How To Install Metal Roofing Over Shingles: The Definitive Professional Installation Guide

Installing metal roofing over existing asphalt shingles is a viable, cost-effective, and sustainable method that eliminates the need for expensive tear-offs, provided the roof deck and structural framing can support the additional weight of the metal system. The process requires installing a high-temperature underlayment over the shingles, followed by a properly spaced wood furring strip system to create a vented thermal break that prevents moisture accumulation and premature corrosion.

Pre-Installation Structural Assessment and Site Readiness

Before committing to an over-roofing project, you must verify that the current roof assembly meets strict structural and municipal standards. Because you are adding weight—typically 1 to 3 pounds per square foot for metal panels—you must confirm that your rafters or trusses can handle the cumulative dead load. Most local building codes allow for a maximum of two layers of roofing; if your home already has two layers of shingles, a full tear-off is mandatory.

• Essential Tools: Cordless impact driver, circular saw with carbide-tipped blade for metal cutting, aviation snips, chalk line, roofing nailer, safety harness and roof brackets, and a heavy-duty electric drill for fastener installation.
• Essential Materials: Synthetic high-temperature underlayment, 1x4 or 2x4 pressure-treated wood furring strips (battens), #10 or #12 wood-to-metal screws with EPDM washers, and perimeter flashing kits compatible with your specific metal profile.
• Prerequisite Standards: Ensure the existing shingles are lying relatively flat. If shingles are curling, missing, or cupping excessively, you must level the surface or proceed with a total tear-off. Confirm that local building codes permit over-roofing in your jurisdiction, as some coastal or high-wind zones prohibit this practice due to potential uplift concerns.
• Time Benchmark: For a standard 1,500-square-foot roof, expect a 3 to 5-day installation timeline for a two-person professional crew.

Systematic Installation Procedures for Metal Over-Shingle Applications

Step 1: Surface Preparation and Inspection

Begin by sweeping the existing roof surface to remove debris, moss, and loose granules. Walk the roof to identify soft spots, which indicate rotted deck sheathing. These must be replaced with new OSB or plywood. Flatten any curled shingles by nailing them down or cutting them away. Ensure all existing vents and chimney flashings are in good condition, as you will need to extend these to account for the new height of the metal roof.

Step 2: Installing the High-Temperature Underlayment

Roll out a high-temperature synthetic underlayment directly over the asphalt shingles. Unlike traditional felt, high-temp synthetic underlayment is engineered to withstand the intense thermal transfer that occurs between metal panels and the substrate. Ensure the underlayment is lapped at least 6 inches on horizontal seams and 12 inches on vertical overlaps. Secure the underlayment using cap nails to prevent moisture ingress.

Step 3: Fastening the Furring Strip Grid

The furring strips (battens) are the most critical component, as they create the required ventilation air gap and provide a solid anchoring point for the metal panels. Install 1x4 pressure-treated lumber horizontally across the roof, spaced according to the metal manufacturer's specifications—typically 16 to 24 inches on center. Use structural-grade screws that are long enough to penetrate the existing shingles, the underlying deck, and at least 1 inch into the roof rafters.

Pro-Tip: If your roof layout requires vertical furring strips to allow for proper drainage, install them first, then layer the horizontal strips on top to create a cross-hatched grid for maximum airflow.

Step 4: Installing Eave and Rake Flashings

Install the drip edge and eave flashing over the furring strips before laying the panels. The flashing must sit flush against the furring to ensure that water shed from the metal roof is directed into the gutters rather than behind them. Use a sealant tape or high-grade exterior caulking at all transition points to create a weather-tight seal.

Step 5: Positioning and Fastening Metal Panels

Align your first panel at the rake edge, ensuring it is perfectly square with the eaves. Secure the panel using the manufacturer’s recommended fastener pattern. Most systems require screws to be placed through the flat of the panel into the furring strips. Ensure you do not over-tighten the fasteners; the EPDM washer should be compressed until it just bulges slightly, but not distorted or flattened to the point of structural failure.

Warning: Avoid using a standard abrasive cut-off wheel on your saw when trimming metal panels. These generate excessive heat and sparks that burn the protective coating of the metal, leading to rapid rust streaks and voiding your material warranty. Always use specialized metal-cutting blades or aviation snips.

Material Selection and Performance Metrics

The following table outlines the comparative requirements for installing various types of metal roofing systems over existing asphalt shingles.

Common Site Failures and Remediation Strategies

• Root Cause: Improper Fastener Tension. If screws are driven too deep, the rubber washer splits, leading to immediate leaks at every penetration point. Actionable Fix: Remove the compromised screw, fill the hole with a color-matched polyurethane sealant, and install an oversized "remedial" screw with a larger rubber gasket to regain the seal.
• Root Cause: Insufficient Ventilation. Failing to provide a clear air gap between the shingles and the metal can lead to condensation buildup on the underside of the panels. Actionable Fix: Ensure the ridge vent is fully unobstructed and that the furring strips provide a continuous path for air to travel from the eaves to the peak.
• Root Cause: Dissimilar Metal Corrosion. Installing copper flashing against steel panels can trigger galvanic corrosion. Actionable Fix: Replace all incompatible flashings with matching steel or aluminum components and ensure all fasteners are coated in a galvanized or zinc-alloy finish.

Frequently Asked Questions

Does the metal roof need a special underlayment over shingles?

Yes, you must use a high-temperature rated synthetic underlayment. Metal roofs reach significantly higher temperatures than asphalt shingles, and standard organic felt underlayment can melt or bond to the metal, causing damage to the system.

Can I install a standing seam roof over shingles myself?

While possible, standing seam roofs require specialized mechanical seaming tools and a high degree of precision in panel alignment. It is generally recommended that DIYers stick to exposed fastener (screw-down) panels, which are more forgiving during the installation process.

Does adding metal over shingles void my insurance or warranty?

Most insurance companies permit this, but you must check your policy. Regarding warranties, you must strictly follow the manufacturer's installation manual; failure to use the specified ventilation gap or fastener type will typically void the material warranty immediately.

Will the extra weight cause my roof to collapse?

If your roof structure is built to modern building codes, the addition of a metal roof (approximately 1.5 lbs/sq ft) is negligible compared to the snow load ratings of most regions. However, if your rafters show signs of existing deflection, consult a structural engineer before proceeding.

Consult with your local building department to ensure compliance with regional wind-uplift codes and secure a professional inspection of your roof deck integrity today.