What Size Furring Strips for Metal Roofs

Choosing the right furring strip size for a metal roof affects ventilation, insulation, and attachment strength. This guide explains common sizes, spacing, fastening, and installation tips for U.S. residential and light commercial metal roofing systems.

Furring Size	Typical Use	Spacing	Notes
1×2 (¾”x1½”)	Minor elevation for thin insulation or underlayment	16″–24″ o.c.	Lightweight; limited structural support
1×3 (¾”x2½”)	Common for roofing panels over solid sheathing	16″–24″ o.c.	Good balance of height and cost
1×4 (¾”x3½”)	Create ventilation channel and thicker insulation gap	16″ o.c. recommended	Preferred where more airflow or insulation clearance needed
2×2 (1½”x1½”)	Grid or cross-furring for panel leveling	12″–16″ o.c.	Stronger in compression; used on uneven substrates
2×3 / 2×4	Structural furring/purlin substitute	Varies by load	Used where heavy loads, snow, or long spans exist

Factors To Consider When Choosing Furring Strip Size

Substrate Type, Roof Profile, Insulation Needs, Ventilation, And Local Weather Are Primary Determining Factors. Solid sheathing (osb/plywood) vs. open purlin systems changes required height and spacing of furring strips.

Panel Profile And Fastener Location Influence Strip Height. Deep-rib metal panels may require taller furring to clear ribs and align with screw lines; low-profile panels need less elevation.

Insulation Thickness And Ventilation Gap Determine Minimum Strip Thickness. To allow airflow and prevent condensation, many installers use 1x4s to create a 3/4″–3½” gap depending on insulation and venting strategy.

Common Furring Strip Sizes And Where To Use Them

1×2 Furring Strips

Best For Minimal Elevation And Light Applications. Use where metal panels mount directly to solid decking and only a small separation is needed for a moisture barrier or thin insulation.

1×3 Furring Strips

Most Popular Choice For Residential Metal Roof Retrofits. Offers a practical compromise: enough height for underlayment and some ventilation while remaining economical and easy to handle.

1×4 Furring Strips

Preferred When Ventilation Or Thicker Insulation Is Needed. This size provides a deeper airflow channel that helps control condensation and permits thicker rigid insulation where used above deck.

2×2, 2×3, And 2×4 Options

Used For Heavier Loads, Uneven Surfaces, Or Longer Spans. These thicker members act like small purlins, providing stronger support for screws and reducing panel deflection on wide spans.

Spacing And Layout Recommendations

Typical Spacing Is 16″ Or 24″ On Center Depending On Load And Panel Span. Use 16″ o.c. for high wind or snow loads and 24″ o.c. for lighter, short-span roofs with solid decking.

Align Furring With Metal Panel Fastener Lines. Place furring so screws penetrate solid wood at the manufacturer’s recommended locations to prevent pull-out and leaks.

Cross-Furring Improves Panel Leveling And Attachment. On uneven decks, employ cross-furring at 90 degrees to primary strips to create a consistent plane for panel installation.

Fasteners And Penetration Depth

Select Screws And Lengths To Penetrate The Furring And Embed At Least 1″ Into The Structural Deck. For example, with 1×4 furring plus metal panel thickness, choose screws long enough to go through metal, furring, and 1″ into plywood or rafters.

Use Corrosion-Resistant Fasteners Rated For Metal Roofs. Stainless steel or coated screws with neoprene washers prevent corrosion and maintain watertight seals under thermal cycling.

Treated Wood, Composite Options, And Moisture Considerations

Exterior-Treated Or Naturally Durable Lumber Is Recommended For Long-Term Exposure. If furring contacts potential moisture or treated lumber is required by code, choose pressure-treated pine or rot-resistant species.

Composite Or Engineered Strips Offer Dimensional Stability. PVC or composite furring resists rot and insect damage but requires compatible fasteners for secure attachment.

Ventilation, Condensation Control, And Thermal Performance

Create A Continuous Air Gap When Possible To Reduce Condensation Risks. Taller furring (1×4 or greater) enables a ventilation channel that allows moisture-laden air to escape at eaves and ridges.

Consider Thermal Bridging And Insulation Placement. Furring can become a thermal bridge; placing insulation above the metal or using continuous insulation under furring helps maintain energy efficiency.

Installation Best Practices

Install Furring Parallel To Roof Slope For Panel Attachment Unless Cross-Furring Is Required. Ensure strips are straight, uniformly spaced, and securely fastened to the deck or rafters.

Use Blocking Or Additional Support At Panel Seams And Ridge Lines. This prevents screw over-stress and provides a solid substrate under fastener locations that receive concentrated loads.

Seal Penetrations And Use Proper Flashing Around Penetrations. Even with washered screws, apply compatible sealants and flashing at transitions, chimneys, and skylights to prevent leaks.

When To Use Purlins Instead Of Furring Strips

Choose Purlins For Long Spans, Heavy Snow Loads, Or Structural Support Needs. Metal or larger dimensional lumber purlins (2×6, engineered sections) are designed for primary support and may replace furring on commercial roofs.

Furring Strips Are Typically Secondary Members For Leveling And Ventilation, Not Primary Structural Elements. Evaluate engineering requirements for snow, wind, and building codes before substituting smaller furring for structural purlins.

Code, Manufacturer, And Warranty Considerations

Always Follow Metal Panel Manufacturer Installation Guidelines To Preserve Warranty. Many manufacturers specify furring heights, spacing, and fastener types; deviating from these details can void warranties.

Local Building Codes May Dictate Fastener Embedment, Material Treatment, And Snow/Wind Design Loads. Consult code officials or a structural engineer for high-wind or high-snow regions to ensure compliance.

Frequently Asked Questions

What Is The Minimum Furring Height For Metal Roofs?

Minimum Heights Range From 3/4″ To 1-1/2″ Depending On Panel Profile And Underlayment Needs. Many installers use at least 1×3 to ensure proper screw seating and minor ventilation.

Can Untreated Pine Be Used For Furring Under Metal Roofs?

Untreated Pine Should Be Avoided In Direct Exterior Contact Without Adequate Moisture Protection. Pressure-treated or rot-resistant lumber is recommended where moisture, condensation, or direct roof drainage may occur.

How Much Clearance Is Needed For Ventilation Under Metal Panels?

A Continuous Gap Of At Least 3/4″ Is Often Recommended; More May Be Needed For Rigid Insulation Systems. The exact clearance depends on insulation thickness and manufacturer recommendations.

Do Furring Strips Affect Snow Load Capacity?

Yes—Smaller Furring Strips Are Not Structural; Heavy Snow Or Large Spans May Require Purlins Or Larger Sections. Design loads should be checked with a structural engineer for snowy climates.

Summary Recommendations

• Use 1×3 For Typical Residential Retrofits Where Moderate Ventilation And Attachment Are Needed.
• Choose 1×4 When A Clear Ventilation Channel Or Thicker Insulation Clearance Is Required.
• Select 2x Members Or Purlins For Structural Support, Long Spans, Or High Snow/Wind Loads.
• Always Match Fastener Length To Penetrate At Least 1″ Into Structural Decking And Use Corrosion-Resistant Screws.

Following These Guidelines Helps Ensure A Durable, Watertight, And Energy-Efficient Metal Roof Installation. For complex roofs or unusual loads, consult the metal panel manufacturer and a licensed structural engineer.