Installing a metal roof over existing asphalt shingles is a practice that generates considerable debate among roofing professionals. The approach, known as a re-cover or overlay, offers cost savings by eliminating the labor and disposal costs of tear-off. However, it also introduces potential problems with moisture trapping, uneven surfaces, and shortened metal roof lifespan. Understanding the conditions under which an overlay is acceptable and the proper installation techniques is essential for anyone considering this approach. This guide provides comprehensive information about installing metal roofing over existing asphalt shingles.
Pros and Cons of Metal Over Shingles
Before deciding to install a metal roof over existing shingles, carefully weigh the advantages and disadvantages of this approach compared to a full tear-off and new installation.
Advantages of the Overlay Approach
The primary advantage of installing metal over shingles is cost savings. A full tear-off of an existing asphalt shingle roof typically costs $1 to $2 per square foot for labor and disposal, which can amount to $1,500 to $3,000 for an average residential roof. By eliminating this step, the overall roofing project cost is reduced by 15 to 25 percent. The overlay also eliminates the risk of exposing the interior to weather during the roofing process, as the existing shingles remain in place as a weather barrier until the metal roof is fully installed. Additionally, the existing shingles provide a secondary weather barrier and an extra layer of insulation that can slightly improve the thermal performance of the roof assembly. Durable roofing materials and installation methods for residential homes compares the long-term value of different roofing approaches including overlays.
Disadvantages and Risks
The most significant disadvantage of installing metal over shingles is the potential for moisture trapping between the old shingles and the new metal roof. The metal roof is impermeable, so any moisture that enters from the edges or through fastener penetrations can become trapped between the metal and the shingles, leading to mold growth and accelerated deterioration of the shingles and the roof deck. The uneven surface of old shingles can cause the metal panels to distort or telegraph the shingle texture beneath, creating an unsightly wavy appearance. The additional weight of the metal roof (typically 1 to 1.5 pounds per square foot) combined with the existing shingles adds approximately 3 to 5 pounds per square foot to the roof structure, which may exceed the design load capacity of the framing. Common causes of asphalt shingle failure and roof deterioration explains how overlaying new roofing over failing shingles accelerates problems by trapping heat and moisture against the deck.
| Factor | Tear-Off + New Metal | Metal Over Shingles |
|---|---|---|
| Initial cost (2,000 sq ft roof) | $12,000 – $18,000 | $9,000 – $14,000 |
| Labor time | 3-5 days | 2-4 days |
| Waste disposal | 2-3 tons to landfill | Minimal |
| Metal roof lifespan | 40-60 years | 30-50 years |
| Moisture risk | Low – proper ventilation | Moderate to high |
| Visual quality | Smooth, even surface | Possible waviness |
| Structural load | 1-1.5 psf (metal only) | 4-6 psf (metal + shingles) |
Conditions That Allow an Overlay
Not every existing shingle roof is a suitable substrate for a metal overlay. Specific conditions must be met for the installation to be successful.
Acceptable Existing Roof Conditions
The existing asphalt shingle roof must be in good condition with no more than one existing layer of shingles. Most building codes limit roofing overlays to a maximum of two total layers (one existing plus one new). The shingles must be fully adhered, not curled, buckled, or missing. There must be no signs of moisture damage, rot, or mold on the roof deck or in the attic below. The roof slope must be 3:12 or steeper to ensure adequate water runoff. The existing shingles must be clean and free of moss, algae, or debris that could create uneven surfaces under the metal panels. If the existing roof shows any signs of failure, including curled shingles, missing granules, or leaks, the overlay should not proceed and a full tear-off is required.
Removing High Spots and Obstructions
Before installing metal over shingles, all high spots and obstructions must be addressed. Ridge caps, hip shingles, and any loose or lifted shingles must be removed or nailed flat. Plumbing vents, chimneys, and skylights must be flashed to extend through the metal roof, requiring removal of existing step flashing and replacement with new flashing integrated with the metal panels. The ridge vent system must be evaluated and replaced if needed. If the existing ridge vent is incompatible with the metal roof ridge cap system, it must be removed and replaced with a metal roof ridge vent. These modifications reduce some of the cost savings of the overlay approach but are essential for proper performance.
Installation Techniques for Metal Over Shingles
Proper installation techniques are critical for the success of a metal roof overlay. The following practices minimize the risks associated with this approach.
Creating a Drainage Plane and Ventilation Channel
The most important step in a metal-over-shingles installation is creating a ventilation and drainage plane between the shingles and the metal panels. This is achieved by installing 2×4 purlins (also called strapping) perpendicular to the roof slope over the existing shingles. The purlins create a 1.5-inch air gap that allows any moisture that enters the assembly to drain and dry. The purlins must be fastened through the shingles into the roof rafters or trusses with screws or ring-shank nails of sufficient length. The air gap also provides a thermal break that reduces heat transfer from the metal roof to the attic below. Over the purlins, a layer of synthetic underlayment is installed before the metal panels are applied. This drainage plane approach is widely recommended by metal roofing manufacturers for overlay installations.
Fastening Through Multiple Layers
Fastening metal panels through shingles requires longer fasteners than standard metal roof installations. Screws must penetrate through the metal panel, the purlin (if used), the shingles, the roof sheathing, and extend at least 3/4 inch into the rafter or truss below. For a typical installation with 2×4 purlins, 1/2-inch sheathing, and asphalt shingles, #10 or #12 self-drilling screws of 3 to 3.5 inches in length are typically required. The screw must be long enough to achieve the required 3/4-inch penetration into solid wood framing. Using screws that are too short results in inadequate pull-out resistance, while using screws that are too long risks damaging interior ceilings or wiring below the roof deck. Choosing the right fasteners and sealants for metal roof installation provides detailed specifications for screw type, length, and spacing for overlay installations.
When an Overlay Is Not Recommended
In several situations, installing metal over existing shingles is strongly discouraged regardless of the condition of the existing roof.
Situations That Require Full Tear-Off
If the existing roof has two or more layers of shingles, a full tear-off is required by building code in most jurisdictions. If the roof deck shows any signs of rot, decay, or moisture damage, the damaged sheathing must be replaced and the cause of moisture entry corrected before new roofing is installed. If the roof structure is not designed to support the additional weight of the overlay, a structural engineer must evaluate and recommend reinforcement. If the metal roof manufacturer’s warranty requires installation over a solid deck or a specific substrate, deviating from this requirement voids the warranty. For insurance purposes, check with the homeowner’s insurance provider, as some insurers require full tear-off for roofing work or provide reduced coverage for overlay installations.
Climate Considerations
In climates with frequent freeze-thaw cycles, the moisture trapped between the shingles and the metal roof can undergo repeated freezing and thawing, accelerating deterioration of both the shingles and the metal panels. In warm humid climates, the trapped moisture promotes mold growth and corrosion of the metal panel edges. In areas with heavy snow loads, the additional weight of snow combined with the overlay can exceed the roof structure capacity. In coastal areas with salt spray, the crevice between the metal and the shingles traps salt-laden moisture that accelerates metal corrosion. For any of these climate conditions, a full tear-off and proper new installation is the recommended approach.
Summary: Installing a metal roof over existing asphalt shingles can reduce initial costs but introduces risks of moisture trapping, visual imperfections, and structural overloading. The overlay approach is acceptable only when the existing roof is in good condition, the roof slope is adequate, and a ventilated drainage plane is created between the shingles and the metal panels. In most cases, a full tear-off and new installation provides superior long-term performance, warranty coverage, and peace of mind that justifies the additional upfront cost.