Common Problems with Installing Metal Roofs Over Shingles

Metal roofs are renowned for their durability, energy efficiency, and longevity, making them a popular choice for homeowners looking to upgrade their roofing systems. However, some homeowners opt to install a metal roof directly over their existing asphalt shingles in an effort to save money and reduce installation time.

While this approach may seem practical, it comes with significant risks that can compromise the roof’s performance and structural integrity. Understanding these challenges can help homeowners make informed decisions when considering a metal roof installation.

Why Installing Metal Roofing Over Shingles Can Be Problematic

1. Poor Ventilation and Trapped Moisture

Proper attic ventilation is essential for regulating temperature and preventing moisture buildup within the roofing structure. When a metal roof is installed over shingles without adequate ventilation, moisture can become trapped between the two layers.

• Impact on Home Efficiency: Excess heat and humidity can cause higher energy bills as the HVAC system struggles to maintain a comfortable indoor temperature.

• Structural Risks: Prolonged exposure to moisture can lead to mold growth, wood rot, and a weakened roof deck.

• Potential Condensation Issues: In colder climates, trapped moisture can freeze and expand, causing structural damage to the roof.

2. Rust and Corrosion Risks

Metal roofing panels are highly resistant to weather-related damage, but their lifespan can be significantly reduced if they are exposed to excessive moisture.

• How Moisture Causes Rust: Asphalt shingles are designed to breathe, allowing moisture to escape. However, when covered with metal panels, trapped moisture accelerates rust formation on the underside of the metal roof.

• Impact on Longevity: Corrosion weakens the metal panels, reducing their lifespan and increasing the likelihood of premature failure.

3. Excessive Weight Load on the Roof Structure

Installing a new metal roof over an existing shingle roof adds additional weight to the home’s structure. Although metal roofing is lighter than asphalt shingles, the cumulative weight of both layers can strain the roof’s support system.

• Risk of Structural Damage: Older homes or those with weakened roof decking may be unable to support the added weight, leading to sagging or, in extreme cases, structural collapse.

• Building Code Compliance: Many local building codes limit the number of roofing layers allowed, requiring the removal of old shingles before a new roof can be installed.

4. Voiding Manufacturer Warranties

Most roofing manufacturers offer warranties that cover defects in materials and installation. However, these warranties often require that the roof be installed according to specific guidelines.

• Warranty Implications: Installing metal roofing over shingles may void the manufacturer’s warranty, leaving the homeowner responsible for any future repairs or replacements.

• Lack of Coverage for Defects: If problems arise due to improper installation, the homeowner may be denied warranty benefits, resulting in significant out-of-pocket expenses.

5. Insurance and Coverage Challenges

Insurance companies have strict guidelines regarding roof installation methods. Installing a metal roof over an existing shingle roof can create issues when filing a claim for damages.

• Policy Limitations: Some insurers may deny claims if the installation method does not meet their standards.

• Higher Premiums or Reduced Coverage: Homes with multiple roofing layers may be considered higher risk, leading to increased insurance costs or limited coverage options.

The Proper Way to Install a Metal Roof

To avoid the risks associated with installing metal roofing over shingles, homeowners should follow best practices to ensure a durable and efficient roofing system.

1. Remove Old Shingles Before Installation

Stripping the old roof down to the decking allows for proper inspection of the roof structure and ensures a smooth, even surface for metal panel installation.

• Benefits of Removal:

  • Prevents moisture entrapment and condensation issues.

  • Reduces overall roof weight, protecting structural integrity.

  • Ensures proper attachment of metal roofing panels.

2. Install a High-Quality Underlayment

A synthetic underlayment provides an additional layer of protection against moisture infiltration and improves energy efficiency.

• Recommended Underlayment Options:

  • Synthetic Underlayment: Lightweight and highly resistant to moisture.

  • Peel-and-Stick Membrane: Ideal for high-moisture areas and regions prone to ice dams.

3. Optimize Ventilation and Insulation

Proper attic ventilation prevents heat buildup and condensation-related issues.

• Ventilation Enhancements:

  • Install ridge vents, soffit vents, or gable vents to facilitate airflow.

  • Ensure insulation is evenly distributed to prevent temperature fluctuations.

4. Hire a Professional Roofing Contractor

Working with an experienced metal roofing contractor ensures that the installation meets industry standards and local building codes.

• Qualities to Look for in a Roofing Contractor:

  • Licensed and insured for metal roofing installations.

  • Experience with removing old roofing materials and preparing the structure.

  • Strong customer reviews and manufacturer certifications.

Comparing Metal Roofs Over Shingles vs. Full Replacement

Factor	Metal Roof Over Shingles	Full Roof Replacement
Cost	Lower initial cost	Higher upfront investment
Longevity	Shortened lifespan due to moisture and ventilation issues	Full lifespan of 40+ years
Energy Efficiency	Reduced efficiency due to trapped heat	Optimal insulation and ventilation
Structural Impact	Increased weight load	Maintains structural integrity
Warranty Coverage	May be voided	Fully covered by manufacturer
Insurance Approval	Potential claim issues	Higher likelihood of approval

A Trusted Roofing Partner in Lehigh Valley

For homeowners considering a metal roof installation, working with a certified roofing contractor ensures long-term performance and protection. East Penn Roofing, located in Lehigh Valley, PA, provides expert roofing solutions backed by top industry certifications. As an OSHA Safety Certified, Owens Corning Preferred Contractor, CertainTeed Shingle Select Contractor, GAF Certified Installer, and HAAG Certified company, East Penn Roofing guarantees high-quality installations tailored to meet local climate conditions and building standards.

Conclusion

Installing a metal roof over an existing shingle roof may seem like a cost-effective solution, but it introduces risks such as moisture buildup, structural strain, warranty voidance, and insurance complications. The best approach is to remove the old roofing materials, install a proper underlayment, and ensure adequate ventilation. Hiring an experienced roofing professional guarantees a long-lasting, efficient, and structurally sound metal roofing system.

Frequently Asked Questions

1. Can a metal roof be installed over two layers of shingles?

Most building codes prohibit installing a metal roof over multiple layers of shingles due to weight concerns and ventilation issues. It is best to remove all existing roofing layers before installation.

2. How long does a properly installed metal roof last?

A well-installed metal roof can last 40–70 years, depending on the material and maintenance. Removing old shingles before installation maximizes its lifespan.

3. Is a metal roof heavier than asphalt shingles?

No, metal roofing is typically lighter than asphalt shingles. However, installing it over an existing shingle roof adds extra weight that can stress the home’s structure.

4. Will installing a metal roof over shingles save money?

While it may reduce initial labor costs, long-term expenses due to ventilation issues, warranty voidance, and potential structural damage often outweigh the short-term savings.

5. What is the best underlayment for a metal roof?

A synthetic underlayment or peel-and-stick waterproof membrane provides the best protection against moisture infiltration and enhances roof durability.

Read also our blog: Best Asphalt Shingle Colors for Homes in 2024