How to Install Metal Roofing Over Shingles

A step-by-step overview of the process for installing metal roofing panels over an existing asphalt shingle roof, including when the approach is permitted, required preparation, and situations where it should not be done.

Specifications

Required Substrate	One existing shingle layer maximum, no prior overlay
Furring Strip Spacing	Typically 24 inches on center, matching panel fastening schedule
Underlayment	Synthetic underlayment over furring strips before metal panels
Ventilation	Air gap created by furring strips allows moisture to escape

Common issues

Skipping furring strips and fastening metal panels directly to old shingles, creating moisture traps and uneven surfaces
Not inspecting the existing deck for soft spots or rot before covering it with a second roofing layer
Failing to flash transitions properly at walls, chimneys, and penetrations when the roof plane is now elevated by the furring strip thickness
Ignoring manufacturer installation requirements, voiding the metal panel warranty

Insurance relevance

If a metal-over-shingle roof is damaged, the claim must account for both layers. Tearoff scope on a layered system is more expensive because both the metal panels and the underlying shingles must be removed. Adjusters should document the layered assembly and include appropriate line items for removal and disposal of both materials. If hidden deck damage is discovered during tearoff, a supplement is typically required.

Before You Start: Inspection and Code

Before installing metal roofing over existing shingles, the first step is verifying that the approach is permitted. Check the local building code. Most jurisdictions follow the IRC, which allows a new roof covering over one existing layer. If the house already has two layers of shingles, a full tearoff is required. Next, inspect the existing roof. Walk the surface and probe for soft spots that indicate deck rot. Check the attic from below for staining, sagging, or daylight showing through the deck. Any compromised decking must be repaired or replaced before the overlay proceeds. Pull a building permit. Most jurisdictions require a permit for a re-roofing project, and the inspection process confirms the work meets code.

Furring Strip Layout

Furring strips are the foundation of a successful metal-over-shingle installation. Use 1x4 or 2x4 pressure-treated or kiln-dried lumber. Snap chalk lines at 24 inches on center (or per the metal panel manufacturer's fastening schedule) and fasten the strips through the existing shingles into the roof rafters or trusses with screws long enough to penetrate the framing by at least 1.5 inches. The furring strips must be level. Use shims where the old shingle surface is uneven. Run strips along the eave, ridge, and all edges in addition to the field rows. At valleys, hips, and transitions, additional blocking provides a fastening surface for trim and flashing. The air channel created between the old shingles and the new metal is essential for moisture management.

Underlayment and Panel Installation

After the furring strips are in place, install a synthetic roofing underlayment over the top of the strips. This provides a secondary water barrier between the metal panels and the furring assembly. Begin panel installation at the eave, aligning the first panel with a 1 to 1.5 inch overhang past the drip edge. Fasten according to the manufacturer's screw pattern, using the recommended screw type with neoprene washers. Work from the eave to the ridge, overlapping panels per the manufacturer's specifications. Side laps should face away from the prevailing wind direction. At the ridge, install a vented ridge cap to allow the air channel to exhaust moisture. All penetrations require metal-roofing-compatible flashing, which may differ in profile from standard shingle flashing due to the raised panel ribs.

When the Overlay Approach Fails

The overlay approach has limits. If the existing shingles are severely deteriorated, the furring strip fasteners may not hold reliably because the underlying decking is compromised. If the old shingles have active leaks that have gone unrepaired, covering them with metal does not fix the problem. Trapped moisture accelerates. If the home is in a jurisdiction that requires a full tearoff for insurance coverage or building code compliance, the overlay is not an option regardless of the structural feasibility. In these cases, a full tearoff, deck inspection and repair, and clean installation is the correct path. The additional cost of tearoff is offset by the opportunity to address hidden damage and the assurance of a warranty-compliant installation on a verified substrate.

Frequently asked questions

The general process is: (1) inspect the existing shingle layer and decking for damage, (2) make any necessary deck repairs, (3) install furring strips (1x4 or 2x4) over the shingles fastened through to the roof deck framing at 24 inches on center, (4) install synthetic underlayment over the furring strips, (5) install the metal panels starting from the eave and working upward per manufacturer instructions, (6) flash all penetrations, transitions, and edges with metal-roofing-compatible flashing. Local building permits are required in most jurisdictions.

Do not install metal over shingles if there are already two layers of roofing on the house, if the existing decking shows signs of rot or structural damage, if the existing shingles are severely curled or buckled and cannot provide a reasonably flat surface, or if the local building code prohibits overlay installations. Some homeowner insurance policies also require a full tearoff for coverage eligibility on the new roof.

It depends on the metal panel manufacturer. Some manufacturers permit installation over one layer of existing shingles as long as furring strips are used and their specific installation instructions are followed. Others require installation on a clean deck only. Always review the manufacturer's warranty terms before proceeding. Installing without furring strips or without proper underlayment voids most manufacturer warranties regardless of the substrate.

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