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  • Writer's pictureDarrin Card

The Perils of Multiple Layers of Asphalt Shingles: Lifespan and Building Code Considerations


When a shingle roof needs to be replaced, homeowners have a decision to make; should they remove the existing layer or install an additional layer over the top? This is a crucial choice that can have significant implications for the longevity and structural integrity of your roof, not to mention potential code violations. In this article, we will delve into the negative effects of multiple layers of asphalt shingles, the expected lifespan of a single layer versus a double layer, and the important fact that it is often against building codes to install more than two layers of shingles.


The Downside of Multiple Layers


Installing a new layer of asphalt shingles over an existing one may seem like a cost-effective and time-saving option. However, it comes with a host of negative consequences that homeowners should be aware of:


Reduced Lifespan


One of the most significant drawbacks of adding multiple layers of asphalt shingles is a substantially reduced lifespan. Asphalt shingles are designed to protect your home from the elements, but they have a finite lifespan. When you add a second layer, the new shingles do not adhere as securely to the roof decking as they would to a clean, smooth surface. The second layer of shingles will also hold more heat which speeds deterioration. This lack of adhesion and the additional heat can lead to premature shingle failure and a shorter overall lifespan for your roof.


Increased Weight


Another issue associated with multiple layers of shingles is the added weight. Roofing materials are not weightless, and when you stack two layers on top of each other, the extra load can strain the structural integrity of your home. This added weight can lead to sagging or even the collapse of your roof over time.


Aesthetic Issues


Multiple layers of shingles can also negatively impact your home's aesthetics. The added thickness can create uneven surfaces which can cause a wavey look on the roof. This can be noticeable from the ground.


Lifespan Comparison: Single Layer vs. Double Layer


To understand the importance of this decision, let's consider the expected lifespan of a single layer of asphalt shingles compared to a double layer:


Single Layer


A properly installed single layer of high-quality asphalt shingles can typically last 20 to 25 years or more, depending on factors like climate, maintenance, and material quality. Dimensional shingle options can even reach 30 years or longer.


Double Layer


When you add a second layer of shingles, you can expect a significantly shorter lifespan. In many cases, the top layer may deteriorate within 10 to 15 years, while the bottom layer deteriorates even faster. This can lead to frequent repairs and premature roof replacement, ultimately costing you more in the long run.


Building Code Considerations


Beyond the negative effects on your roof's performance and longevity, it's crucial to consider building code regulations. In many areas, it is against the building code to install more than two layers of shingles on a roof. Violating these codes can have serious consequences, including fines, mandatory removal of the additional layers, and potential legal liabilities if your roof fails and causes damage.


In conclusion, while adding a second layer of asphalt shingles may seem like a cost-effective solution, it often comes with significant drawbacks. The reduced lifespan, increased weight, and decreased energy efficiency can lead to more significant expenses down the road. Moreover, it is important to adhere to building codes and regulations to ensure the safety and structural integrity of your home. When it comes to roofing, it's often best to consult with a professional roofing contractor who can provide expert guidance and ensure your roof is installed correctly and in compliance with local codes. In the long run, investing in a single, high-quality layer of shingles is likely to be the wiser choice for your home and your wallet.

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