While single-ply membrane roofing systems are effective solutions for various applications, their installation over metal buildings poses significant structural risks. According to Charlie Smith, McElroy Metal's National Recover Manager, this increasingly common practice fundamentally conflicts with metal building design principles and has been linked to several roof collapses.
Smith emphasizes, "A metal building is designed for a metal roof. It's not designed for a smooth surface roof, like membrane roofing." This distinction is crucial, particularly in managing water across the structure.
Metal roofs are engineered to distribute water load evenly. "Every panel carries its share of the water from the ridge to the eave," Smith explains. Smooth surface roofs are inherently designed to drain differently and require more robust engineering and loading capabilities.
The incompatibility between membrane roofs and metal buildings arises from fundamental differences in their construction methods. Here's a comparison:
Installing membrane roofs on buildings designed to have a metal roof can lead to dangerous water accumulation. Ponding water typically starts at the lowest point on the roof surface and continues to grow outward from the original point. As the affected area grows, additional weight is applied to the structure beyond what the initial building was designed to support. While initially that may not seem problematic, it's important to remember that just five gallons of water weighs over 40 lbs.
As might be expected, the risk of ponding water becomes particularly acute in buildings with slopes below 1:12 and those with internal gutters, scuppers, and parapet walls.
The problem in these applications usually starts with a clogged scupper, causing water to back up. As the water ponds on the roof surface, the first purlin deflects below the eave strut. When the purlin in the first bay is pushed down, it causes the purlin in the next bay to rise because they are connected. This action draws water from the two adjacent bays into the affected bay, and the roof can collapse due to the additional weight of the water.
The severity of this issue has gained attention within the construction industry. "There have been about four articles written on the subject," Smith notes, highlighting the growing awareness of these risks.
While membrane roofing systems have their applications, they should not be considered a quick or economical fix for metal buildings. The fundamental engineering principles of metal buildings necessitate roofing systems that effectively channel and distribute water load. Smith succinctly concludes, "A metal building is designed for a metal roof. It's not designed for a single ply."