How To Put Metal Roof On Over Shingles: The Complete Professional Installation Guide
Installing a metal roof directly over existing asphalt shingles is a viable, time-saving roofing method that eliminates tear-off costs and landfill waste, provided structural load limits and moisture profiles are verified. Success hinges on precise purlin layout, thermal expansion management, and strict adherence to local building codes regarding maximum shingle layer thickness.
Initial Structural Assessment and Material Planning
Executing a retrofit metal roofing system over an existing asphalt shingle roof requires rigorous up-front evaluation to ensure the underlying framing can support the combined dead load of two roofing systems. Most residential roof rafters are engineered to handle a secondary metal layer (which typically adds only 1 to 1.5 pounds per square foot), but local jurisdiction limits—often capped at a maximum of two total layers—must be checked before proceeding.
- Essential Gear, Tools, and Materials: 1x4 wood purlins (strapping) or specialized hat channels, synthetic underlayment or high-temp self-adhering ice and water shield, exposed-fastener or standing-seam metal panels, pneumatic nail gun, variable-speed impact driver with magnetic socket adapters, metal-cutting shears or a nibbler, butyl tape, color-matched silicone exterior sealant, and personal fall arrest safety systems.
- Prerequisite Knowledge and Standards: Review ASTM E108 fire ratings, UL 2218 impact resistance classifications, and local wind-uplift rating requirements. Ensure the existing shingle roof does not exhibit advanced structural sagging, active leaks, or more than one prior layer of shingles.
- Time and Budget Benchmarks: For an average 2,000-square-foot roof, expect a three- to five-day installation window for a two-person crew. Material costs typically range from three to seven dollars per square foot depending on whether you choose exposed-fastener corrugated panels or premium concealed-fastener standing seam systems.
Step-by-Step Retrofit Installation Workflow
Step 1: Surface Preparation and Minor Repairs
Begin by thoroughly inspecting the entire shingle surface. Walk the roof to identify soft spots, localized decking rot, or deeply cupped and curling shingles. Replace any severely damaged plywood or OSB decking from the underside if attic access permits, or replace isolated shingle mats where buckling is extreme. Sweep the roof clean of all loose granules, leaves, and debris to ensure the underlayment or strapping can lay completely flat against the shingle plane.
Warning: Never install metal roofing over curling or heavily warped architectural shingles without leveling the surface first; high spots will telegraph through metal panels, causing oil-canning and uneven fastener tension.
Step 2: Underlayment and Moisture Barrier Application
Unroll a high-temperature synthetic underlayment or a self-adhering ice and water barrier directly over the existing asphalt shingles, working horizontally from the eave edge upward with a minimum 4-inch side lap and 6-inch end lap. Secure the underlayment using cap nails or roofing staples spaced according to the manufacturer's specifications. This vital layer acts as a slip sheet to prevent galvanic or abrasive friction between the abrasive shingle granules and the underside of the metal panels, while providing secondary waterproofing.
Step 3: Purlin Layout and Installation
Install 1x4 wood purlins (strapping) flat across the roof deck perpendicular to the rafters, starting flush with the eave edge and spacing them evenly up the slope according to your metal panel manufacturer's instructions (typically 12 to 24 inches on center). Fasten the purlins directly through the shingles and into the roof decking using ring-shank nails or structural wood screws long enough to penetrate the underlying OSB or plywood by at least three-quarters of an inch.
Pro-Tip: Snap chalk lines horizontally across the underlayment to maintain absolute parallel alignment of your purlins; even a minor deviation at the eave will compound upward and misalign your entire metal roof system.
Step 4: Eave Trim and Drip Edge Placement
Install pre-formed metal eave drip edge and rake trim over the purlin ends at the perimeter of the roof. Secure these edge flashings with galvanized roofing nails spaced every 8 to 12 inches. Apply a continuous bead of butyl tape along the top edge of the eave flashing where it interfaces with the first metal panels to create a watertight seal against wind-driven rain and insect intrusion.
Step 5: Hanging and Fastening Metal Panels
Position your first metal panel flush with the eave and rake edge, ensuring a square overhang of approximately one to two inches past the drip edge. Drive specialized metal roofing screws equipped with EPDM neoprene washers through the pre-drilled or self-tapping panel faces into the wood purlins below. Maintain uniform torque when driving fasteners; over-tightening compresses and deforms the rubber washer, leading to premature cracking and leaks, while under-tightening fails to compress the seal.
Step 6: Ridge Cap and Closure Strip Installation
Install high-density foam inside-closure strips along the top ridge beneath where the final metal panels meet. Lay the ridge cap flashing centrally over the ridge peak, overlapping adjacent sections by a minimum of 6 inches and sealing the lap joints with polyurethane exterior sealant. Secure the ridge cap using long stitch screws with neoprene washers driven through the top of the panels and directly into the uppermost purlins.
How To Put A Metal Roof Over Shingles - ManMadeDIY
Technical Specifications and Material Comparison Matrix
| Roofing Parameter | Exposed-Fastener Panels | Standing Seam Panels | Direct-to-Deck Shingle Retrofit |
|---|---|---|---|
| Fastener Visibility | Exposed through panel face | Concealed clips underneath | Hidden beneath overlapping courses |
| Thermal Movement | Restricted by face screws | Accommodates expansion/contraction | N/A (Standard shingle behavior) |
| Expected Lifespan | 30 to 40 years | 50+ years | 20 to 25 years |
| Slope Requirement | Minimum 3:12 pitch | Minimum 2:12 pitch | Minimum 2:12 pitch |
| Wind Uplift Resistance | Moderate (up to 110 mph) | High (up to 150+ mph) | Moderate |
Common Site Failures and Field Fixes
- Root Cause: Fastener over-tightening crushing the EPDM rubber washer.
- Actionable Fix: Back out the compromised screw, inject a dab of polyurethane roof sealant into the oversized hole, and replace it with a slightly wider oversized retro-fit fastener equipped with a fresh sealing washer.
- Root Cause: Thermal expansion and contraction causing oil-canning (waviness) across wide panel flats.
- Actionable Fix: Loosen the face fasteners slightly on exposed systems to allow natural metal shifting, or switch to narrow-pan architectural standing seam profiles featuring sliding clips for future installations.
- Root Cause: Galvanic corrosion occurring where dissimilar metals (such as aluminum flashing touching untreated steel panels) react chemically in the presence of moisture.
- Actionable Fix: Remove the reactive metal components immediately, separate the contact points using heavy-duty PVC tape or specialized isolation coatings, and replace them with code-compliant, chemically compatible fasteners and flashings.
- Root Cause: Improper purlin spacing leading to panel deflection under heavy snow or foot traffic loads.
- Actionable Fix: Strip back the affected panels and install intermediate sub-purlins to bridge the structural span gaps, adhering strictly to the manufacturer's maximum allowable load charts.
Frequently Asked Questions
Can I install a metal roof over two layers of asphalt shingles?
Most building codes and structural engineers prohibit installing a metal roof over more than a single existing layer of shingles due to excessive dead-load weight limits and the inability of fasteners to achieve adequate withdrawal resistance in deteriorated decking. Always strip down to the original wood deck or limit your installation to a single shingle layer.
Do I need underlayment between shingles and metal roofing?
Yes, a high-temperature synthetic underlayment or ice and water shield is mandatory. It acts as a thermal break, dampens interior rain noise, prevents abrasive shingle granules from wearing away the protective paint coating on the underside of the metal panels, and provides temporary waterproofing during installation.
How do I walk on a metal roof without damaging it?
Always wear soft-soled, clean shoes or boots to prevent scratching the finish. Step exclusively on the flat portions of the panels directly over the underlying wood purlins or decking supports, never in the middle of wide panel spans, to avoid denting or creasing the metal.
Will a metal roof over shingles cause overheating in my attic?
No, provided your attic maintains proper continuous intake (soffit) and exhaust (ridge) ventilation. In fact, metal roofs reflect a significant portion of solar radiation compared to dark asphalt shingles, often lowering overall attic temperatures and reducing summer cooling loads.
Start your metal roof retrofit project today by verifying local weight limits and sourcing high-grade purlins and panels designed for long-term durability.
