Installing a Metal Roof Over Asphalt Shingles: A Comprehensive Guide

Introduction: Weighing Your Roofing Options

Deciding on a new roof is a significant investment and a pivotal moment for any homeowner. The roof is, after all, your home’s primary defense against the elements. When the time comes to replace an aging or damaged asphalt shingle roof, many homeowners find themselves considering the advantages of metal roofing. Metal roofs, known for their longevity, durability, and energy efficiency, represent a compelling alternative to traditional materials. One particularly appealing option is installing a metal roof directly over the existing asphalt shingles. While not suitable for every situation, this approach can save time, money, and the hassle of a complete tear-off. This comprehensive guide will explore the process of installing a metal roof over asphalt shingles, discussing the benefits, potential drawbacks, necessary preparations, and step-by-step instructions.

Understanding the Advantages of Metal Roofing

Before delving into the specifics of installing a metal roof over shingles, it’s essential to understand the core benefits that make metal roofing such an attractive choice. These advantages often outweigh the higher upfront cost, making metal roofing a long-term investment in your home’s value and protection.

Longevity and Durability

One of the most significant advantages of metal roofing is its exceptional lifespan. Unlike asphalt shingles, which typically last 15-20 years, a metal roof can endure for 40-70 years or even longer with proper maintenance. This extended lifespan translates to fewer replacement cycles and lower overall costs over the long term. Metal roofs are also highly resistant to various weather conditions, including strong winds, heavy snow, hail, and fire. They are less susceptible to cracking, curling, or deteriorating compared to asphalt shingles, ensuring lasting protection for your home.

Energy Efficiency

Metal roofs are known for their energy efficiency, particularly those with reflective coatings. These coatings reflect a significant portion of the sun’s radiant heat, reducing the amount of heat absorbed by the roof and subsequently transferred into the attic and living spaces. This can lead to lower cooling costs during the summer months, saving homeowners money on their energy bills. In colder climates, metal roofs can also help retain heat inside the home, contributing to lower heating costs. The energy efficiency of metal roofing contributes to a more comfortable indoor environment and a reduced carbon footprint.

Aesthetic Appeal

Metal roofs are available in a wide variety of styles, colors, and finishes, allowing homeowners to enhance the aesthetic appeal of their homes. From traditional standing seam panels to metal shingles that mimic the look of slate or tile, there’s a metal roofing option to complement virtually any architectural style. The clean lines and modern appearance of metal roofs can add a touch of sophistication and elegance to a home’s exterior, increasing its curb appeal and potentially boosting its resale value.

Reduced Maintenance

Compared to asphalt shingle roofs, metal roofs require minimal maintenance. They are resistant to moss, algae, and mildew growth, which can stain and degrade asphalt shingles. Regular inspections and occasional cleaning are typically all that’s needed to keep a metal roof in top condition. This reduced maintenance translates to less time and money spent on upkeep, allowing homeowners to focus on other aspects of homeownership.

Environmentally Friendly

Metal roofing is a sustainable choice for environmentally conscious homeowners. Many metal roofs are made from recycled materials, and at the end of their lifespan, they can be recycled again. This reduces the demand for new materials and minimizes waste. The energy efficiency of metal roofs also contributes to a smaller carbon footprint by reducing energy consumption. Additionally, metal roofs do not contain the harmful chemicals found in some other roofing materials, making them a safer option for the environment and your family.

Evaluating the Suitability of Installing Metal Over Shingles

While installing a metal roof over asphalt shingles can offer numerous advantages, it’s crucial to assess the suitability of your existing roof and the overall situation before proceeding. This method is not a one-size-fits-all solution, and certain factors can make it impractical or even detrimental.

The Condition of the Existing Shingles

The most critical factor to consider is the condition of the existing asphalt shingles. If the shingles are severely damaged, deteriorated, or have multiple layers, installing a metal roof over them is generally not recommended. A solid, relatively flat, and stable surface is essential for proper metal roof installation and performance. Multiple layers of shingles can trap moisture, leading to rot and decay, and create an uneven surface that can compromise the integrity of the metal roof. If the existing shingles are in poor condition, a complete tear-off is the better option.

Local Building Codes and Regulations

Before starting any roofing project, it’s imperative to check with your local building department to determine if there are any restrictions or regulations regarding installing a metal roof over asphalt shingles. Some municipalities may prohibit this practice altogether, while others may have specific requirements regarding the type of metal roofing allowed or the condition of the existing shingles. Ignoring local codes can result in fines, delays, and the need to remove the newly installed roof.

Weight Considerations

While metal roofing is generally lighter than tile or slate, it still adds weight to the existing structure. It’s crucial to assess whether the roof framing can handle the additional weight of the metal roof and the existing shingles. If the roof framing is weak or compromised, it may need to be reinforced before installing the metal roof. Consulting with a structural engineer is recommended if you have any concerns about the structural integrity of your roof.

Ventilation Issues

Proper ventilation is essential for maintaining a healthy and energy-efficient roof system. Installing a metal roof over asphalt shingles can potentially impede ventilation if not done correctly. Trapped moisture between the shingles and the metal roof can lead to condensation, mold growth, and structural damage. It’s crucial to ensure that the roof system has adequate ventilation to prevent these problems. This may involve installing additional vents or using a vented underlayment that allows airflow between the shingles and the metal roof.

Potential for Future Problems

While covering up old shingles can seem like a quick and easy solution, it’s important to consider the potential for future problems. Any existing issues with the underlying roof structure, such as leaks or rot, will be hidden beneath the metal roof and may worsen over time. These hidden problems can eventually lead to more extensive and costly repairs in the future. A thorough inspection of the existing roof is crucial to identify and address any potential issues before installing the metal roof.

Preparing for Metal Roof Installation

Proper preparation is paramount for a successful metal roof installation over asphalt shingles. This involves a thorough inspection of the existing roof, necessary repairs, and the installation of an underlayment to create a smooth and protective surface for the metal roofing panels.

Roof Inspection and Repairs

Begin with a comprehensive inspection of the existing asphalt shingles. Look for signs of damage, such as missing shingles, cracks, curling, or granule loss. Identify any areas of rot or decay in the underlying roof structure. If you find any significant damage, it’s crucial to repair it before proceeding with the metal roof installation. This may involve replacing damaged shingles, repairing rotted sheathing, or reinforcing weak areas of the roof framing. Addressing these issues early on will ensure a solid foundation for the metal roof and prevent future problems.

Cleaning the Existing Shingles

Once you’ve completed any necessary repairs, clean the existing asphalt shingles thoroughly. Remove any debris, such as leaves, branches, and dirt, using a broom or leaf blower. If there’s any moss or algae growth, use a cleaning solution specifically designed for asphalt shingles to remove it. Avoid using harsh chemicals or pressure washers, as these can damage the shingles. A clean surface will allow the underlayment to adhere properly and provide a smooth surface for the metal roofing panels.

Installing Underlayment

Underlayment is a crucial component of any roofing system, providing an extra layer of protection against moisture and wind. When installing a metal roof over asphalt shingles, the underlayment also helps to create a smooth and even surface for the metal roofing panels. There are several types of underlayment available, including synthetic underlayment, felt paper, and self-adhering membranes. Synthetic underlayment is a popular choice due to its durability, water resistance, and ease of installation. Felt paper is a more traditional option, but it’s less durable and more susceptible to tearing. Self-adhering membranes provide the best protection against moisture, but they are also the most expensive. Choose an underlayment that is appropriate for your climate and budget.
Install the underlayment according to the manufacturer’s instructions. Typically, this involves overlapping the edges of the underlayment by several inches and securing it to the roof with roofing nails or staples. Ensure that the underlayment is smooth and free of wrinkles or creases. Pay particular attention to areas around chimneys, vents, and other roof penetrations, as these are common areas for leaks.

Step-by-Step Guide to Installing a Metal Roof Over Shingles

With the necessary preparations complete, you can now proceed with the installation of the metal roof. This process involves measuring and cutting the metal roofing panels, installing the panels according to the manufacturer’s instructions, and flashing around chimneys, vents, and other roof penetrations.

Measuring and Cutting the Metal Roofing Panels

Accurate measurements are essential for a successful metal roof installation. Start by measuring the length and width of the roof, taking into account any overhangs or extensions. Use these measurements to determine the number of metal roofing panels you’ll need. When ordering the panels, it’s always a good idea to add a few extra to account for waste and any unexpected issues.
Metal roofing panels can be cut using a variety of tools, including metal shears, nibblers, and circular saws with metal-cutting blades. Choose a tool that is appropriate for the type of metal roofing you’re using and follow the manufacturer’s instructions. Always wear safety glasses and gloves when cutting metal to protect yourself from sharp edges and flying debris.
When cutting the panels, it’s important to make clean, straight cuts. Use a metal cutting guide or straight edge to ensure accuracy. Avoid using abrasive blades, as these can create excessive heat and damage the finish on the metal panels.

Installing the Metal Roofing Panels

The installation process will vary depending on the type of metal roofing you’re using. Standing seam metal roofs, for example, are typically installed using clips that are attached to the roof deck. The panels are then snapped into the clips, creating a watertight seal. Metal shingles are installed in a similar manner to asphalt shingles, using roofing nails or screws to secure them to the roof deck.
Regardless of the type of metal roofing you’re using, it’s crucial to follow the manufacturer’s instructions carefully. These instructions will provide specific details on the proper spacing of fasteners, the overlap of panels, and any other special considerations.
Start by installing the first row of panels along the eave of the roof. Ensure that the panels are aligned properly and that the edges are straight. Overlap the panels according to the manufacturer’s instructions. Secure the panels to the roof deck using the appropriate fasteners.
Continue installing the panels row by row, working your way up the roof. Maintain consistent spacing and overlap between the panels. Periodically check your work to ensure that the panels are aligned properly and that the roof is square.
When you reach the ridge of the roof, you’ll need to install ridge caps to cover the exposed edges of the metal panels. Ridge caps are typically made from the same material as the roofing panels and are designed to provide a watertight seal.

Flashing Around Chimneys, Vents, and Other Roof Penetrations

Flashing is a critical component of any roofing system, as it prevents water from entering the roof around chimneys, vents, and other roof penetrations. When installing a metal roof over asphalt shingles, it’s important to install new flashing to ensure a watertight seal.
Remove the existing flashing around the chimney, vent, or other roof penetration. Clean the area thoroughly and apply a layer of roofing sealant. Install the new flashing, overlapping the metal roofing panels and the underlayment. Secure the flashing to the roof deck using roofing nails or screws.
Ensure that the flashing is properly sealed to prevent water from entering the roof. Apply a bead of roofing sealant around the edges of the flashing. Inspect the flashing regularly to ensure that it remains in good condition.

Addressing Potential Challenges

Even with careful planning and execution, installing a metal roof over asphalt shingles can present certain challenges. Being prepared to address these challenges will help ensure a successful outcome.

Uneven Roof Surface

One of the most common challenges is dealing with an uneven roof surface. Asphalt shingles can settle and deform over time, creating an uneven surface that can affect the appearance and performance of the metal roof.
To address this challenge, you may need to use shims or furring strips to create a level surface for the metal roofing panels. Shims are thin pieces of wood or plastic that can be inserted under the panels to level them out. Furring strips are longer pieces of wood that are attached to the roof deck to create a level plane.
When using shims or furring strips, it’s important to ensure that they are securely fastened to the roof deck and that they are properly spaced to provide adequate support for the metal roofing panels.

Ventilation Issues

As mentioned earlier, installing a metal roof over asphalt shingles can potentially impede ventilation if not done correctly. This can lead to condensation, mold growth, and structural damage.
To address this challenge, you may need to install additional vents to increase airflow in the attic. Ridge vents are a popular choice, as they allow warm, moist air to escape from the attic. Soffit vents are another option, as they allow cool, dry air to enter the attic.
It’s also important to ensure that the underlayment is breathable, allowing moisture to escape from the roof system. Synthetic underlayments are generally more breathable than felt paper.

Fastener Selection

Choosing the right fasteners is crucial for a successful metal roof installation. The fasteners must be compatible with the type of metal roofing you’re using and they must be able to withstand the effects of weather and corrosion.
Stainless steel fasteners are generally the best choice for metal roofing, as they are highly resistant to corrosion. However, stainless steel fasteners can be more expensive than other types of fasteners.
It’s also important to choose fasteners that are the correct length. The fasteners must be long enough to penetrate the metal roofing panels, the underlayment, and the roof deck, but they should not be so long that they protrude through the other side of the roof deck.

Maintenance and Care of Your New Metal Roof

Once your metal roof is installed, proper maintenance and care will help ensure its longevity and performance. Regular inspections and occasional cleaning are typically all that’s needed to keep a metal roof in top condition.

Regular Inspections

Inspect your metal roof at least twice a year, typically in the spring and fall. Look for signs of damage, such as loose panels, damaged flashing, or corrosion. Pay particular attention to areas around chimneys, vents, and other roof penetrations.
If you find any damage, repair it promptly to prevent further deterioration. Loose panels can be re-secured with new fasteners. Damaged flashing can be replaced or repaired. Corrosion can be treated with a rust inhibitor.

Cleaning Your Metal Roof

Clean your metal roof periodically to remove dirt, debris, and moss or algae growth. Use a soft brush and a mild detergent to clean the roof. Avoid using harsh chemicals or pressure washers, as these can damage the finish on the metal panels.
Rinse the roof thoroughly with clean water after cleaning. Ensure that all soap residue is removed.

Preventative Measures

Take preventative measures to protect your metal roof from damage. Trim trees and shrubs that overhang the roof to prevent branches from falling and damaging the panels. Remove snow and ice buildup in the winter to prevent excessive weight on the roof.
Keep gutters and downspouts clean to ensure proper drainage. Clogged gutters can cause water to back up onto the roof, leading to leaks and damage.

Conclusion: A Durable and Sustainable Roofing Solution

Installing a metal roof over asphalt shingles can be a cost-effective and sustainable way to improve the durability, energy efficiency, and aesthetic appeal of your home. By carefully evaluating the suitability of your existing roof, preparing the surface properly, and following the manufacturer’s instructions, you can achieve a successful metal roof installation that will provide years of reliable protection. Remember to prioritize safety throughout the entire process and consult with professionals when needed. With proper maintenance and care, your new metal roof will not only enhance the value of your home but also contribute to a more sustainable future.

Disclaimer

This guide is intended for informational purposes only and should not be considered a substitute for professional advice. Roofing projects can be dangerous, and it’s essential to prioritize safety and follow all applicable building codes and regulations. Always consult with qualified professionals before undertaking any roofing work.