Installing a Metal Roof Over Asphalt Shingles: A Comprehensive Guide

Considering a metal roof for your home? One option that often comes up is installing it directly over your existing asphalt shingles. This method, sometimes referred to as a metal roof overlay, can seem appealing due to its potential to save time and money compared to a complete tear-off. However, it’s crucial to understand the advantages, disadvantages, and proper installation techniques before making a decision. This comprehensive guide will walk you through everything you need to know about installing a metal roof over asphalt shingles, helping you determine if it’s the right choice for your home.

Understanding the Basics: Metal Roofing and Asphalt Shingles

Before diving into the specifics of overlaying a metal roof, let’s establish a clear understanding of the two primary roofing materials involved: metal and asphalt shingles.

Asphalt Shingles: The Ubiquitous Roofing Choice

Asphalt shingles are the most common roofing material in North America, prized for their affordability, ease of installation, and variety of styles and colors. They consist of a fiberglass or organic mat saturated with asphalt and coated with mineral granules. These granules protect the asphalt from UV rays and add color and aesthetic appeal.

There are two main types of asphalt shingles: three-tab shingles and architectural shingles (also known as dimensional or laminated shingles). Three-tab shingles are the simpler and more economical option, while architectural shingles offer a more textured and visually appealing look, along with greater durability.

The lifespan of asphalt shingles typically ranges from 15 to 30 years, depending on the quality of the shingles, the climate, and proper installation and maintenance.

Metal Roofing: Durability and Longevity

Metal roofing, on the other hand, is known for its exceptional durability, longevity, and energy efficiency. Metal roofs can last 50 years or more, often outperforming asphalt shingles significantly. They are also highly resistant to fire, wind, and impact damage.

Metal roofs come in a variety of materials, including steel (galvanized, galvalume, and weathering steel), aluminum, copper, and zinc. Each material offers different benefits in terms of cost, durability, and aesthetics.

Metal roofing can be formed into different styles, such as standing seam, corrugated, and metal shingles or shakes. Standing seam roofs, characterized by their vertical panels and concealed fasteners, are particularly popular for their sleek appearance and weather tightness.

The Allure of a Metal Roof Overlay: Why Consider It?

The idea of installing a metal roof directly over asphalt shingles is appealing for several reasons, primarily related to cost and convenience.

Cost Savings: Avoiding the Tear-Off

The most significant advantage of a metal roof overlay is the potential to save money on labor and disposal costs associated with removing the existing asphalt shingles. Tearing off an old roof can be a time-consuming and labor-intensive process, adding significantly to the overall project expense. By avoiding the tear-off, you can potentially reduce the total cost of your new roof.

Time Savings: A Faster Installation

Overlaying a metal roof can also save time on the installation process. Without the need for demolition, the installation crew can focus solely on preparing the existing roof surface and installing the new metal panels. This can shorten the project timeline and minimize disruption to your daily life.

Reduced Landfill Waste: An Environmentally Friendly Option

Another benefit of a metal roof overlay is that it reduces the amount of waste sent to landfills. Asphalt shingles are a significant source of construction and demolition debris. By leaving the old shingles in place, you’re contributing to a more sustainable building practice.

Additional Insulation (Potentially): A Layer of Protection

While not a primary benefit, the existing layer of asphalt shingles can provide a small amount of additional insulation to your home. This can help to improve energy efficiency and reduce heating and cooling costs.

The Drawbacks of a Metal Roof Overlay: Potential Problems to Consider

Despite the apparent advantages, there are several potential drawbacks to installing a metal roof over asphalt shingles that must be carefully considered. These issues can outweigh the benefits in certain situations.

Concealing Existing Problems: A Recipe for Disaster

One of the biggest risks of overlaying a metal roof is that it can conceal underlying problems with the existing roof deck. Leaks, rot, or structural damage may be hidden beneath the asphalt shingles, and you won’t be able to address them properly without removing the old roof. Installing a new roof over a damaged deck is like building a house on a weak foundation – it’s likely to lead to problems down the road.

Moisture Trapping: A Breeding Ground for Mold and Rot

Another concern is the potential for moisture to become trapped between the metal roof and the asphalt shingles. This trapped moisture can create a breeding ground for mold, mildew, and rot, which can damage the roof deck and compromise the structural integrity of your home. Proper ventilation is crucial to prevent this issue.

Added Weight: Structural Considerations

Adding a metal roof over asphalt shingles increases the overall weight load on your roof structure. It’s essential to ensure that your roof framing is strong enough to support the added weight. Consulting with a structural engineer is recommended, especially if you have any concerns about the structural integrity of your roof.

Reduced Lifespan (Potentially): Impact on Metal Roof Longevity

While metal roofs are known for their longevity, overlaying them over asphalt shingles can potentially reduce their lifespan in certain climates. The trapped heat and moisture can accelerate corrosion and degradation of the metal, particularly in areas with high humidity or extreme temperature fluctuations.

Aesthetics: Potential Unevenness

Depending on the condition of the existing asphalt shingles, the surface may be uneven or bumpy. This unevenness can telegraph through the metal roof, creating a less aesthetically pleasing appearance. While underlayment can help to mitigate this issue, it may not completely eliminate it.

Warranty Issues: Potential for Voided Coverage

Some metal roofing manufacturers may void their warranties if their products are installed over asphalt shingles. Be sure to check the manufacturer’s specifications and warranty requirements carefully before proceeding with an overlay installation. It’s crucial to ensure that the installation method complies with the manufacturer’s recommendations to maintain warranty coverage.

Building Codes: Local Regulations and Requirements

Local building codes may restrict or prohibit the installation of metal roofs over asphalt shingles. Check with your local building department to determine if overlaying a metal roof is permitted in your area and if there are any specific requirements that must be met. Some jurisdictions may require a tear-off if the existing roof has more than one layer of shingles.

Determining If a Metal Roof Overlay is Right for You: A Checklist

Now that you understand the pros and cons of installing a metal roof over asphalt shingles, how do you determine if it’s the right choice for your home? Here’s a checklist of factors to consider:

The Condition of Your Existing Roof: A Thorough Inspection

The most important factor is the condition of your existing asphalt shingles. If your roof is old, damaged, or has any signs of leaks, rot, or structural issues, a tear-off is almost always the best option. Attempting to overlay a metal roof on a compromised roof deck is simply not worth the risk.

Look for signs of the following:

• Missing or broken shingles
• Curling or buckling shingles
• Granule loss
• Water stains on the ceiling
• Sagging roof deck
• Visible signs of rot or mold

If you notice any of these problems, it’s essential to have a professional roofer inspect your roof thoroughly to assess the extent of the damage. They can provide you with a detailed report and recommend the best course of action.

The Slope of Your Roof: Minimum Requirements

Metal roofs generally require a minimum roof slope to ensure proper water runoff. Check the manufacturer’s specifications for the specific metal roofing product you’re considering. If your roof slope is too shallow, a metal roof overlay may not be suitable.

Local Climate: Considerations for Humidity and Temperature

The climate in your area can also influence the suitability of a metal roof overlay. In areas with high humidity or extreme temperature fluctuations, the potential for moisture trapping and accelerated corrosion is higher. In these climates, a tear-off and proper ventilation are often recommended to ensure the longevity of the metal roof.

Structural Integrity: Assessing the Weight-Bearing Capacity

It’s crucial to assess the structural integrity of your roof framing to ensure that it can support the added weight of the metal roof and the existing asphalt shingles. If you have any concerns about the structural capacity of your roof, consult with a structural engineer. They can perform an inspection and provide you with recommendations for reinforcing the roof structure if necessary.

Budget Considerations: Weighing the Costs and Benefits

While a metal roof overlay can potentially save money upfront, it’s important to weigh the long-term costs and benefits. If you end up having to deal with hidden problems down the road, the cost of repairs can easily outweigh the initial savings. Consider the potential for reduced lifespan and warranty issues when making your decision.

Warranty Requirements: Ensuring Compliance

Carefully review the warranty requirements for the metal roofing product you’re considering. Ensure that the overlay installation method complies with the manufacturer’s specifications to maintain warranty coverage. If the manufacturer requires a tear-off for warranty validation, it’s best to follow their recommendations.

Building Codes: Adhering to Local Regulations

Check with your local building department to determine if metal roof overlays are permitted in your area and if there are any specific requirements that must be met. Adhering to local building codes is essential for ensuring the safety and legality of your roofing project.

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

If you’ve carefully considered the pros and cons and determined that a metal roof overlay is the right choice for your home, here’s a step-by-step guide to the installation process. Important Note: Installing a metal roof is a complex and potentially dangerous task. It’s highly recommended to hire a qualified and experienced roofing contractor to ensure a safe and proper installation. This guide is for informational purposes only and should not be considered a substitute for professional advice.

Phase 1: Preparation and Inspection

The preparation phase is critical to ensure a successful metal roof overlay. This involves a thorough inspection, cleaning, and preparation of the existing asphalt shingle surface.

1. Thorough Inspection: Identifying and Addressing Problem Areas

Begin by conducting a thorough inspection of the existing asphalt shingles. Look for any signs of damage, such as missing or broken shingles, curling or buckling shingles, granule loss, or water stains. Identify and mark any areas that need to be repaired or addressed before proceeding with the installation.

2. Cleaning the Existing Roof: Removing Debris and Dirt

Clean the existing roof surface thoroughly to remove any debris, dirt, moss, or algae. Use a broom, leaf blower, or a pressure washer (with a low-pressure setting) to clean the shingles. Be careful not to damage the shingles during the cleaning process.

3. Addressing Unevenness: Creating a Smooth Surface

If the existing asphalt shingles are uneven or bumpy, you may need to address these issues to create a smoother surface for the metal roof. One option is to install a layer of underlayment over the shingles. This can help to even out the surface and provide a barrier against moisture. Another option is to remove any particularly uneven shingles and replace them with new ones.

4. Repairing Damaged Areas: Addressing Any Underlying Issues

Repair any damaged areas of the existing roof before proceeding with the metal roof installation. Replace any missing or broken shingles, and repair any leaks or damaged flashing. Addressing these issues now will help to prevent problems down the road.

Phase 2: Underlayment Installation (Recommended)

While not always required, installing a layer of underlayment over the existing asphalt shingles is highly recommended. Underlayment provides an additional layer of protection against moisture and helps to create a smoother surface for the metal roof.

1. Choosing the Right Underlayment: Selecting a Suitable Material

Select an underlayment that is specifically designed for use with metal roofs. Synthetic underlayments are generally preferred over traditional felt underlayments, as they are more durable, water-resistant, and resistant to tearing.

2. Installing the Underlayment: Overlapping and Securing

Install the underlayment according to the manufacturer’s instructions. Overlap the edges of the underlayment by at least 6 inches, and secure it to the roof deck with roofing nails or staples. Ensure that the underlayment is installed smoothly and without any wrinkles or bubbles.

3. Sealing the Seams (Optional): Adding Extra Protection

For added protection against moisture, you can seal the seams of the underlayment with roofing sealant. This will help to prevent water from penetrating the underlayment and reaching the asphalt shingles below.

Phase 3: Metal Roof Installation

The final phase involves installing the metal roofing panels according to the manufacturer’s instructions. This requires careful attention to detail and proper fastening techniques.

1. Installing Furring Strips (Optional but Highly Recommended): Creating an Air Gap

While not always required, installing furring strips (also known as battens) over the underlayment is highly recommended, especially in climates with high humidity or extreme temperature fluctuations. Furring strips create an air gap between the metal roof and the asphalt shingles, which helps to improve ventilation and reduce the risk of moisture trapping. This can significantly extend the lifespan of both the metal roof and the existing asphalt shingles.

The furring strips are typically made of wood and are installed vertically, spaced according to the manufacturer’s specifications. The metal roofing panels are then attached to the furring strips.

2. Cutting the Metal Panels: Accurate Measurements and Precision Cuts

Measure and cut the metal roofing panels to the correct size and shape, according to the dimensions of your roof. Use a metal cutting saw or shears to cut the panels cleanly and accurately. Wear safety glasses and gloves to protect yourself from sharp edges.

3. Installing the Metal Panels: Following Manufacturer’s Instructions

Install the metal roofing panels according to the manufacturer’s instructions. Overlap the panels according to the specified overlap requirements, and secure them to the roof deck or furring strips with the appropriate fasteners. Use the correct type and size of fasteners to ensure a secure and watertight connection.

4. Fastening Techniques: Ensuring a Secure Attachment

Proper fastening techniques are crucial for ensuring a secure and watertight metal roof installation. Use the correct type of fasteners for the metal roofing material and the roof deck. Space the fasteners according to the manufacturer’s specifications. Over-tightening the fasteners can damage the metal panels, while under-tightening can lead to leaks.

5. Installing Flashing: Preventing Water Intrusion

Install flashing around chimneys, vents, skylights, and other roof penetrations to prevent water intrusion. Use the appropriate type of flashing for each application, and ensure that it is properly sealed to the metal roofing panels.

6. Finishing Touches: Adding Ridge Caps and Trim

Install ridge caps and trim to complete the metal roof installation. Ridge caps are installed along the ridge of the roof to provide a watertight seal. Trim is installed along the edges of the roof to create a clean and finished look.

Phase 4: Final Inspection and Cleanup

After the metal roof is installed, conduct a final inspection to ensure that everything is properly installed and that there are no leaks or other problems.

1. Thorough Inspection: Checking for Leaks and Issues

Carefully inspect the entire roof for any signs of leaks, loose fasteners, or other problems. Pay particular attention to areas around flashing, chimneys, and vents.

2. Cleanup: Removing Debris and Materials

Clean up any debris or materials from the roof and the surrounding area. Dispose of any waste materials properly.

Maintaining Your Metal Roof: Ensuring Longevity

Once your metal roof is installed, it’s important to maintain it properly to ensure its longevity and performance.

Regular Inspections: Catching Problems Early

Inspect your metal roof at least once a year, preferably in the spring and fall. Look for any signs of damage, such as loose fasteners, corrosion, or dents.

Cleaning: Removing Debris and Grime

Clean your metal roof periodically to remove any debris, dirt, or grime. Use a soft brush and a mild detergent solution. Avoid using harsh chemicals or abrasive cleaners, as they can damage the metal.

Removing Snow and Ice: Preventing Damage

In areas with heavy snowfall, it’s important to remove snow and ice from your metal roof to prevent damage. Use a roof rake to remove the snow, and be careful not to damage the metal panels. Avoid using sharp tools or chemicals to melt the ice, as they can also damage the metal.

Addressing Issues Promptly: Preventing Further Damage

If you notice any problems with your metal roof, address them promptly to prevent further damage. Repair any leaks, replace any loose fasteners, and remove any corrosion.

Conclusion: Making an Informed Decision

Installing a metal roof over asphalt shingles can be a viable option in certain situations, but it’s crucial to carefully consider the pros and cons before making a decision. Assess the condition of your existing roof, the climate in your area, the structural integrity of your roof framing, and your budget. Consult with a qualified and experienced roofing contractor to get professional advice and ensure a safe and proper installation.

Remember that while saving money upfront is tempting, the long-term costs of a poorly executed overlay can far outweigh the initial savings. Prioritize the structural integrity of your roof and the longevity of your roofing system to ensure a safe and sound home for years to come. By making an informed decision, you can enjoy the benefits of a durable and beautiful metal roof without compromising the integrity of your home.