Metal roofs are extremely durable and have greater longevity than asphalt roofs, but they're more expensive. The low cost of asphalt roofs is the reason behind their popularity—asphalt shingles are inexpensive and they're quick and easy for roofing contractors to install.
Thankfully, there's a way to decrease the cost of installing a metal roof: simply install it over your existing asphalt roof. You'll avoid the need for a lengthy teardown, which makes the installation process quicker and less expensive. However, installing a metal roof over asphalt shingles isn't a possibility in every circumstance. To find out if you'll be able to install a metal roof over your existing asphalt shingles, read on to learn the things you need to consider.
1. Local Building Codes
If you already have two roofs on your home, you'll need to tear them both down before you can install a metal roof. This is a near-universal part of building codes.
The reason behind this is that homes aren't architecturally designed to have so much weight placed on the roof trusses. The trusses distribute the weight of the roof evenly across the structure of your home, but they'll begin to sag and break if you place too much weight on them. Even though metal roofs are lighter than asphalt ones, you're still limited to a maximum of two roofs on your home.
Some jurisdictions go one step further and only allow homes to have one roof on them. You need to check your local building codes to make sure that this isn't the case in your area. If you're limited to one roof, there's no option available to you except to tear off your current asphalt roof before you install a metal one.
2. Condition of Your Roof Decking
Your roof decking needs to be in good condition in order to support the addition of a metal roof. Metal roofing panels are attached to the roof decking with screws, and they need a sturdy connection in order to resist being pulled up by the wind. If the roof decking is rotten, the screws can easily tear through the wood during heavy storms, and your roof can fail.
Unfortunately, it isn't possible to get a 100% accurate idea of your roof decking's condition without tearing off all of your existing asphalt shingles. Instead, your roofing contractor will rely on clues in order to estimate its structural integrity. If you have water stains on the attic ceiling or if you have missing or torn shingles that may have exposed the roof decking to rain, then there's a chance that the decking may be rotten.
The condition of your asphalt shingles doesn't matter, but your decking does. If your decking appears to be undamaged, you'll be able to easily install a metal roof on top of your existing asphalt roof. If the roofing contractor suspects that the decking has started to rot, then they will recommend tearing off the roof and replacing the decking in order to ensure that the screws used to secure the metal roofing panels can withstand heavy winds.
3. Manufacturer's Warranty
Finally, you need to pay attention to the manufacturer's warranty when you're installing a metal roof over an existing asphalt shingle roof. Since installing a metal roof over asphalt is so common, many manufacturers support it as long as the metal roof is installed using purlins. Purlins are wood beams that are attached to your existing asphalt roof, and the screws that hold the metal roofing panels down will be driven into them instead of into the asphalt shingles. This provides more depth for the screws, which increases the roof's wind resistance. If you're installing a metal roof over an asphalt one, make sure that you read the warranty to see if a special installation process is required.
Overall, installing a metal roof over an existing asphalt shingle roof is typically not a problem. When installing a metal roof this way, the installation process will be faster and you'll pay less money. The roof will be just as strong as one that's installed directly onto the roof decking, and you'll even get the added bonus of an extra layer of thermal insulation provided by the old asphalt shingles.
If you're looking to upgrade to a metal roof and don't want to go through the hassle of tearing off your current one, contact a roofing contractor in your area and have them come examine your roof. They'll be able to determine if your roof is in good enough condition to install a metal roof on top of it.