (6-inch depth): Spans up to 18 feet – Standard residential and light commercial applications
• C8 (8-inch depth): Spans up to 25 feet – Medium commercial applications
• C10 (10-inch depth): Spans up to 30 feet – Heavy-duty commercial applications
• C12 (12-inch depth): Spans up to 35+ feet – Industrial applications

Z Purlin Sizes

• Z6 (6-inch depth): Continuous spans up to 30 feet
• Z8 (8-inch depth): Continuous spans up to 40 feet
• Z10 (10-inch depth): Continuous spans up to 50 feet
• Z12 (12-inch depth): Continuous spans up to 60+ feet

Always remember that actual spanning capability depends on variables such as loading conditions, purlin spacing, gauge thickness, and local building code requirements. Always consult with a structural engineer before finalizing your design.

Purlin Installation Systems

The installation method, called the “run system,” significantly impacts purlin size requirements. Review the different types of span systems below to determine which makes the most sense to implement on your next project:

1. Single Span System

One purlin spans the entire length between supports. This simple system works well for smaller buildings but limits span capabilities.

2. Double Purlin Span System

Two purlins are supported at both ends and at a center support point. This allows for longer overall spans while making use of smaller purlin sizes.

3. Double Lapped System

Two purlins overlap slightly at the center support to provide continuous load transfer. This system offers improved structural performance over simple spans.

4. Continuous Lapped System

Purlins are supported at multiple points with overlapping connections, creating a continuous structural system. This is considered the most efficient system for long-span applications.

Key Factors Affecting Purlin Size Selection

Building Span and Frame Spacing

The distance your purlins must span between structural frames directly determines size requirements. Longer spans require deeper purlins to control deflection and maintain the structural integrity of the building. 

Load Requirements

Your project specs will determine any design load requirements that will need to be supported by your metal building purlin selection including:

• Dead loads: Consider the weight of any roofing, insulation, and equipment that your purlins will need to buttress. 
• Live loads: These loads include maintenance access and any temporary storage you’ll need atop your build.
• Wind loads: Factoring in wind loads is absolutely critical in hurricane-prone regions such as Louisiana, Mississippi, Alabama and Texas.
• Snow loads: Generally minimal in the Gulf Coast service area, but notable exceptions have occurred, as was the case in early 2025. 

Note that all purlin sizing must comply with local building codes and wind load requirements, particularly important in Louisiana and Mississippi’s hurricane-prone regions.

Purlin Spacing

Standard purlin spacing is typically 4 to 6 feet on center. The spacing you choose affects both purlin size and roofing panel requirements:

• 4-foot spacing: Allows smaller purlins but increases material count
• 5-foot spacing: Balanced approach for most applications
• 6-foot spacing: Requires larger purlins but reduces installation time

Frequently Asked Questions About Metal Building Purlins

1. What’s the difference between purlins and girts?

Purlins support the roof system and run horizontally between rafters or trusses. Girts support the wall system and run horizontally between wall columns. While similar in construction, they serve different structural functions and have different load requirements.

2. How far apart should purlins be spaced?

Standard purlin spacing ranges from 4 to 6 feet on center, with 5 feet being most common. The exact spacing depends on your roofing material requirements, load conditions, and cost considerations. Closer spacing allows smaller purlins but increases material costs. When budgeting for your next project, ADCO Metals offers free estimates to help keep your financials on track with zero upfront financial commitment. 

3. Do I need an engineer to size my purlins?

For most standard applications, experienced suppliers can provide valuable guidance in working through proper purlin sizes. However, custom applications, heavy loads, large spans, or local code requirements may require professional engineering. When in doubt, always remember that consulting a structural engineer will help you uphold safety and code compliance.

4. Can I mix different purlin sizes on the same project?

Yes, it’s common practice to use different purlin sizes on the same building based on varying load conditions. For example, you might use larger purlins at the eaves where wind uplift forces are highest and smaller purlins toward the ridge where forces naturally lessen.

5. What gauge steel is right for my metal building purlins?

Common purlin gauges include 14, 16, and 18 gauge steel. Heavier gauges (lower numbers) provide more strength but cost more accordingly. Your supplier will recommend the appropriate gauge based on your span, spacing, and load requirements.

6. How do I prevent purlin corrosion?

Choose galvanized purlins with appropriate coating thickness for your environment and ensure proper purlin installation to avoid damaging protective coatings during the construction phase. In coastal areas with high levels of humidity and moisture year-round, consider upgrading to a zinc-based coating for enhanced corrosion protection.

7. Can purlins support the weight of equipment such as HVAC units?

Standard purlins are designed for distributed loads, not concentrated point loads from equipment. HVAC units typically require additional structural framing or reinforcement. Always consult with an engineer before mounting heavy equipment to purlins.

8. How do I know if my existing purlins need replacement?

Signs that indicate it’s time for a purlin system replacement include any visible sagging, corrosion, or damage. You’ll also need to replace your purlins if you’re opting to use heavier roofing materials on a new roof for your structure. Have a structural professional evaluate any existing purlins before major roof renovations.

9. What happens if I choose the wrong purlin size?

Undersized purlins for metal roofs can lead to excessive deflection, code violations, and in extreme cases, structural failure. Oversized purlins waste money and may create problematic connection issues. Professional sizing ensures optimal performance and cost-effectiveness; ADCO Metals’ experts are here to help ensure that your purlin sizes are perfect for whatever project you have in mind. 

ADCO Metals: Your Local Experts in Purlins for Metal Roofs 

Knowing what purlins for metal buildings you require for your next commercial, agricultural, or residential project is the first step in creating a structure that lasts for years to come. Because understanding purlin materials, sizing, spacing, and building requirements can often be a challenge, the teams at ADCO Metals are here to support you each and every step of the way. 

With nearly six decades of experience serving the communities of the Gulf Coast in LA, MS, AL, TX, and beyond, ADCO Metals provides comprehensive purlin solutions with expert guidance, quality materials, custom fabrication, and complete end-to-end installation support. 

Want to learn more? Our FAQs are available on our website! Get in touch by filling out our contact form for a free estimate, or by calling (985) 882-3222 or (601) 798-4600 to speak with our team directly.