Can You Install a Metal Roof Over an Existing Roof?

One of the things most people and roofers associate with metal roofs is the price. Metal roofs are some of the longest lasting roofing materials that you can get, but they do come with a heftier price tag than your average asphalt shingle roof.

Sometimes to cut down some of that cost, homeowners may try to avoid tearing off their old roof, and install the metal roof right over top of their old one. But is that even possible? And if so, is it a good idea?

Unfortunately, the answer is not a “yes” or a “no.” Instead, it’s more of a “yes, but...”

Yes, you can get install a metal roof over an existing roof, but there are a number of criteria that need to be met to make sure it’s the right decision, and that it won’t cause more issues in the future.

In this article, we’ll go over the things you need to know before putting a metal roof over a shingle roof, how the process works, and some pros and cons to doing so. Let’s get started!

What to Know Before Putting Metal on an Old Roof

What Does the Building Code Say?

The building code in each state will likely be the first thing that determines whether or not you can install a metal roof over an asphalt shingle roof. Some states don’t have solid regulations on it, while others mandate that each time a new roof is put on, the previous material must be torn off.

Here in North Carolina, our code states that a metal roof can only be installed over a pre-existing roof if it is installed in accordance with manufacturer specifications, there is no water damage, and the roof is not made of slate, clay, or cement. Another important note from the code, is that if a roof already has two or more layers of roofing material on it, that material has to come off before a new roof can be installed.

What is the State of the Roof & Decking Underneath?

If you’ve got one layer of asphalt shingles on your roof, cool! That means that if you install a metal roof over it, your building will still be up to code. Well, as long as there isn’t much water damage or deterioration to your roof and decking, that is.

This is one of the trickiest parts of installing a roof without pulling off the old material. When we tear off an old roof, the next step is always to inspect the roof decking, which are the boards that attach the roofing material to your home’s frame. Often as a roof ages and develops leaks or holes, the decking is the first thing that starts to show the damage. So even if you don’t have an active leak that you can see, your decking might be soaking up all that water and starting to rot.

So in order to install the new roof on top of your old one, you’ll need to make sure that your decking is in great condition so that it doesn’t end up coming back to haunt you later on. Sometimes you can see damage like that from the underside in your attic, but you may not know for sure without seeing under the shingles.

How is it Installed Over an Asphalt Shingle Roof?

So maybe you have a pretty new roof that’s in great condition, and you want to opt for metal for its energy efficiency, and the longevity of it. This is the circumstance where being able to install over the old roof comes in handy.

The process is pretty similar to a normal metal roof installation, just without the initial tear off. The main difference after that is that you’ll need to have some kind of barrier, or underlayment, between the surfaces, as well as a small gap to ensure that there is enough airflow going through the roof.

What are the Pros and Cons?

PRO: Saves Time and Money During Installation

The main positive behind this method is that it saves you, and the roofing company, time and money. The less time it takes to install a roof, the less costs the contractor faces, and the less they have to charge you. It also prevents a lot of waste and debris that comes from tearing off the old roof.

The company won’t need to rent as large of a dumpster and won’t need to pay as much to rent it, since it will weigh a LOT less than it would if it was carrying an entire discarded roof in it. Though a 3,000 square foot roof doesn't sound like it might have too much material on it, the weight of the shingles on the roof can actually be over 3 TONS, and since dumpster rental companies charge based on the weight they carry, you can save a good bit of money by not needing one.

So, simply put, not having to tear off the old roof will save you money and time up-front.

CON: Doesn’t Offer the Chance to Inspect Decking

As for the drawbacks, well, it might end up costing you more in the long run. If the new roof isn’t installed exactly up to specifications, you could end up with a metal roof that degrades much faster than it should from rubbing up against the shingles. If the roof isn’t flat enough, and the metal is installed without the recommended gap in between, it may end up warping, which wouldn’t look great.

The chance to inspect your roof decking is also a pretty big drawback, as roofers often find rotten decking that needs replacing, even on homes that didn’t have any visible leaks.

Even though you are saving up front by installing your metal roof over the old one, it does open you up to potential risks, and expenses, than you would have with a normally, and properly installed metal roof.

Metal roofs are usually chosen by homeowners who want to have a roof that will last them for several decades, so we typically don’t recommend installing it over an existing roof, since it might decrease that lifespan significantly.

Want to Learn About Metal Roofing in Raleigh? 

If you want to learn more about metal roofing, or roofing in Raleigh, NC, check out some more blogs in our learning center!

We’ve got several blogs on the subject of metal roofing, including: Are Metal Roofs Noisy When it Rains?

On Tops Roofing has been replacing and repairing metal and asphalt shingle roofs in the Raleigh area for over 30 years, so whatever your roofing project requires, we are here to help!