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How to Install Metal Roofing Over Shingles

Metal roofs offer a ton of advantages. In sunny climates like San Diego, exposure to UV light can greatly reduce the lifespan of asphalt shingles. A metal roof is more resilient, and it is much better at absorbing the extra heat, which should help your house stay cool. Termites are much less inviting to rot, mildew, and termite infestations. They can also protect your home from spreading fires.

Though they are more expensive initially, they have a much longer lifespan than a traditional shingle roof. Add that to their durability, and you won’t have to spend nearly as much money on repairs. In many instances, metal can last for the remainder of your home’s life. So, how to install metal roofing over shingles?

Because they are so lightweight, they can easily be installed over old shingles. This can greatly reduce the time, labor, and overall cost of installation, as well as provide better insulation for your home. Take a look at the average metal roof cost versus just having shingles. It can also help dampen the noise of rain on your new metal roof. 

There’s no good way to dispose of asphalt shingles. Currently, they make up as much as 3% of landfill waste. Leaving them installed on your roof is a good way to keep them out of landfills, and your new metal roof will protect them from deteriorating further. 

So where do you get started, and what does the process look like?

Hire a Roof Inspector First

It’s crucial to have your roof inspected by a professional before you install metal roofing over existing shingles. Unless you purchased your home, you might not know whether or not there are already multiple layers of shingles. 

A roofer will be much better at spotting the signs of leaking shingles, rot, or structural damage. If your current roof is in bad shape, it’s better to tear it away and start new.  They’ll also be more familiar with local code requirements that may prevent you from installing metal roofing over your existing shingles. A roofing contractor will generally offer free estimates, and they will inspect your roof in the process.

Even the handiest of homeowners shouldn’t rely on their own skills to install a metal roof. If you install your metal roof over a leaky or buckling roof, it will lead to an expensive repair job. A professional can prevent you from making costly or dangerous mistakes. 

Metal roofing is much more difficult to install than asphalt shingles. If you don’t know how to cut metal roofing panels, you can warp them or damage the protective coating that prevents rust. You will also need to invest in some specialty tools to help you get the job done.

Repair Leaks and Rot 

With roofs that have minor damage, you don’t necessarily need to tear it away.  You can simply repair an existing roof leak and replace any missing shingles. Rotting rafters and beams should be replaced before you install your new metal panels.

Roofs with moderate buckling may not be able to support additional roofing on top. Though metal roofing is much lighter weight than a clay tile or asphalt shingle roof, it adds a significant amount of stress to an already struggling roof. Again, a roof inspector will be much better at spotting issues with your roof decking and prevent you from making a costly mistake. 

Protect Against Condensation

Throughout the day, the temperatures in your attic can skyrocket. As the weather outside cools overnight, condensation will form under your metal roof. Not only can this lead to corrosion in your new metal roof, but it can also cause rot and mildew on the wood underneath. 

Insulation and ventilation can protect your roof from condensation. Insulation can be used to prevent your metal roof from reaching the dew point— the temperature that will cause water vapor from the air to turn into droplets. 

Ventilation works to keep enough airflow in your roof to allow water to quickly evaporate. Additionally, you can use a vapor barrier to prevent water that forms from diffusing into the wooden support under your roof. 

An experienced roofer is well versed in the methods to prevent condensation from corroding your roof or causing structural damage. Over time, condensation can affect more than just your roof. Water damage can spread throughout your whole house, which can weaken overall structure and lead to mildew and mold. 

Underlayment vs Purlins Installation

When you’re installing your new roof, it’s important that you don’t place the metal panels directly on the asphalt shingles. The abrasive surface from your old roofing can scratch the underside of the sheet metal, which will remove the protective coating that prevents corrosion. There are two options to choose from.

You can use an underlayment to cover your old roof.  You can use a water-resistant asphalt-saturated felt or a water-resistant synthetic underlayment. For areas with extreme winters or heavy rain fail, you can use waterproof rubberized asphalt. The underlayment will protect your new roof from scratching and provide a moisture barrier.

Purlins can be used to provide space between the old roofing and the new metal roofing panels. You can stall them on your old roof deck and screw your metal roofing panels to the purlins. This will provide ventilation while protecting your new roof from the abrasive shingles. It will also give you a nice, flat surface to install your panels on. 

There are pros and cons to each method, and every roof installation is unique. Using purlins is the more expensive option, but it’s not always the best one. A roofing contractor will have the expertise and experience to make the right call for your situation. 

Measure Your Roof

Before you purchase roofing material, you’ll need to find the exact area of your roof. For a flat roof, this is as simple as multiplying the length and width of your roof. If you have a sloped roof it’s a little more complicated. 

You’ll need to use a carpenter’s level on the underside of one of the rafters in the attic. Measure the vertical distance from the 12-inch mark on the level to the underside of the rafter you chose. 

Divide the vertical distance by 12, and you’ll have your slope. Multiply the area by the slope, and the result is the total square footage of your roof. When you order your roofing materials, buy about 10% more for waste.

Installing Edging, Panels, and Flashing

You’ll need to install edging around the roof before you install your panels. You’ll need to use corner pieces anywhere the roof bends, and make sure the edging extends about a ½” over any gutters that are installed. 

Once it’s time to install your panels, it’s crucial that you follow the manufacturer’s guidelines for installation. Metal panels need to be cut and spaced and screwed in a very specific pattern, and that pattern will vary depending on how your home is oriented with prevailing winds and the type of panels you’re using. It’s also imperative that you use the screws or fastener clips that are recommended, and save your nails for a different project.

If the screws aren’t properly placed and fastened, they won’t be able to withstand the load they are designed for. This placement varies depending on the type of roof. Panels with exposed fasteners and standing seam panels use totally different anchor systems. Aluminum panels will require a much tighter screw spacing than heavier sheet metal. 

Experienced roofers will be familiar with your local weather conditions. They also tend to be loyal to roofing materials that they trust and will know exactly how to safely and effectively install it over your old roof. 

The first panels need to overlap with your edging by about ½”. Don’t over tighten the screws—you shouldn’t see the waterproof gasket squishing out from under the screw head. Each panel should overlap You’ll need to use a silicone sealant where the panels overlap to create a watertight seal.  

In addition to the panels, you’ll need to install flashing and ventilation to protect your roof and home from water damage. Amateur flashing and trim jobs are easy to spot and it can lower your home’s curb appeal. You’ll also develop leaks very quickly with a poor installation. A professional roofing company that has installed thousands of roofs will know how to make your roof beautiful while protecting your home from the damage that may not be covered by insurance if it’s caused by faulty installation. 

Metal Roofing Installations in San Diego

Preman Roofing Solar has serviced San Diego for over 20 years. We don’t use independent contractors—all of our roofers are employees trained to safely and professionally install your roof. 

We service, repair, and install both metal and shingle style roofs, and all of our estimates are guaranteed. And it’s not just the cost you can count on—we stick to our schedule so you always know when to expect us and exactly when you can count on the work to be completed. Contact us today for a quote on roofing work or solar panels and explore our financing options.