Can You Put a Metal Roof on Top of Shingles – Causes, Tests & Fixes

Installing metal roofing over asphalt shingles is a topic many homeowners consider. There are clear pros and cons depending on the roof condition and the chosen metal system. Building code and manufacturer guidance should be checked before moving forward.

Generally, some situations allow an overlay while others require removal of the old shingles. This intro summarizes when an overlay is commonly permitted and when it is not, so you know what questions to ask and what to verify from official sources.

Code, Warranty, and Insurance Considerations

Before you proceed with installing a metal roof over your existing shingles, it’s crucial to check local building codes. Some areas may prohibit this practice due to safety concerns.

Manufacturer warranties may also be void if the new roof is installed directly over old shingles. Always review and understand the warranty terms before proceeding.

Additionally, insurance policies might not cover damage to a reroofed structure as well as they would if the old roof had been removed. It’s essential to consult with your insurance provider to ensure you’re adequately covered.

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Start with a careful visual inspection of the current roof. Look for curling, cupping, or missing patches that might indicate a weak base. You should also note any signs of rot or moisture intrusion into the sheathing.

Decide whether the shingle layer looks sound enough for an overlay or if removal is the safer route. If you see widespread damage, plan for a full tear-off instead of overlaying.

Visual Checkpoints and Red Flags

The first step in assessing your shingle roof is a visual inspection. Use this checklist to spot any signs of damage or issues that may require removal before installing a metal roof.

Structural and Decking Inspection

Before you start any work, it’s crucial to ensure your roof deck can support the additional weight of a metal roof. Use this checklist to inspect the structural integrity of your roof.

• Sagging: Check for any sagging in the roof. This could indicate weak spots or damaged joists that need immediate repair.
• Warping or Cupping: Inspect the roof deck for any warped or cupped boards. These can cause issues with the new metal roof and should be replaced.
• Fastener Integrity: Check that all nails or screws are securely fastened and not rusted or missing. Replace any damaged fasteners.
• Joist Spacing: Ensure your joists are spaced no more than 16 inches apart. If they’re further apart, you may need to add additional support before installing a metal roof.
• Sheathing Condition: Inspect the sheathing for any signs of rot, damage, or delamination. Replace any damaged sections.

Quick rule: If you find any significant structural issues, consult a structural engineer or professional roofer before proceeding with your metal roof installation.

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Common metal options include standing seam, corrugated panels, and metal shingles. Each type behaves differently when installed over asphalt shingles. The choice affects drainage, ventilation, and the potential for moisture traps.

Check compatibility notes from the metal system’s label or manufacturer instructions before deciding. Local rules or the roof structure may also influence which type is appropriate for an overlay.

Standing Seam vs. Corrugated vs. Metal Shingles

The three main types of metal roofing differ in profile, installation method, and underlayment needs when overlaying shingles.

Standing Seam: Has vertical panels with seams raised above the level of the panel’s flat area. Uses hidden fasteners, reducing risk of leaks. Requires a solid substrate (like plywood) or battens for proper installation over shingles.

Corrugated: Features an exposed-fastener system with rounded or ribbed panels. Can be installed directly onto existing shingles but may require additional underlayment to prevent leaks at screw holes.

Metal Shingles: Mimic the look of asphalt shingles, slate, or tile. Typically have exposed fasteners and can be installed over shingles with proper underlayment and flashing.

Material Thickness, Weight, and Fastening Systems

The gauge (thickness) of metal, its weight, and the chosen fastening system significantly impact overlay installations.

• Gauge: 26-29 gauge is standard for residential use. Thicker gauges offer better durability but add more load to your roof structure. Look for ratings like ’26-gauge Galvalume’ and avoid going too thin (like 32-gauge) as it may dent easily.
• Weight: Heavier metals put more stress on your roof’s structure. Consider your roof’s load-bearing capacity before choosing a metal type.
• Fastening Systems: Hidden fasteners reduce the risk of leaks but require a solid substrate. Exposed fasteners are simpler to install but may leak if not properly sealed and maintained.
• Expansion Issues: Metal expands and contracts with temperature changes. Use proper spacing between fasteners and consider expansion joints for larger installations to prevent buckling or warping.
• Avoid Overlapping: Installing metal over shingles without removing them first can trap moisture, leading to mold, mildew, and structural damage. Always remove old shingles before installing a new metal roof.

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Poor ventilation can lead to trapped heat and moisture under the metal. This condition can cause latent issues that show up after the install. A flat or low-slope roof also increases the risk of leaks with overlays.

Flashing conditions and existing penetrations play a big role in overlay success. If flashing is compromised, a tear-off plus proper re-flashing may be the better path.

Trapped Moisture and Condensation Risks

Overlaying a metal roof on top of shingles can trap moisture, leading to serious issues like mold, rot, or corrosion. Here’s what you need to know:

Moisture from rain or snow can get trapped between the old shingle roof and the new metal one. Without proper ventilation, this moisture has nowhere to go.

Signs of trapped moisture: Look for water stains on the ceiling, peeling paint, or musty odors coming from your attic. These could indicate that moisture is building up under your roof.

To mitigate these risks, ensure your existing roof has proper ventilation before installing a metal overlay. If you’re unsure, consult with a professional roofer.

Flashing, Valleys, and Penetration Issues

Existing flashing around chimneys, vents, and valleys can be compromised when you overlay a metal roof. This can create new leak paths, leading to water damage in your home.

When you install a metal roof over shingles, the new roof’s edges won’t seal tightly against these penetrations. This leaves gaps where water can enter.

Check for compromised flashing: Inspect the areas around chimneys, vents, and valleys on your existing roof. Look for damaged or missing flashing, and ensure they’re in good condition before installing a metal overlay.

If you find any issues, repair them before proceeding with the overlay. It’s crucial to have a solid base to prevent leaks from occurring later on.

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Use simple moisture checks to gauge the roof deck condition. A moisture meter can help identify hidden damp spots under the shingles. If moisture is found, plan accordingly.

A nail-pull or fastener test can reveal the integrity of the shingle layer and decking. For more precision, consider a thermal scan conducted by a professional to map heat and moisture patterns.

Moisture Meter and Infrared Scan Use

Before you start, use these tools to check for hidden moisture. It’s crucial to find any dampness before overlaying.

• Check decking with a moisture meter: Insert probes into the decking at multiple points. readings above 15-17% indicate moisture. If found, skip overlying until resolved.
• Scan insulation with an infrared camera: Look for cold spots (blue/purple) which may indicate missing or damp insulation. If present, address before overlaying to prevent condensation issues.

Quick rule: If moisture is detected, don’t proceed with the overlay until the issue is fixed.

Fastener and Deck Soundness Tests

Before you install new fasteners, ensure your shingles and decking can hold them. Here’s how to check:

• Test nail-pull resistance: Remove a few nails from the roof edge. If they pull out easily or with minimal effort, your roof might not be strong enough for overlay.
• Probe decking for soundness: Use a flathead screwdriver to probe the decking at multiple points. If it goes in easily and stays put, your decking may be too weak or rotten for an overlay.

Quick rule: If your roof or decking fails these tests, consider a full tear-off instead of an overlay.

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Clean the roof surface to remove dirt, debris, and loose granules. Repair or replace damaged shingle sections to create a solid base. Inspect and repair any roof deck issues before proceeding.

Apply an appropriate underlayment or slip sheet as required by the system you choose. Consider adding battens or furring if the overlay design calls for it, following the manufacturer guidance.

Underlayment and Ventilation Upgrades

Upgrading underlayment and ventilation is crucial for a successful overlay. It helps prevent moisture buildup, reducing risks of mold and rot.

Use a breathable underlayment or slip sheet. This allows moisture to escape while keeping out water from outside. Common types include felt paper and synthetic underlayments.

Improve ventilation with ridge vents along the roof’s peak, combined with continuous soffit vents at the eaves. This creates a ‘balanced’ system, allowing hot air to escape through the ridge while cool air enters via the soffits.

Pro tip: Ensure your attic is properly insulated and sealed from the living space below. This helps maintain proper ventilation and prevents ice dams in winter.

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Follow a logical installation sequence that matches the metal system you selected. Use the recommended fastening pattern and spacing as per the product instructions. Ensure all laps and seams align for a watertight finish.

Replace flashing where needed and maintain consistent clearance at roof edges and penetrations. Use clear visual checkpoints to confirm straight lines, proper alignment, and secure attachment.

Flashing and Transition Details

Before installing the metal roof, ensure all flashings are in good condition to prevent water intrusion at critical points.

Fastening Patterns and Thermal Movement Considerations

Proper fastening is crucial for a durable metal roof installation over shingles.

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Leaks or moisture-related symptoms are common concerns after an overlay. Identify the source and address it with proper flashing or gasket improvements. If in doubt, consult a pro for a thorough leak investigation.

Noise, buckling, or premature wear may indicate ventilation or installation issues. Plan corrections promptly and rely on professional help for complex fixes or when structural questions arise.

Leak Diagnosis and Repair Steps

If you’re finding leaks after installing a metal roof over shingles, don’t panic. Here’s how to diagnose and fix them.

First: Check your attic during the next rain. Look for water stains or drips. Mark these spots.

Next: Once it stops raining, go outside with a ladder and inspect the roof at the marked spots. You’re looking for damaged panels, gaps in seams, or issues around penetrations like vents and chimneys.

Temporary repairs can be made using roofing tape or sealant. But these are just band-aids. For permanent fixes, you’ll need to replace damaged panels, re-seal seams, or address penetration issues. Always use quality materials and follow manufacturer’s guidelines.

Noise, Squeaks, and Panel Movement

Metal roofs shouldn’t make noise when it’s windy or raining. If yours is making a racket, here’s what you can do.

Cause: Insufficient fastening or lack of insulation can lead to panel movement and noise. During installation, if panels weren’t secured properly or there wasn’t enough insulation underneath, they can rattle when it’s windy.

Solution: If you’re hearing squeaks or rattling, first check your fasteners. Make sure they’re tight and haven’t backed out. You might need to re-fasten some panels.

Adding insulation between the roof deck and the metal panels can also help reduce noise. This acts as a buffer, absorbing vibrations and reducing sound transmission. If you decide to add insulation, make sure it’s compatible with your existing roof system and follows manufacturer guidelines.

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