How Many Furring Strips Are Needed for a Metal Roof

Calculating how many furring strips are needed for a metal roof is essential for proper attachment, ventilation, and insulation. This guide explains spacing standards, factors that influence quantities, step-by-step calculations, material choices, and practical tips to determine the exact number of furring strips for most U.S. residential and light-commercial metal roof projects.

Roof Type	Common Furring Spacing	Furring Orientation
Standing Seam Metal	24″ O.C. or 16″ O.C.	Perpendicular To Ribs
Corrugated Metal	16″ O.C. Typically	Perpendicular To Corrugations
Metal Shingles/Panels	12″–16″ O.C.	Perpendicular To Panel Seams

Why Furring Strips Matter Under A Metal Roof

Furring strips create a secure nailing surface, maintain a ventilation gap, and help level irregular sheathing. They improve longevity of the metal roofing by reducing trapped moisture and providing structural support for fasteners. Properly placed furring strips prevent panel distortion and reduce thermal bridging in insulated assemblies.

Key Factors That Affect Furring Strip Quantity

Several variables influence how many furring strips are needed: roof slope, metal panel type, span between rafters or trusses, local building codes, and whether insulation or a secondary substrate is used. Each factor changes spacing and orientation requirements, which directly determines total linear feet of furring strips.

Standard Spacing Guidelines By Panel Type

Spacing varies with panel profile and manufacturer recommendations. Use these general practices as a starting point, but always confirm with panel manufacturer and local code.

• Standing Seam Panels: Often installed over solid substrate or continuous furring at 16″–24″ O.C.; clips may reduce furring need.
• Corrugated Panels: Typically require furring or purlins at 16″ O.C. for light loads; heavy-duty panels may allow wider spacing.
• Exposed Fastener Panels / Metal Shingles: Usually need solid support at 12″–16″ O.C. depending on wind load.

Step-By-Step Calculation Method

This method converts roof dimensions into the exact number of furring strips required. It works for rectangular roofs and can be adapted for complex shapes.

1. Measure Roof Length (Eave To Ridge): Use the actual field measurement along the slope, not plan view.
2. Determine Furring Spacing: Choose spacing based on panel type and local code (for example, 16″ O.C.).
3. Calculate Number Of Rows: Convert spacing to feet (16″ = 1.333 ft) and divide roof width by spacing, rounding up to ensure full coverage.
4. Multiply By Roof Length: Multiply number of rows by roof length to get total linear feet of furring strip required.
5. Account For Waste And Overlap: Add 10%–15% for waste, cuts, and overlaps.

Worked Example

For a 30 ft x 24 ft gable roof with corrugated panels and 16″ O.C. furring spacing: Roof Width = 24 ft; Spacing = 16″ = 1.333 ft; 24 ÷ 1.333 = 18 rows (rounded up). Each row spans the 30 ft length, so 18 x 30 = 540 linear feet. Adding 10% waste yields 594 linear feet.

Orientation: Perpendicular Vs. Parallel Installation

Furring strips are typically installed perpendicular to the metal panel seams or corrugations to provide direct fastener support. Perpendicular orientation ensures each panel fastener bears on a solid member, reducing sag and wind uplift risk. Parallel installation is uncommon and only used where manufacturer instructions permit.

Material, Size, And Fastening Recommendations

Common furring materials include pressure-treated lumber (1×3 or 1×4) and engineered purlins. Use corrosion-resistant fasteners and consider metal-to-metal clip systems for standing seam roofs. Pressure-treated wood is standard for roof decks exposed to moisture; 1×4 provides better stiffness and fastening surface than 1×3.

Ventilation, Insulation, And Thermal Considerations

Furring strips create an air gap that promotes ventilation under the metal roof, reducing condensation risk. When adding insulation, ensure furring depth accommodates airflow or use vented furring channels. Continuous ventilation at eaves and ridge combined with proper furring spacing preserves thermal performance and prevents moisture buildup.

Local Codes, Wind Loads, And Manufacturer Requirements

Building codes and wind zone requirements significantly affect furring spacing and fastener schedules. High-wind or hurricane zones often require closer spacing and stronger fasteners. Always follow panel manufacturer installation guidelines and local code requirements; noncompliance can void warranties.

Estimating Quantities For Complex Roof Shapes

For hips, valleys, and multiple planes, break the roof into rectangles and apply the calculation method to each section. Add extra furring around valleys and eaves where fastening density increases. Always round up on each section and include a higher waste allowance (12%–15%) for complex cuts.

Cost And Time Considerations

Cost depends on material, size, and labor. Lumber furring strips are inexpensive per linear foot, but installation time adds labor cost. Estimate materials by linear feet and include fasteners, flashing, and vapor/venting components in total cost calculations.

Common Mistakes To Avoid

• Ignoring Panel Manufacturer Instructions: Leads to improper spacing or voided warranty.
• Using Non-Treated Wood In Wet Areas: Causes rot and fastening failures.
• Underestimating Waste For Complex Roofs: Results in mid-job material shortages.

Tools And Materials Checklist

Essential items include measuring tape, chalk line, circular saw, pressure-treated 1×3 or 1×4 lumber or metal purlins, corrosion-resistant screws, and a framing square. A metal roof kit or manufacturer-specified clip system simplifies layout and reduces the need for continuous furring.

FAQ

How Closely Should Furring Strips Be Spaced For Standing Seam Metal?

Spacing commonly ranges from 16″ to 24″ O.C., depending on whether panel clips are used. Follow panel manufacturer instructions and local code for final spacing decisions.

Can Furring Strips Be Installed Over Existing Shingles?

Yes, but check rafter capacity and local code. Adding furring over shingles increases load and may require reinforcement or removal before installation.

Is Metal Purlin Better Than Wood Furring?

Metal purlins offer greater durability, dimensional stability, and resistance to rot. Wood is economical and easy to work with, but metal purlins are preferred in high-moisture or high-wind environments.

Final Practical Checklist For Accurate Counts

• Confirm Panel Type And Manufacturer Spacing Recommendations.
• Measure Roof Slope Length For Accurate Row Lengths.
• Divide Roof Width By Chosen Spacing To Get Row Count, Round Up.
• Multiply Rows By Length Then Add 10%–15% Waste.
• Adjust For Complex Roof Features And Local Code Requirements.