Metal Roofing Tips: Save Money by Installing Over Shingles

metal roofing installation

In this guide, we'll discuss...

red metal roof tiles installed with drill

Installation Methods

Due to metal roofing's surprisingly lightweight nature, one of its greatest advantages is its ability to be installed over an existing shingle roof.

There are two common installation methods when installing metal roofing over an existing shingle roof:

1. Installation with New Underlayment

The first option is to completely cover the existing shingle roof with new underlayment. This acts as a second line of defense against water damage should moisture permeate the roof.

It also acts as a buffer between the metal and shingles. Shingles have a granular surface that would wear away at the back of metal roofing panels as the roof expands and contracts over time. This buffer layer protects both.

2. Installation with Purlins

A second option is to install the metal using purlins or battens. Purlins are screwed or nailed into the roof deck, and the metal is then attached. This installation method has a few benefits.

For one, attaching the metal to a flat surface provides a uniform area without the irregularities that shingles provide. Furthermore, using purlins creates a gap between the metal and shingles.

This gap acts as a buffer when the roof heats up in hot weather, reducing the amount of expansion and contraction, and moderating the internal temperature of the house.

Before you move forward with installing metal roofing over an existing roof, it’s important to make sure the underlayment or plywood is in good shape and not rotten. If the underlayment isn't in good condition, the metal roof won’t be secure and problems will arise.

Determining Your Roof's Condition

A metal roof should only be installed over one (at most two) layers of existing roofing material. But before installing the new roof, the underlying frame and decking need to be in sound condition. One way to assess the condition is to carefully walk around on the roof. 

Be on the lookout for...

  • soft spots 
  • signs of compromised components
  • weather damage

If there’s any doubt about the integrity of the roof, a full removal must be performed. If it appears sound, an over-top installation is doable.

The next step is to inspect the roofing planes from the ground. If they look straight, it’s likely the structure beneath the roofing planes is secure. If you notice depressions or raised sections, an investigation into the roof’s components should be undertaken to assure there are no broken rafters or trusses.

Raising the depth of the roof by adding another layer will impact skylights, flashings, HVAC equipment, and other rooftop components. It’s important to consider this extra height to avoid potential problems.

If the roofing structure appears secure, the new installation will require nails or screws long enough to penetrate the existing shingle layer and the underlayment beneath to assure a sound attachment. If it’s difficult to determine a secure attachment over the preexisting layer, removing the old materials is highly recommended.

KEEP READING: The Homeowner's Complete Metal Roofing Guide

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roofer installing roof membrane

Underlayment Options

Installing a layer of underlayment over the existing shingles will keep the granules from rubbing against the new metal material, which can cause corrosion.

Tar Paper

Tar paper, sometimes referred to as felt paper, is the industry standard for underlayment. Standard tar paper often sticks to the back of metal material, which can cause paper to tear when the roof contracts or expands.


Synthetic underlayments work well with metal roofing, as their design exceeds felt paper standards. They move with the expanding roof and won’t tear or rip.

Reflective Barrier Insulation

A reflective barrier underlayment acts as a cushion between the existing roof and the new metal material, and is a great option for reducing energy costs.

Environmental Benefits

There is an environmental benefit to installing a metal roofing system over a preexisting asphalt shingle roof.

Leaving the asphalt shingles where they are will not only save you money, but it also reduces the amount of roofing waste sent to landfills.

Roofing tear-offs are one of the largest contributors of landfill waste in America. And while the reuse of old asphalt shingles for roads, bike paths, and more is often cited as a common recycling practice, the reality is that a low amount of shingles are ever reused.

KEEP READING: The Ultimate Shingle Roofing Guide

Installing a metal roofing system over an existing roof is a smart choice both economically and environmentally. Plan it right, choose a trusted contractor, and the new metal roof should last a lifetime. 

Find a metal roofing contractor near you