What is the best way to install a metal roof over shingles

Metal roofing offers unprecedented durability and longevity that asphalt shingles simply can’t match, but the prospect of reroofing a home with aging asphalt shingles can be daunting. While installing a metal roof itself is a job many homeowners can handle, removing the old asphalt shingles is a massive undertaking that discourages many homeowners from taking on this job.

One solution is to not remove the asphalt shingles at all. By installing metal roofing over existing shingles, you can save money on the cost of a roof removal, spare yourself the labor of removing the roof, and cut the time it takes to install a new roof in half.

In this article, we’ll examine how to install metal roofing over shingles, review the benefits of this type of roof installation, and identify the pitfalls to avoid.

Quick Navigation

• Why Install Metal Roofing Over Existing Shingles?
  • Installing Metal Roof Over Shingles Pros and Cons
• Can You Put Metal Roofing Over Asphalt Shingles?
• How to Install Metal Roofing Over Shingles
• 1. Prepare the Roof for Metal Roofing Installation
  • Assess the Roof
  • Take Safety Precautions
  • Install Underlayment or Framing
• 2. Install Metal Roof Panels Over Shingles
  • Step 1: Measure the Roof
  • Step 2: Install Edging
  • Step 3: Install Panels or Shingles
  • Step 4: Install Flashing
• Average Cost to Install Metal Roof Over Shingles
• Metal Roof Over Shingles Problems
• Conclusion

Why Install Metal Roofing Over Existing Shingles?

While metal roofing isn’t a suitable option for all homes-it’s notorious for being noisy during heavy rains-it has numerous benefits over standard asphalt shingles. First and foremost, it lasts longer, a lot longer. Whereas you’ll get around 20 years at best out of an asphalt roof, a metal roof can last 40 to 70 years, depending on the material.

This is mostly because metal roofs are more durable and can better withstand severe weather. Some metal roofs will hold up against sustained winds of up to 140 miles per hour. Depending on the type of metal roof, they can also be impact resistant, and they won’t crack like asphalt shingles.

Their slick versus rough texture also makes metal roofs less like to grip debris such as leaves and twigs or. And, while asphalt tends to across the sun’s rays and heats up, metal roofs will reflect the sun’s UV rays, which can result in up to 25 percent reduction in the costs to cool a home.

Metal roofs are also an environmentally friendly option as most metal roofs have between 25 and 95 percent recycled content. And, since they are all metal, they are 100 percent recyclable at the end of their life. Asphalt shingles, on the other hand, typically end up in a landfill.

When it comes to installing a metal roof over shingles, there are many advantages. First and foremost, it eliminates the need to tear off the old roof, which is a more laborious than installing the new roof itself.

Removing an old roof is also surprisingly costly, as it requires the rental of a large dumpster, not to mention the per pound cost of dumping the waste at a landfill. By preserving eh asphalt shingles, the metal roof adds layer of insulation, enhancing the energy-saving properties of a new roof.

Installing Metal Roof Over Shingles Pros and Cons

Before proceeding with a metal roof installation over existing asphalt shingles, it’s important to first consider the pros and cons of this project.

Pros

• Saves the cost of disposing of an old roof. Disposing of asphalt shingles requires the costly expenses of a dumpster rental and disposal fees.
• Saves the labor or tearing off old shingles. Removing old shingles involves removing hundreds if not thousands of pounds of old material. That’s a lot of work.
• Preserving the layer of asphalt shingles keeps the roof better insulated, saving energy costs

Cons

• Old asphalt shingles have inconsistencies that can make the roof uneven. This can make it difficult to properly install a metal roof over the top of it.
• Removing old asphalt shingles allows you to assess the condition of the roof. You won’t be able to see rotted or damaged areas under the asphalt shingles without removing them
• Adding a metal roof over the top of shingles will add additional weight, reducing the live load a roof can handle. While the weight may not be enough to cause problems, it’s worth considering, especially for roofs that see significant amounts of snowfall.

Can You Put Metal Roofing Over Asphalt Shingles?

Most of the time you can; however, there are some cases where you can’t. First, check local building codes to determine if installing a metal roof over shingles is within code.

Keep in mind that most localities will not allow more than two layers of shingles on a home. So, if your house already has a double layer of asphalt shingles, you won’t be able to install another layer on top of it. This rule is there for a reason. Three layers of roofing are simply too much weight for the roof to safely bear.

You can put metal roofing over asphalt shingles if the local building code allows and if you only have a single layer of asphalt shingles on the roof.

Assessing the current condition of the roof, taking proper safety precautions, and prepping the roof with underlayment or framing are all crucial steps before installing a metal roof. Once those steps are complete, accurately measuring the roofing and choosing the right kind of metal roofing is key to a successful install.

1. Prepare the Roof for Metal Roofing Installation

Assess the Roof

Before you do anything, assess the roof as best you can. While you won’t be able to identify problem areas with the roof structure as easily as you would if you removed the asphalt shingles, there are still telltale signs.

Soft areas or bowing in the roof, dark spots, or places where you know water is infiltrating the roof (because there is a leak inside the house when it rains) indicate areas of potential rot. From the inside, check the attic space. Moldy or thin insulation that has collected moisture are clear signs of structural problems that should be addressed before installing a new roof.

While the new roof may stop water from breaching these areas of the roof, it won’t fix any structural damage. It’s especially important to address these issues, especially if the rot has made its way to any roof joists. If you find signs of rot, you’ll need to remove that section of asphalt shingles and make repairs.

Take Safety Precautions

Any work done on the roof is inherently dangerous. Take safety seriously. Invest in a good pair of shoes that will provide plenty of traction while moving around on the roof. This is especially crucial for metal roofs, which are smoother and therefore slicker than asphalt roofs. Wear gloves and safety goggles to protect your hands and eyes when handling sharp edges and installing fasteners.

Ensure the ladder is well-anchored to the ground and check the area around the roof for any low-hanging power lines that could be a hazard. If you’re working on a roof with a steeper pitch, use fall protection equipment as an additional safety measure.

Install Underlayment or Framing

Many people think you can simply slap metal roofing right over asphalt shingles. This would be a mistake. Most old asphalt roofs have shingles that are peeling upward or pooching. Even roofs in good condition will increase about 1/8 of an inch in height with each course, creating a surface that isn’t flat.

This uneven surface makes it difficult, if not impossible, to lay the metal panels flat on the shingles. Framing between the two roofs is required to flatten the roof.

Not having a buffer between the two roof materials can also cause damage to your new roof. Without at least an underlayment, the shingles can also damage the metal roofing.

Asphalt shingles are gritty and rough. As metal expands and contracts in the heat, it will rub against that sandpaper-like surface, slowly wearing away the metal roofing over time. Underlayment acts as a buffer to prevent this from happening.

Before beginning installation of the roof itself, decide how you’ll prep the roof. You have two options: underlayment or framing.

Use Underlayment

Create a buffer between the two roofing surfaces by installing roofing underlayment right on top of the old asphalt shingles. Check building code to see which methods are acceptable in your municipality.

You can use felt or tar paper underlayment or a synthetic material. Roll the tar paper out over the roof and attach with 1-inch roofing cap nails (these are nails that have rings around the shaft and a plastic washer around the head) or a staple gun. Make sure to overlap each course of roofing paper by about 2 inches.

While tar paper is one option, it does tend to stick to metal roofing, which could cause it to tear as the metal expands and retracts. New synthetic underlayments such as roofing foil are just as easy to install but won’t stick.

And while foil underlayment costs more, this type of underlayment reflects heat, helping to make your home more energy-efficient.

Use a 4×4 Frame

While underlayment is a suitable option, it does nothing to level the inconsistencies in an asphalt shingle roof, making it challenging to install the metal roofing.

A 2×4 frame is a good solution for this problem and is relatively simple to execute. This process involves installing 2×4 dimensional lumber parallel to the eaves of the roof to serve as a framework to support the roof’s metal panels.

Begin by measuring the length and width of each roof side. You’ll need enough 2x4s to run parallel to the eaves of the roof in increments of every 24 inches. Begin at the eves and attach the 2x4s to the existing roof using nails or screws.

Make sure you use fasteners that are long enough to penetrate deep into the roof sheathing. Each course of 2x4s should be no more than 24 inches apart.

2. Install Metal Roof Panels Over Shingles

Once you’ve prepped the roof by installing either underlayment or framing, it’s time to begin installing the panels.

Step 1: Measure the Roof

Begin by determining how much material you’ll need to install the new roof. Do this by measuring the area, length times width, of each roof plane. Add all of the areas together to come up with the total square footage for the roof. Order your roofing supplies based on that square footage, adding 10 percent to account for waste. Make sure to order roofing screws that match the color of the metal roof.

Step 2: Install Edging

Install edging around the edge of the roof using strips of metal flashing. Make sure the corner of the flashing sits flush to the edge of the roof. Attach using 1-1/4-inch nails.

Use corner pieces to fit bends on the roof, such as at the peak. If the eaves have gutters, make sure the eve flashing overhangs over the lip of the gutter by about 1/2 inch.

Step 3: Install Panels or Shingles

You have three options when choosing a metal roofing material: metal panels, metal shingles, and standing seam metal roofing.

Metal Panels

Panels are one of the easiest types of metal roofing to install. These sheets come in panels that are 3 feet wide and 14 feet long and weigh around 30 pounds. Given the weight and size of the panels, you’ll need another set of hands to safely and properly install the metal panels.

Begin by laying the first sheet so that it overhangs the edge of the roof by between 1/2 to 3/4 of an inch. If you plan on adding trim and fascia to the gabled end, then install the roof panel flush to the side of the roof.

Next, make sure the panel is square to the roofline. Hold the panel in place and have your partner measure both ends of the panel to the drip edge to ensure the panel runs square to the drip edge. This step is critical. If the first panel is not square, the rest of the panels won’t be either.

That said, most roofs are not perfectly square, to begin with, so accept the fact that there will be inconsistencies in the layout of the panels that you’ll have to live with. Plan the roof so the inconsistencies are in places they won’t be seen.

Install roofing screws through the metal roofing and into the existing roof. If you built a 2×4 frame for the roof, make sure to attach the roofing to the ribbing.

Attach screws to each course of ribbing at intervals of between 18 to 24 inches along each rib. Do not overtighten the screws as this could damage the plastic washer under the screw head that seals to the surface of the roof panel to create a watertight seal.

Add the next panel so that it overlaps the first panel and attaches using the same method as the first panel. While moving up to the roof peak, make sure panels that are on high courses overlap panels on lower courses to ensure proper rain runoff.

Continue adding panels to cover the roof. Use a circular saw to cut panels to size at the edge of the roof or to fit around vents, chimneys, and other structures on the roof.

Metal Shingles

If you’re installing metal shingles, the process is a little different. While metal shingle designs vary, most use a starter strip that is installed along the eave. This strip sometimes takes the place of edge flashing. The first course of shingles locks into this starter strip.

Begin by laying a starter row of metal shingles at the drip edge of the roof. As with the panels, use a tape measure to make sure the row remains square to the drip edge as you install the first course. Use metal roofing nails to attach the shingles to the roof sheathing.

Because shingles are smaller than panels, they are more forgiving when installing on a roof that isn’t perfectly square, as you can make small adjustments to each shingle to account for the variances in the roof.

Once the first course is complete, begin installing shingles beginning at a lower corner and moving upward from eave to ridge, staggering the length of each course. This will allow for an even-looking installation across the entire roof.

This staggered pattern is often specific to the brand of shingle, so make sure to consult the manufacturer’s instructions for the proper installation technique.

Unlike asphalt shingles, which involve driving nails through tabs, most metal shingles use clips that are nailed into the roof. Metal shingles also use a locking system that holds the shingles together. The clips and locking system vary depending on the brand of shingle, so make sure to follow the manufacturer’s instructions for proper installation.

Standing Seam Metal Roofing

Unlike panels and shingles that go from eaves to ridge, standing seam metal roofing is installed from gable to gable. As with shingles, you’ll need to begin by installing an extended eave piece over the eave flashing that runs along the edges from eave to roof peak. The caps and flashing are typically specific to the brand of standing seam metal roofing.

Once that’s in place, it’s time to install the first panel of metal roofing. Each panel of roofing is designed to run from the eave to the roof peak, so you’ll need to cut each panel to size depending on the dimensions of the roof.

Plan so that the panels on either end of the roof will be the same width, then cut the first side to size.

The first piece of metal roofing will need to have a bend-up on its cut side, which requires a special bending tool. These tools can be expensive, so renting one is a good option if you go with this type of metal roofing. The bend-up creates a lip at the edge of the gable that channels the water to the eave.

Once the first panel is in place and squared to the eave edge and gable edge of the roof, fasten the panel to the roof using metal screws along with the tab, sometimes called a male leg, that runs the length of the panel.

Once that first piece is secured, add the next panel by locking the hem of the new panel onto the male leg of the first panel. Secure the panel to the roof by applying metal screws to the second panel’s male leg.

With each panel installation, use a pair of hand benders to clamp the eave cap and the ends of each panel tightly together.

Once you reach the other gable, measure the width for the last piece. If you measured properly, it should be the same width as the other gable piece. Cut the piece to size and use the bending tool to bend up the cut edge 1 inch, then install the last piece.

Step 4: Install Flashing

Metal flashing fits in the joints between the areas where two planes of the roof meet, helping to channel away water from the seams between the joints. This prevents the roof from leaking at these joints.

Begin by covering the peak of the roof. Bend the roof flashing into a v-shape that matches the angle between the roof, then fit it over the top of the roof ridge and attach it with fasteners. Make sure the flashing overlaps the roof panels or shingles by 1 inch, so it directs water away from the seam between the two surfaces.

Cut and install flashing for other joints where two roof surfaces meet.

Average Cost to Install Metal Roof Over Shingles

Metal roofing costs vary depending on the type of material. Metal roofing ranges between $1.50 and $2 a square foot, depending on the brand of the roofing. This compares to asphalt shingles which cost around $1.50 per square foot.

Installing a metal roof yourself can save a significant amount of money. The cost of installing metal roofing costs between $7 and $14 a square foot for standing seam roofing and from $8 to $10 per square foot for metal panels or shingles.

Metal Roof Over Shingles Problems

While installing metal roofing over shingles is a convenient and cost-saving means of adding a new metal roof, it isn’t without its pitfalls. In this section, we’ll cover some of the problems you’ll encounter with this type of metal roof installation.

Roof sheathing is covered – typically, the old shingles are removed before installing a new roof. This gives the homeowner a chance to inspect the roof’s deckings for any problems such as rot to determine if structural repairs need to be made before proceeding with a new roof installation.

There is no thorough way to conduct this inspection when the roof is covered, which may lead to unwelcome surprises later if there are structural problems that are hidden beneath the shingles.

Complicates future roof repairs – If there is a roof leak in the asphalt shingles, adding a metal roof will provide a patch for those leaks. However, if there is a leak in the metal roofing, water will run between the two roofs until it reaches the original leak, then leaks through the roof. This scenario can make the leak difficult to diagnose and fix.

Condensation – The space between the original asphalt roof and the metal roof won’t be airtight. This means damp air can get in between these two layers, causing condensation to form, which can lead to mold growth or rot.

Conclusion

Installing a metal roof over shingles is an excellent way to add a new roof to a home without the cost and headache of removing an old asphalt shingle roof. By following some simple procedures for prepping the old roof and adhering to some important guidelines, you can successfully install metal panels, shingles, or standing seam metal roofing over an asphalt roof.

Regardless of which method you choose, remember that working on a roof is inherently dangerous. With this in mind, make sure to follow the safety procedures described above to prevent injury.

Eugene has been a DIY enthusiast for most of his life and loves being creative while inspiring creativity in others. He is passionately interested in home improvement, renovation and woodworking.