Metal roofs are a popular choice for homeowners due to their durability, energy efficiency, and long lifespan. However, like any other type of roof, they aren’t indestructible and can leak. Here are five common reasons why metal roofing can leak, even when installed properly.
1. Metal Roofing Screws
Roofing screws cause the majority of leaks on metal roofs. Metal roofing screws are customized screws made to seal out water by compressing a rubber washer at the screw head. When the screw is drilled into the metal roofing panel, the rubber washer creates a “gasket” between the panel and the screw head. It appears simple enough, but several things can go wrong, such as under-driving screws, over-driving screws, driving screws at an incorrect angle, and screws that miss the framing member. All of these can create a gap for water to leak through.
Over-driven screws: Many roofing companies will overdrive the screw to ensure a tight seal between the metal panels and the screw head. The excessive torque breaks the rubber washer and causes it to spin to the side.
Under-driven screws: These issues are caused by insufficient screw torque, which fails to properly seat the rubber washer to the metal roofing panel. As a result, a gasket cannot be formed because the rubber washer is not compressed.
Screws are driven at the wrong angle: These prevent the rubber washer from sitting flat on the metal roofing. A portion of the screw is sealed, but another portion is not.
Screws that have missed the metal strut or wood framing: When the screw misses the underlay, it has nothing to seal against. These can be difficult to find because the screw is still present. Without physically checking, it is impossible to know whether the screw hit anything and created a seal or missed its target altogether.
Even if the screws were installed correctly with the proper amount of torque, the rubber washer is still unsafe. Hot summers followed by harsh winters are rough on rubber washers, especially in Victoria, British Columbia. They deteriorate and lose their seal, making it difficult to tell which screw is leaking and which is not. Since the instalment of metal roof screws is so vital to the longevity of the roof, it’s crucial to hire a reputable team like Victoria Metal Roofing.
2. Missing Sealants
Metal roof sealants never last as long as the metal panels they’re installed on. Therefore sealants need to be replaced regularly to keep up with the longevity of the roof. Sealants beneath trims like metal ridge caps and Z flashings, around roof transitions, counter flashings, reglets, and pitch pans need to be “topped off” as they wear.
Tip: Always use a metal roof sealant designed specifically for metal roofing. This is important for many reasons. Other silicone caulkings might not adhere to the paint on the roofing panels and trims. Plus, metal roofing experiences a daily expansion and contraction cycle, and any sealants that are to last must be able to remain flexible and stretch with metal without breaking their seal.
3. Stack Flashings
The area around stack flashings is another high-risk zone for leaks on a metal roof. Stack flashings, also known as “boots,” are flashings that surround pipes protruding from the metal roof, such as HVAC vents, air vents, and plumbing pipes. The stack flashing serves to protect the pipe from water damage. Most stack flashings are made of rubber or rubberized material, which sits flat on the metal roof and forms a seal while also “squeezing” around the pipe to form another seal. The daily expansion and contraction cycle of a metal roof put stack flashings to the test.
The sun also deteriorates the rubber flashings, which only last about half as long as the metal roof. The more caulking, sealant, or tar used behind and around the stack, the more they appear to hold water and cause leaks. Plan on replacing worn or rotten stack flashings to keep your building dry.
4. Seams and Over Laps
Seams where two pieces of metal roofing overlap frequently leak due to a condition known as a capillary draw. Capillary Draw is the ability of water to move uphill between two tightly joined pieces of metal. Sealant or butyl tape between the two pieces of metal can break the capillary draw, but if not used correctly, it can worsen leaks.
5. Curb Flashings
HVAC units on metal roofs are commonly installed on curbs. Installing metal roof panels is a relatively simple process, but it’s the flashing that truly puts a metal roofer’s skills to the test. HVAC flashing leaves little room for error, and keeping a curb dry can be easier said than done. Curbs on the uphill side, and the two upper corners of the curb flashing, are challenging to reach, especially with larger HVAC units. Water frequently becomes trapped behind the unit and “stands” behind the flashing, eroding sealants and causing leaks. The more caulk, seal, or tar you apply to the uphill side, the more water it will hold. Without removing metal roofing panels and starting over, this Catch-22 is hard to resolve.