Choosing Between Metal and Asphalt Shingle Roofs: Which is Better?

When it comes to replacing an existing roof, the decision between installing metal roofing over asphalt shingles or replacing both entirely is a significant one. This guide will provide you with the essential information to make an informed decision, ensuring your roof is both functional and aesthetically pleasing. We will cover the correct procedures for installing metal roofing over asphalt shingles and explain why it is generally not recommended to replace both.

Installing Metal Roofing Over Asphalt Shingles

While it is possible to install a metal roof over an existing asphalt shingle roof, it is crucial to follow the appropriate procedures to ensure longevity and prevent issues such as rusting and degradation. Here are the steps typically recommended:

Install a Moisture Barrier: A moisture barrier is necessary to prevent moisture from penetrating the new metal roof and causing damage beneath. Install Wood Strapping or Furring Strips: These strips provide a secure base for the metal roofing to be fastened to. Secure the Metal Roof: The metal roofing should be securely fastened to the strapping to ensure stability.

It is unequivocal that metal roofing should not be installed directly over shingles or roof sheathing/plywood. There needs to be an air gap between the old roof and the new metal roofing to prevent sweat and rusting. This is explicitly stated in metal roofing warranties. If someone suggests otherwise, it is likely that they lack experience or are trying to sell a cheap metal roofing job. Unfortunately, this practice is increasingly common and should be avoided.

Replacing the Entire Roof

When considering a complete replacement of the existing roof, there are several benefits to doing so. Removing the old asphalt shingles and replacing them with a new metal roof can provide long-term cost savings and increased insulation benefits. However, it is vital to carefully consider the cost and environmental impact of removing and disposing of the old roof.

Environmental Considerations

While asbestos shingles are no longer used, many homes built in the mid-20th century may still have asbestos-containing materials. If your home has these materials, removing them safely and properly is crucial. Additionally, the cost of removing the old roof and hauling it to a landfill can be substantial. An amortized cost analysis considering future benefits should be conducted before making a final decision.

Longevity of Metal Roofing

A proper metal roofing installation can extend the life of your roof significantly. It is worth noting that some metal roofs are designed to outlive the structure they are installed on. Therefore, if you expect your home to last for decades, a metal roof can be a worthwhile long-term investment.

Why Galvanized Steel Roofing is Best

While some may consider the old shingle roof as a layer of insulation, the benefits of a standalone asphalt shingle roof are generally outweighed by the superior performance and maintenance requirements of metal roofing. Here are a few reasons why galvanized steel roofing is preferred:

Prevent Electrolysis: Ensuring that galvanized steel is not in contact with moisture-retentive materials such as concrete or wood prevents electrolysis, which can eat through the zinc coating and cause rapid rusting. Protect Against Damage: Asphalt shingles have sharp, hard granules that can damage the undercoating of metal roofing, causing rapid rust and degradation. Temperature Expansion: The different expansion coefficients of metal and asphalt shingles can lead to high rates of temperature change, further exacerbating the damage from the granules.

Using a thin layer of plywood or a heavy plastic sheet as a lubrication layer can help mitigate some of these issues, but it is not an ideal solution. Long screws and appropriate underlayment are essential to ensure a proper bond and prevent damage.

Conclusion

In conclusion, while it is possible to install a metal roof over an existing asphalt shingle roof, this method should be approached with caution and proper installation techniques. For the best results, consider replacing the entire roof with a high-quality metal roofing system. This approach not only ensures a longer-lasting roof but also provides better insulation and a more durable solution for your home.