How to install a metal roof over an existing tile or wood shake roof

Upgrading your roofing system in Minnesota offers significant benefits, particularly when considering the unique challenges posed by our harsh winters and heavy snow loads. Instead of the labor-intensive and costly process of tearing off existing tile or wood shake roofs, installing a metal roof over an existing substrate provides a faster, more efficient solution that maintains the structural integrity of the underlying building while delivering the durability and aesthetic appeal of modern metal roofing. This approach is particularly popular in the commercial sector where time is money, and minimizing roof deck exposure to the elements is critical for preventing water infiltration and mold growth during the transition period.

Assessing Structural Integrity Before Installation

Before any cutting begins, a thorough structural evaluation is non-negotiable, especially given the weight dynamics involved in Minnesota's climate. Metal roofing systems are generally lighter than traditional tile or shake roofs, which can actually reduce the load on the roof framing, provided the existing deck is sound. However, if you are installing over wood shake, you must ensure that the shakes are not severely rotted, warped, or insect-damaged, as placing a metal roof directly over compromised wood can lead to premature failure. For tile roofs, the underlying wood deck must be inspected for rot, which is a common issue in the humid winters of the North Central region. If the deck is compromised, it may require replacement before the metal panels can be safely attached.

One critical consideration for Minnesota contractors is the snow load capacity. Metal roofs are excellent at shedding snow due to their smooth surface and low friction, but the underlying structure must still be rated to support the accumulated weight of wet snow or ice dams, particularly in older buildings. Our experience in Minnesota teaches us that while metal is lightweight, the transition layer and fasteners must be engineered to handle freeze-thaw cycles that can expand and contract materials, potentially loosening fasteners over time.

Preparing the Substrate and Installing Underlayment

Once the structural integrity is confirmed, the focus shifts to preparing the existing surface. For tile roofs, the primary concern is ensuring the tile does not interfere with the flashing details or the attachment of the metal panels. In many cases, the tiles can remain in place, but the underlayment layer must be carefully applied to bridge the gaps between tiles and create a continuous waterproof barrier. For wood shake roofs, the surface needs to be cleaned of debris, moss, and loose particles to ensure proper adhesion of the underlayment.

The choice of underlayment is vital for performance in our snowy conditions. Synthetic underlayments are often preferred in Minnesota because they resist UV degradation better than felt paper and offer higher tear strength, which is crucial when dealing with the uneven surfaces of old wood shake. They also do not become as soggy and brittle as traditional felt when exposed to the intense cold and snow loads of winter. The underlayment should be installed in overlapping layers, with the shingles or tiles acting as the secondary water barrier.

Installing the Metal Roofing System

With the underlayment securely in place, the actual installation of the metal roofing panels begins. This phase requires precision and adherence to the manufacturer's specifications to ensure weather tightness. Start by measuring and cutting the metal panels to fit the roof dimensions, taking care to account for the existing roof's irregularities. When working over wood shake, the fastener holes must be carefully positioned to avoid damaging the wood fibers, which could lead to rot. For tile roofs, fasteners may need to be drilled through the tile into the deck, which requires specialized tools and techniques to prevent cracking the ceramic material.

It is crucial to maintain a consistent pitch for the metal roof, even if the underlying roof is uneven, to ensure proper water drainage. In Minnesota, poor drainage can lead to ice dam formation, which is a major cause of roof leaks. The metal panels should be installed with adequate overhangs at the eaves to prevent ice from melting and dripping onto the ground, and sufficient ventilation at the roof line to allow warm air to escape and prevent ice dams.

Managing Flashing and Weatherproofing Details

The most common failure points for any metal roofing system are the flashing details, and this is especially true when installing over existing materials. Flashing must be installed at all penetrations, including vents, chimneys, skylights, and wall penetrations. In Minnesota, thermal expansion and contraction play a significant role, so flexible flashing systems that accommodate movement are essential to prevent leaks caused by the building shifting due to temperature changes.

For roof-to-wall transitions, step flashing or curb flashing should be used to direct water away from the wall and onto the metal roof. When working over tile, the flashing must be designed to clear the tile profile without dislodging it. Proper sealing with high-quality roofing sealants at the termination points of the flashing is mandatory to ensure a watertight seal against the relentless wind and precipitation common in our region.

Sealing and Final Inspection

Before the project is declared complete, all seams, joints, and penetrations must be sealed with appropriate roofing sealants designed for metal roofs in cold climates. Pay special attention to the flashings around chimneys and vents, as these are the most likely areas for leaks. Once the sealants are applied, conduct a thorough final inspection, ideally after a simulated rain event or during a heavy snowfall if possible, to ensure no water is penetrating the system.

Our team at Metro Steel Roofing emphasizes that a successful installation is not just about the panels going up; it is about the entire system working in harmony with the building and the environment. Regular maintenance after installation, such as clearing debris from valleys and ensuring gutters are free of ice dams, will extend the life of the roof. By following these practical steps and respecting the specific challenges of Minnesota's climate, you can achieve a durable, energy-efficient, and aesthetically pleasing metal roof that protects your commercial property for decades to come. Remember, proper installation is the key to maximizing the value and longevity of your metal roofing investment.

To ensure a flawless installation, consider adhering to the following critical checklist for your project:

  • Verify local building codes regarding roof replacement limits and permit requirements in your specific Minnesota jurisdiction.
  • Confirm that the existing roof deck has a minimum thickness of 7/16 inch before proceeding with panel installation.
  • Use ice and water shield underlayment in all eaves, rakes, and valleys to prevent ice dam leakage.
  • Ensure all fasteners are sealed with neoprene washers compatible with Minnesota's freeze-thaw cycles.
  • Schedule a final inspection after the first snowfall to check for any loose panels or sealant failures.

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