What size purlins for metal roofing?

Common wood purlin sizes are 2×4 and 2×6 for residential. Steel purlins are typically 16-gauge C-channel, 1.5″ to 4″ depth. The size depends on the span between trusses/rafters and the roof load.

What is the standard purlin spacing for metal roofing?

Most standing-seam and corrugated metal roofing panels are rated for 24″ OC purlin spacing. Some lighter panels require 16″ OC. Always check the panel manufacturer's installation guide.

Do I need purlins over plywood sheathing?

Not always. If the roof has solid sheathing (plywood or OSB), purlins are usually unnecessary for asphalt shingles. Purlins are mainly needed for metal and tile roofing on open-frame structures without solid decking.

Can I install purlins on existing rafters?

Yes. In re-roofing with metal over existing shingles, purlins are often nailed over the old roof to create a flat nailing surface and air gap. This is called a purlin overlay or furring strip installation.

How do I handle purlin splices?

Splice purlins at rafter/truss locations with at least 6 inches of overlap, or butt them with a scab plate. Each piece must bear on the supporting rafter for load transfer.

What is the difference between purlins and girts?

Purlins are horizontal roof members spanning between rafters/trusses. Girts are horizontal wall members spanning between columns. Both serve as attachment points for exterior cladding.

Purlin Spacing Calculator

Calculate the number of purlins and total linear feet for metal roofing or tile installations. Supports various purlin spacings.

Rafter Length (slope)

Purlin Spacing

Building Length

Stock Length

Purlin Rows

eave to ridge

Total Linear Feet

360

all rows combined

Pieces to Buy

16-ft stock

Planning notes, formulas, and examples

About the Purlin Spacing Calculator

Purlins are horizontal members that span between rafters or trusses to support roofing materials, particularly metal panels and tiles. Proper purlin spacing depends on the roofing material, wind loads, and local codes. Metal roofing typically requires purlins at 24″ on center, while some tile systems may need tighter spacing.

This purlin spacing calculator computes the number of purlin rows needed along the length of the rafter (from eave to ridge) and the total linear feet of purlin material based on the building length. It helps you create an accurate material list for purchasing and scheduling.

Purlins may be wood (2×4, 2×6) or light-gauge steel (C or Z channel). The calculator works for any material—just enter the rafter length and desired spacing to get your counts.

When This Page Helps

Accurate purlin counts prevent material shortages and delays during roofing installation. This calculator gives you both the number of purlin rows and the total linear feet needed for the entire roof.

How to Use the Inputs

Enter the rafter length (eave to ridge, on the slope).
Enter the desired purlin spacing in inches.
Enter the building length (perpendicular to rafters).
Read the number of purlin rows and total linear feet of material.
Adjust spacing if needed for your specific roofing product.

Formula used

Purlin Rows = floor(Rafter Length × 12 / Spacing) + 1
Total LF = Purlin Rows × Building Length
Total pieces = Total LF / Purlin stock length

Example Calculation

Result: 9 purlin rows, 360 LF of material

A 16-ft rafter length at 24″ OC needs floor(16×12/24)+1 = 9 rows of purlins. Each row spans the 40-ft building length, so total material is 9 × 40 = 360 linear feet.

Tips & Best Practices

Check metal roofing manufacturer specifications for maximum purlin spacing—it varies by panel profile and gauge.
Use pressure-treated or galvanized purlins in high-moisture environments.
Align purlin splices at rafter or truss locations for proper bearing.
Add an extra purlin row at the eave and ridge regardless of spacing for edge support.
For steel purlins, check bearing capacity tables—C-channels and Z-channels have different allowable loads.
Purlins must be installed perpendicular to the roof slope (parallel to the eave).

Purlin Materials: Wood vs. Steel

Wood purlins (2×4, 2×6) are common in residential construction and agricultural buildings. They're easy to install with standard framing tools. Steel purlins (C-channel, Z-channel) are used in commercial and pre-engineered metal buildings for greater span capacity and fire resistance.

Purlin Spacing and Wind Loads

In high-wind zones, purlin spacing may need to be reduced from the standard 24″ to 16″ or even 12″ to provide adequate fastening for metal panels. The design wind speed and exposure category determine the required uplift resistance, which affects purlin spacing.

Anti-Sag Rods

Steel purlin systems often include anti-sag rods—diagonal tension members that prevent purlins from rotating or sagging between supports. These are tensioned during installation and are critical for long-span purlin systems.

Sources & Methodology

Last updated: February 8, 2026

Frequently Asked Questions

Common wood purlin sizes are 2×4 and 2×6 for residential. Steel purlins are typically 16-gauge C-channel, 1.5″ to 4″ depth. The size depends on the span between trusses/rafters and the roof load.