Comprehensive Guide: How To Install Metal Roofing Over Shingles Successfully

Installing metal roofing directly over existing asphalt shingles is a viable, cost-effective, and code-compliant method that minimizes debris disposal while creating a durable secondary weather barrier. Success requires strict adherence to structural load capacity verification, the use of proper furring strips or sub-purlins, and the integration of high-temperature synthetic underlayments to prevent galvanic corrosion and thermal degradation.

Strategic Preparation and Structural Validation

Before initiating a metal overlay project, you must determine if your existing roof structure can accommodate the additional dead load. Asphalt shingles typically weigh between 2.0 and 2.5 pounds per square foot, while standing seam or exposed-fastener metal roofing adds approximately 1.0 to 1.5 pounds per square foot. Most residential roof trusses are engineered to handle this marginal increase, but homes with structural sagging or localized rot must be remediated first.

• Essential Material Inventory:
  
  
  • 1x4 or 2x4 pressure-treated wood furring strips (or metal hat channels).
  • High-temperature self-adhered synthetic underlayment (rated for at least 250 degrees Fahrenheit).
  • Corrosion-resistant fasteners (stainless steel or coated exterior-grade screws) sized to penetrate the roof deck by at least one inch.
  • Metal flashing kits, ridge caps, and eave trim specifically matched to your panel profile.
  • High-quality butyl sealant or thermoplastic rubber caulk for panel penetrations.
• Mandatory Tools:
  
  
  • Variable speed impact driver with magnetic bits.
  • Circular saw equipped with a metal-cutting blade (carbide-tipped).
  • Handheld metal shears or "snips" for fine trimming.
  • Fall protection equipment including OSHA-compliant harnesses and static lines.
  • Chalk line and pneumatic roofing nailer for furring strip installation.

Step-by-Step Installation Workflow for Metal Overlays

Step 1: Roof Inspection and Shingle Flattening

Conduct a thorough inspection of the existing shingle roof. Ensure that all loose shingles are nailed down flat and that damaged or curling shingles are replaced or removed to provide a level surface. Protruding nails must be driven flush or removed entirely to prevent the metal panels from resting on irregular points, which can lead to oil-canning or fastener backing.

Step 2: Underlayment Application

Apply a high-temperature synthetic underlayment directly over the prepared asphalt shingles. This layer serves two purposes: it acts as a slip sheet to prevent the metal panels from bonding or abrading against the shingle granules, and it provides a critical secondary moisture barrier should any condensation accumulate beneath the metal. Ensure at least a four-inch overlap on all seams and extend the underlayment over the drip edge.

Step 3: Installing Furring Strips or Hat Channels

Install furring strips horizontally across the roof slope, typically spaced 16 to 24 inches on center depending on the metal panel manufacturer’s span tables. Secure these strips through the shingles and into the structural rafters or trusses using long, structural-grade screws.

Warning: Never rely on the sheathing alone to hold the furring strips if the roof slope is steep or if you are in a high-wind zone; always anchor into the rafters to ensure structural integrity.

Step 4: Installing Drip Edges and Eave Flashing

Install the perimeter flashing before laying the metal panels. Place the starter drip edge at the eaves, ensuring it laps over the underlayment. This redirects water away from the fascia and prevents moisture from wicking into the newly installed furring strips.

Step 5: Panel Attachment and Sequence

Starting from the gable end, lay your metal panels perpendicular to the furring strips. Secure the panels using recommended fasteners, typically placed in the flat of the panel for exposed-fastener systems or concealed clips for standing seam systems.

Pro-Tip: Always install a closure strip at the ridge and eaves to prevent wind-driven rain, debris, and insects from entering the void space between the metal and the underlying shingles.

Technical Specifications and Material Comparisons

Field Troubleshooting and Failure Prevention

• Issue: Oil-Canning (Wavy appearance in flat panels)
  
  
  • Root Cause: Uneven substructure or overtightening of fasteners.
  • Fix: Ensure furring strips are perfectly coplanar using a string line. If using exposed fasteners, back off the torque on your impact driver to allow the panel to expand and contract naturally.
• Issue: Galvanic Corrosion
  
  
  • Root Cause: Contact between dissimilar metals (e.g., copper flashing with steel panels).
  • Fix: Use compatible materials throughout. If incompatibility is unavoidable, apply a non-conductive barrier or heavy-duty bituminous coating between the two metal surfaces.
• Issue: Condensation Beneath Panels
  
  
  • Root Cause: Inadequate ventilation or thermal bridging.
  • Fix: Ensure the ridge vent allows for continuous airflow through the channel created by your furring strips, effectively purging the cavity of moist air.

Frequently Asked Questions

Does installing metal over shingles violate building codes?

Most jurisdictions allow up to two layers of roofing materials, but you must check your local municipal building codes. Some regions specifically prohibit metal over shingles due to fire-rating concerns, so verify your local requirements before purchasing materials.

Do I need to remove the existing shingles?

Removing the shingles is optional but recommended if the existing roof has multiple layers or significant structural rot. Leaving the shingles in place provides added thermal mass and sound dampening, but you must account for the added weight and ensure the substructure can support it.

How do I handle heavy snow loads with metal roofing?

Metal roofs shed snow quickly, which can be dangerous for pedestrians below. It is highly recommended to install snow guards or snow fences in regions with heavy snowfall to prevent dangerous shedding and to protect gutters from ice dams.

Will metal roofing over shingles make my house louder?

Contrary to common belief, a metal roof installed over an existing shingle roof is no louder than any other roofing material. The layer of asphalt shingles acts as a significant acoustic dampener, effectively absorbing the sound of rain and hail.

Modernize Your Property Today

Upgrade your home with a resilient metal roofing system that offers superior energy efficiency and long-term protection against the elements. Contact a certified roofing professional today to assess your structure and request a professional material estimation for your overlay project.