CAN A METAL ROOF BE INSTALLED OVER AN EXISTING ROOF?

There are several advantages to metal roofs - 40-year warranties, Roof Panel Forming Machine water-shedding properties and energy-efficiency being some of the more publicized examples. A lesser discussed but equally attractive advantage is that a metal roofing system can often be installed right over an existing roof.

INSTALLING A METAL ROOF OVER AN EXISTING SHINGLE ROOF
The pros:

Reduced cost
Reduced labor
Increased thermal resistance (R-value) of completed roof system
Faster completion time
Minimizes unnecessary landfill waste
A properly installed metal roof lasts for multiple decades
The cons:

It shouldn't be done if there are more than two layers of existing roofing materials
Underlying shingles can experience moisture and other potential wear and tear issues if the metal roof is not installed properly
Potential problems with the local building department
The cons shouldn't be much of a deterrent if you have done your homework, work with an experienced metal roofing manufacturer and are familiar with building codes in your area.

THINGS TO CONSIDER BEFORE INSTALLING A METAL ROOF OVER A SHINGLE ROOF
Is it okay with your local building department?


Odds are the answer is, "yes," but every roll former larger-scale repair, remodel or renovation requires a check in with the local building department before any work is started. Your roofing project will require permits and inspections. They will probably send out a building inspector before the project begins to ensure the existing roof is strong enough and in good enough shape to support the prospective metal roof system. Your building department may require that materials are approved before they are installed.

Failure to secure the necessary permits and approvals beforehand, or neglecting to meet their inspection requirements, can be detrimental to your upcoming roofing project. You could experience steep fines and may have to un-do some of your hard work so it can be inspected properly and then re-installed.

WHAT IS THE STATUS OF YOUR EXISTING ROOF SYSTEM?
While metal roofs are routinely installed over existing shingle and asphalt roofs, those existing roofs must be up to par for you to reap the long-term benefits. You will need to do a thorough inspection of your current roof.

How many layers are there?

If you have more than two layers of existing roofing material something's gotta give. Adding a third layer, even a metal one, is not a good idea. In this case you will probably want to remove the existing roofing materials and treat the project as a roofing system replacement.

In most cases, demolition work can be safely executed by yourself - and a couple of friends will expedite the process. Use best safety practices when working on the roof to make sure everyone leaves the project as intact as when they started. Also, check in with your local waste disposal company to see whether or not the old roofing materials can be recycled. In most cases they can and the company will deliver a special container that can be picked up when you say the word. This prevents unnecessary materials from cluttering up landfills.

Walk the roof.

Walk all over the roof's surface and look/feel for:

Soft spots
Evidence of rot
Weak or compromised structural components
Evidence of leaks
Sections that are noticeably uneven
If you notice any of the above issues, odds are you will need to repair the existing roof before you can cover it with a metal roof. Installing a metal roof over a roof that's in bad shape can result in serious issues down the road, including the voiding of your new metal roof's warranties.

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Plan to tear the current roof off if you notice

Roof lines that are so warped or uneven that they will compromise the look of the new metal roof
More than a few small areas of rotten or broken roofing components
The existing roof layers exceed what is recommended or allowed by current building codes and/or the metal roofing manufacturer
Current materials/layers are so thick that they prevent nails or screws from adequately penetrating and anchoring into the structural component
Inspect the roof from below.

Walk around the perimeter of the building and insect the existing roofing components from below. Do the roof planes look generally straight, even and true? If so, the roof is probably in good shape. If you notice obvious depressions or "bellies" it's a sign the roof may have rafters or trusses that are broken and/or overloaded.

How will the new roof impact existing features?

One obvious, but often overlooked, fact is that adding another layer of roofing material affects the overall depth of your total roof covering. This can impact certain structural and building features such as your HVAC equipment, dormers, skylights, roof-to-wall flashings and so on. It's always best to troubleshoot these items before a project commences.

Adding a high-quality metal roof over an existing roof is a smart way to save money and time while gaining a solid roof that will last for decades as long as you have done your research ahead of time.