What are the most common questions about installing metal roofing over shingles?
What shingle conditions prevent roofing over them with metal?
Several shingle conditions make roofing over them with metal inadvisable, primarily focusing on stability, moisture, and excessive unevenness. Critically, any signs of rot, significant granule loss exposing the asphalt mat, multiple layers of existing shingles, or severe curling, warping, or sagging necessitate a complete tear-off before installing a metal roof.
The fundamental principle behind successfully installing a metal roof over shingles is creating a solid and stable substrate. If the existing shingles are deteriorated, they cannot provide this necessary foundation. Rotting shingles will continue to decay under the metal, potentially compromising the integrity of the new roof system and leading to leaks and structural damage. Excessive granule loss signals that the shingle's protective layer is gone, and the exposed asphalt is vulnerable to further degradation. Furthermore, uneven surfaces created by curling, warping, or sagging shingles will translate through the metal roofing, creating an unsightly and potentially weakened roof. Multiple layers of shingles significantly increase the weight on the roof structure and trap moisture, accelerating deterioration and making it extremely difficult to properly fasten the metal roofing. Before considering a metal roof-over, a thorough inspection is vital to assess the condition of the existing shingles and address any underlying problems to ensure the long-term performance of the new roof.Do I need underlayment between shingles and the metal roof?
Yes, you absolutely need underlayment between your existing asphalt shingles and the new metal roof. Installing a proper underlayment layer is crucial for the longevity and performance of your metal roof system, even when installing over existing shingles.
Underlayment serves several critical purposes. First, it provides a protective barrier between the abrasive surface of the shingles and the underside of the metal panels. Shingles, especially older ones, can contain granules that can rub against the metal, leading to premature wear and corrosion, particularly from galvanic corrosion if dissimilar metals are in contact. Secondly, the underlayment helps to minimize thermal expansion and contraction noises. Metal roofs expand and contract with temperature changes, and without underlayment, this movement can create unwanted sounds as the metal rubs directly against the shingles. Finally, it can provide a degree of fire resistance, particularly if you choose a fire-rated underlayment product. Choosing the right underlayment is important. Synthetic underlayments are generally preferred over traditional felt paper because they are more durable, tear-resistant, and waterproof. Look for an underlayment specifically designed for metal roofing, as these often have features like a textured surface to improve grip and prevent slippage during installation. Consider the climate you live in, as some underlayments are better suited for extreme temperatures or heavy precipitation. Be sure to follow the manufacturer's instructions for proper installation, including overlapping seams and securing the underlayment with appropriate fasteners.How does roofing over shingles affect the metal roof warranty?
Roofing over existing shingles often voids or significantly alters the metal roof warranty. Most manufacturers require direct application to a properly prepared substrate to ensure the metal roofing system performs as intended and to maintain warranty coverage. Deviating from their installation guidelines, especially regarding underlayment and attachment methods when installing over shingles, releases them from liability for issues that may arise.
Warranties are designed based on specific installation procedures that manufacturers have rigorously tested. These procedures account for proper ventilation, secure fastening to prevent wind uplift, and effective drainage to prevent water damage. Installing over shingles can compromise these factors. For example, the shingles can create uneven surfaces that affect the metal's attachment, preventing proper sealing around fasteners. Shingles can also trap moisture, increasing the risk of corrosion to the underside of the metal roof and potentially accelerating deterioration. These conditions are generally outside the scope of what a warranty covers. Furthermore, some warranties explicitly state that they are void if the metal roof is installed over any existing roofing material. Even if the warranty doesn't explicitly forbid it, proving that any issues arising are *not* due to the underlayment of old shingles can be extremely challenging. The manufacturer may argue that the underlying shingles contributed to the problem, making a warranty claim difficult, if not impossible, to pursue successfully. Always thoroughly read and understand the warranty documentation before proceeding with any installation. Contacting the manufacturer directly to clarify their stance on installing over shingles is highly recommended.What's the best way to attach the metal roofing to the shingles?
The best way to attach metal roofing over shingles is typically *not* directly to the shingles themselves. Instead, install a substructure of furring strips or purlins through the shingles and into the roof rafters or trusses. This provides a solid, level, and adequately ventilated base for the metal roofing panels.
Attaching directly to the shingles is generally discouraged for several reasons. Shingles are inherently uneven and flexible, which can lead to an uneven metal roof prone to dents, oil canning, and fastener failure. Furthermore, directly attaching can trap moisture between the metal and the shingles, accelerating corrosion and potentially causing rot in the underlying roof deck. The lack of an air gap also hinders proper ventilation, impacting energy efficiency and potentially shortening the lifespan of both the metal and shingle roofing systems. The preferred method involves installing vertical furring strips, often 1x4 or 2x4 lumber, spaced according to the metal roofing manufacturer's specifications. These strips should be fastened through the shingles and securely anchored to the roof framing. This creates a level plane and an airspace for ventilation. Alternatively, a grid of purlins can be used, especially for more complex roof shapes or when enhanced support is required. Always consult with local building codes and the metal roofing manufacturer's instructions for specific requirements regarding fastener type, spacing, and ventilation.What are the code requirements for metal roofing over existing shingles?
Generally, building codes permit installing metal roofing over one layer of existing asphalt shingles, provided the existing roof is in relatively good condition and meets specific criteria. The key code requirements often focus on the flatness of the existing roof, the secure attachment of the metal roofing, and ensuring the overall roof system can handle the added weight and maintain proper drainage. Always consult your local building department for specific requirements and permit procedures, as codes can vary significantly by jurisdiction.
Building codes prioritize structural integrity and safety. The existing roof's flatness is crucial to prevent distortion of the metal panels and ensure a uniform appearance. Excessive bumps, dips, or curling shingles can create uneven surfaces that compromise the performance and aesthetics of the new metal roof. Before installation, a thorough inspection is essential to identify and address any damaged or deteriorated areas of the existing shingle roof. In some cases, minor repairs or even a complete tear-off might be required to comply with code and manufacturer recommendations. Beyond flatness, proper attachment is paramount. Codes typically specify the type and spacing of fasteners needed to secure the metal roofing to the underlying structure. Fasteners must penetrate through the existing shingles and into the roof decking with sufficient holding power to withstand wind uplift. Furthermore, weight is a significant consideration. The combined weight of the existing shingles and the new metal roofing must not exceed the roof's structural load capacity. Local codes may require a structural engineer's assessment to verify the roof can handle the increased load. Proper ventilation is also crucial; adding a metal roof over shingles can impact airflow in the attic, so ensure adequate ventilation to prevent moisture buildup. Remember to check local regulations for necessary permits, inspections, and approved materials. Ignoring code requirements can result in fines, delays, and potentially unsafe roofing conditions.How does the added weight affect the existing roof structure?
Adding a metal roof over existing shingles increases the load on the roof structure, potentially causing stress and, in extreme cases, failure if the original framing wasn't designed to handle the extra weight. While metal roofing is typically lighter than asphalt shingles, the cumulative weight of both materials, plus any underlayment and fasteners, needs to be assessed against the roof's load-bearing capacity.
The impact of the added weight depends heavily on the existing roof's condition and construction. If the underlying structure—the rafters, trusses, and sheathing—is already weakened by rot, water damage, or poor construction, even a relatively light metal roof can push it beyond its limits. A thorough inspection by a qualified roofing contractor or structural engineer is crucial before proceeding. They can evaluate the integrity of the existing structure and determine if it can safely support the additional load. This inspection should include checking for sagging, water stains, and any other signs of structural distress. Furthermore, local building codes often dictate maximum allowable roof loads. Exceeding these limits can lead to structural problems and potential code violations. It's essential to consult with your local building department to understand the requirements for your area and to ensure that your roof meets those standards. If the existing structure is deemed inadequate, reinforcement may be necessary before installing the metal roof, which can significantly increase the overall project cost. In some instances, removing the existing shingles might be a more cost-effective and structurally sound option.Alright, that's the gist of it! Metal roofing over shingles can be a rewarding DIY project, giving your home a long-lasting and stylish upgrade. Thanks for taking the time to learn with me – I hope this guide has been helpful. If you have any other questions, or just want to explore other home improvement tips, come on back anytime. Happy roofing!