Why It’s Bad To Put A Metal Roof Over Shingles

Installing metal roofing over shingles can be quicker but also lead to more problems down the road for your house.
Read Time: 5 minutes
Dec 19, 2024
Getting a new roof can be very costly, but it’s often necessary. An old or faulty roof will leave you and your house unprotected and vulnerable to further damage. However, many people understand that they need a new roof, but do not want to go through the entire process of replacing it. So instead, they choose to put a metal roof over their asphalt shingle roof, thinking it can save them time and money. Unfortunately, this choice is not smart in the long run. Let’s break down why.

There’s No Way to Inspect Underneath

Man looking for roof leaks. If you simply put a metal roof on top of your asphalt shingle roof, you and your contractors won’t get to see what’s underneath the shingles. Most roof problems occur underneath the first layer. If there’s a problem with the roof, you won’t know before putting another roof on top of it. Here are some of the things that you could find if you started by removing the original roof instead:

Hidden Faulty Roof Decking

Roof decking is made up of wooden boards that form the foundation of your entire roof system. Typically, if you wanted to replace a roof, you’d rip everything down to the decking to see if there is anything wrong with it, such as broken boards, rotting, or mold. Roof decking problems are fairly common. But if you don’t do a full assessment of the decking, you won’t know. Men on a roof install underlayment, a layer that goes on top of the roof decking. Not taking the time to see what’s hidden underneath that needs replacing is dangerous. For example, your underlayment might need to be replaced. Underlayment goes over your decking as the roof’s last line of defense. Most metal roofs use a special underlayment designed specifically for metal roofs. But without removing the asphalt shingle roof, you won’t be able to put in that special underlayment.

Additional Layers Add Weight of the Roof

Installing a metal roof over top of a shingle roof can also be problematic because of the extra weight you’ll be adding. While metal roofs are generally lightweight, the combined weight of the shingles and metal might push the structure beyond safe limits leading to structural failure. If you are unsure of the structure underneath this failure could happen right away or years down the road. It’s just not worth it.

Layers Make It Harder to Find Future Leaks

A close-up of roof decking that has rotted, with a large hole and water damage. If you install a metal roof over a shingle roof, it could be harder to find and fix leaks in the future. Down the road, when you get a leak in your roof, you won’t know where it’s coming from: the first roof or the metal roof. Not only does this make it harder to find, but also harder to fix. In the case that the leak is coming from the original shingle roof, a contractor will have to go through the metal roof to repair it.

Metal Roofing Over Shingles Can Mask Health Concerns

Roof leaks can cause mold and other health concerns If there’s anything wrong with your old roof, putting a metal roof over the existing shingles will just cover up the problems rather than solve them. For example, if there is mold or mildew within the first roof, putting a new roof on top of it will only cover up the problem for a while. But the mold will continue to spread underneath the metal roof, causing damage to your house and potentially your health.

Roofing Layers Can Create Uneven Surfaces

When placing a metal roof on top of a shingle roof, most contractors use furring boards as the foundation between the two. But because shingle roofs aren’t flat to begin with, the furring boards can create an even more uneven surface for the metal roof to sit on. Additionally, the furring has the possibility of absorbing humidity and warping over time. This unevenness could lead to gaps, loose panels, or reduced protection against water infiltration. A poor fit not only affects the roof’s functionality but also detracts from its aesthetic appeal.

Metal Over Shingles Might Not Be Legal

In some areas, local building codes might prohibit the installation of a metal roof over an existing layer of shingles. These codes are often designed to ensure that roofs meet safety, structural, and aesthetic standards. While not all regions enforce this rule, it’s worth checking with local authorities or a trusted roofing contractor to ensure compliance. Skipping this step could lead to fines or challenges when selling your home in the future. So be safe and contact your local government before taking on such a project.

Metal Roofing Installed Over Shingles Can Shorten the Roof’s Lifespan

Metal roofs are known for their durability and long life, but installing over shingles might reduce these benefits. Multiple layers can trap heat and moisture between the two, potentially causing damage to the metal panels or the underlying structure. Over time, this buildup might lead to warping, corrosion, or mold growth. This potential for a reduction in expected lifespan is why some manufacturers frown or even refuse to warranty roofs utilizing this practice.

Roofing Warranties Might Be Voided

Most roofing manufacturers offer warranties for their products… but only under proper installation. Manufacturers often specify that their products must be installed under particular conditions and processes to maintain coverage. If the shingles underneath cause damage to the metal roof or prevent proper installation, you could lose warranty protection, leaving you responsible for repair or replacement costs.

How to Minimize Problems of Metal Roof Installations

  • Remove old shingles whenever possible
  • Repair any damage before installation
  • Read and understand the metal roofing company’s warranty
  • Always check local building codes
  • When in doubt, enlist an experienced roofing professional for an expert assessment
Two workers in hardhats install sheets of red metal roofing.

When Installed Correctly There Are Many Benefits of Metal Roofing

Metal roofs have many advantages when they’re installed properly. These include:

Energy Efficiency

Metal roofs reflect sunlight and heat in the summer, keeping your home cooler. As a result, your air-conditioner will often use less electricity.

Fire-resistance

Most metal roofs are fire-resistant, so they can potentially make your home safer in higher-risk, remote areas like the mountains.

Durability

When you install a metal roof the right way, it can last for over 50 years. This durability against hail and limb damage is one of the leading reasons for metal roofs’ recent popularity.

Different Styles and Colors

Metal roofs come in many different styles, colors, and textures, so you can choose one that matches your home’s exterior and neighborhood vibe perfectly.

Lightweight

Metal roofs weigh much less compared to asphalt shingle roofs. Because of this, they don’t create as much of a structural liability as shingle roofs unless installed on top of one, of course.

Have questions about installing metal roofing?

So, there are certain instances that you can put a metal roof over shingles, but the list of potential problems almost always outweighs the benefits. When in doubt, it is always advisable to work with an experienced metal roofer like Smith Roofing who understands these and other issues that can arise when installing metal roof panels. It is often much cheaper, with fewer headaches, in the long run.
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