How to Properly Flash Metal Roofs Around Skylights and HVAC Units

In the harsh Minnesota climate, where sub-zero temperatures and relentless snow loads test the integrity of any building, the distinction between a functional roof and a catastrophic leak often lies in the details of the flashing installed around penetrations. For commercial metal roofing contractors, ensuring that skylights and HVAC units are sealed correctly is not merely a procedural step but a critical investment in energy efficiency, structural safety, and occupant comfort. When done poorly, these junctions become the primary entry points for water intrusion, leading to costly repairs, mold growth, and premature deterioration of the roof deck. Proper flashing installation creates a continuous waterproof barrier that directs water away from sensitive areas, protecting the building envelope from the elements. ## The Critical Importance of Proper Sealing in Cold Climates Minnesota's winter conditions present unique challenges for metal roofing systems. The expansion and contraction of metal panels due to extreme temperature fluctuations can create gaps if flashing is not meticulously aligned. Furthermore, heavy snow accumulation adds significant static load, which can compromise weak flashing joints, causing water to infiltrate and freeze, potentially leading to ice dam formation that seeps into the building interior. Unlike residential applications, commercial buildings house essential machinery and sensitive equipment, meaning the consequences of a leak are far more severe. A failed seal around an HVAC unit can ruin electrical conduits, while a compromised skylight can flood the roof deck and damage the structure underneath. Therefore, the flashing must act as a rigid, impermeable shield that withstands wind uplift, thermal movement, and heavy precipitation without failure. ## Preparing the Roof Deck and Installing the Base Flange Before any flashing material is applied, the underlying roof deck and the penetration itself must be thoroughly cleaned and inspected. Debris, rust, or loose fasteners on the existing metal roof can prevent a proper seal from forming. For skylights and HVAC units, the manufacturer's specific instructions regarding the base flange are paramount. The base flange is the circular or square metal ring that sits directly on the roof deck under the roof penetration. It must be pressed down firmly until it reaches the deck and is secured with appropriate fasteners, often requiring a washer to distribute the load and prevent tearing the metal. In commercial settings, it is common to use self-drilling screws specifically designed for metal-to-metal connections to ensure a tight, leak-free joint. ### Choosing the Right Flashing Profile for Commercial Applications Once the base flange is secured, the next step involves selecting the correct flashing profile for the specific type of penetration and the local weather conditions. Commercial metal roofs typically utilize standing seam systems, which offer superior weather resistance compared to exposed fastener systems due to their concealed fastener design. The flashing material itself must be compatible with the roof membrane; using dissimilar metals can lead to galvanic corrosion, which weakens the seal over time. Common materials include aluminum, copper, or specialized coated steels. For Minnesota, low-profile, heavy-gauge flashing is often preferred to prevent wind uplift during severe storms. The flashing should extend well beyond the base flange, wrapping around the sides of the skylight or HVAC unit to create a drip edge that sheds water away from the penetration rather than allowing it to run down the face of the unit. ## Installing the Counter-Flash and Sealant The installation of the counter-flashing is the most critical phase in creating a watertight seal. This piece of flashing is attached to the side of the skylight or HVAC unit, overlapping the base flashing underneath. The key to success here is ensuring that the water flow is directed over the base flashing and away from the penetration, never allowing it to run under the flashing. In standing seam systems, the sealant bead should be applied to the roof deck where the base flange meets the deck, not necessarily on the flashing itself, allowing the metal to expand and contract without cracking the sealant. However, a continuous, high-quality silicone or polyurethane sealant bead is often applied along the perimeter where the counter-flashing meets the base flange to provide an additional layer of defense against wind-driven rain. The counter-flashing must be mechanically fastened to the building penetration to ensure it does not shift or fall during high winds. ## Final Inspection and Maintenance Protocols After installation, a thorough inspection is required to verify that all seams are watertight and that there are no exposed fasteners or gaps. In a commercial context, this often involves a water test or a visual inspection from the ground using binoculars to check for streaks of water running down the roof. Regular maintenance schedules should be established for commercial properties, with inspections conducted at the end of each season before the harsh winter sets in. Looking for signs of rust, loose fasteners, or sealant deterioration allows for minor repairs before they escalate into major leaks. Properly flashed penetrations significantly extend the life of the commercial roof system, reducing the frequency of maintenance calls and ensuring that the building remains dry and safe for its occupants throughout Minnesota's unpredictable weather patterns. To guarantee long-term performance in the northern climate, contractors should adhere to the following best practices during the installation process: 1. Clean the base flange and deck surface to remove all grease, paint, and oxidation before fastening. 2. Ensure the base flange is seated fully against the roof deck to eliminate voids where water could pool. 3. Apply sealant only to the specified areas to maintain the flexibility required for thermal expansion. 4. Verify that the counter-flashing overlaps the base flange at a minimum of one inch to create a positive drainage path. 5. Check all mechanical fasteners for tightness immediately after installation to prevent loosening caused by vibration. 6. Inspect the drip edge to ensure it directs water away from the building penetration effectively. 7. Document all work performed for future warranty claims and property management records. ## Related reading - [Mastering Metal Roofing Coatings for Commercial Construction in Minnesota](/blog/a-buyers-guide-to-galvanized-galvalume-and-painted-metal-roofing) - [Elevating Energy Independence: The Metal and Solar Synergy in Minnesota](/blog/can-i-add-solar-panels-to-a-metal-roof-a-feasibility-guide) - [One-Day Metal Roofing? The Reality for Small Homes in Minnesota](/blog/can-you-install-a-metal-roof-in-less-than-a-day-on-a-small-home) - [Maximizing Roof Lifespan Through Strategic Winter Maintenance](/blog/can-you-snow-remove-your-metal-roof-safely-without-damaging-it) - [Securing Your Commercial Roofing Investment for Minnesota's Harsh Climate](/blog/commercial-metal-roofing)