Table of Contents
- Most Common Causes of Metal Roof Leaks
- Metal roofing screws
- Stack flashings
- Missing sealants
- Curb Flashings
- Seams and overlaps
Few roofing products can compete with metal roofing in terms of durability, strength, and performance. However, metal roofs are not indestructible and can leak. Here are five typical reasons why even correctly constructed metal roofs can leak.
The majority of leaks on metal roofs are caused by leaking roofing screws. Metal roofing screws keep water out by compressing a rubber washer at the base of the screw head. When the screw is inserted into the metal roofing plate, the rubber washer creates a “gasket” between the panel and the screw head. It seems straightforward enough, but some items may go wrong, such as under-driving screws, over-driving screws, driving screws at an angle that is not correct, and screws that bypass the framing member.
Over-driven screws: Many roofing companies would overdrive the screw to achieve a close seal between the metal roofing and the screw head. The excessive torque damages the rubber washer, which causes it to rotate to the right.
Under-driven screws: This is caused by insufficient screw torque to seat the rubber washer to the metal roofing plate correctly. There is no compression of the rubber washer and the formation of a gasket.
Screws inserted at an incorrect angle: This prevents the rubber washer from sitting flat on the metal roofing. A portion of the screw is secured, but another portion is not.
Screws that have slipped into the metal strut or wood framing underneath: There is nothing against which these can be sealed. These can be hard leaks to locate because the screw is always there, but without touching it, you wouldn’t realize that it didn’t strike something and hence failed to seal.
Even if the screws were correctly mounted and with the proper amount of torque, the rubber washer is still not secure. Hot summers accompanied by harsh winters are harsh on rubber washers. They deteriorate and break their seal, making it impossible to tell which screw is leaking and which is not.
The area around stack flashings is another area on a metal roof that is vulnerable to leaks. Stack flashings are the “boots” or flashings that go around pipes that protrude from the metal roof. HVAC windows, ventilation vents, and plumbing pipes would all vent and come through the metal roof. The stack flashing protects the pipe from water. The majority of stack flashings are made of rubber or rubberized material that lies flat on the metal roof and forms a seal while still “squeezing” through the pipe to create another seal. The displacement of the metal roofing due to expansion and contraction is constantly putting these seals to the test.
The sun even degrades the rubber flashings, which only last for half as long as the concrete roof. To keep the building safe, plan on removing worn or rotting stack flashings.
Metal roof sealants installed in combination with a metal roof only last as long as the metal roofing panels and must be replaced as part of routine roof maintenance. Sealants under trims such as metal ridge caps and Z flashings, over roof transitions, counter flashings, reglets, and pitch pans will all need to be “topped off” from time to time as they wear. Use a metal roof sealant designed specifically for metal roofing. Some silicone caulking might not stick to the paint on the roofing panels and trims. Metal roofing undergoes a daily expansion and contraction period, and any sealants that are to last must be able to stay stable and expand with metal without losing their seal.
HVAC systems on metal roofs are usually mounted on curbs. Installing metal roof panels is a relatively simple operation, but the flashing puts a metal roofer’s skills to the test. When it comes to HVAC, there is no space for error in fabricating the flashing, and keeping a curb dry can be better said than done. The uphill side of the curbs and the two upper corners of the curb blinking are difficult places to work on, particularly with larger HVAC units. Water also becomes stuck behind the device and “stands” behind the flashing, eroding sealants and causing leaks. The more caulk, patch, or tar you apply to the uphill foot, the more water it can carry. This Catch-22 is impossible to resolve without scraping metal roofing panels and starting from scratch.
Because of a phenomenon is known as capillary pull, seams where two parts of metal roofing overlap each other often leak. When water flows uphill between two closely connected pieces of metal, this is known as a capillary draw. Sealant or butyl tape between the two pieces of metal will split the capillary draw, but if not used properly, it will potentially cause leakage worse than if used no sealant or butyl tape at all.
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