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.
The Fundamental Design Mismatch
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.
Structural Engineering Differences
The incompatibility between membrane roofs and metal buildings arises from fundamental differences in their construction methods. Here's a comparison:
Traditional Building Design:
- Utilizes bar joists manufactured with built-in camber (upward bow).
- Bar joints are not connected end-to-end.
- Bar joists flatten naturally under dead loads, such as ponding water, creating an ideal flat surface for membrane roofing.
Metal Building Design:
- Utilizes straight purlins that deflect more than bar joists.
- Purlins are connected end-to-end, allowing the deflection in one bay to impact another.
- It is designed to account for channelized water flow down the ribs of the metal panels.
The Danger of Ponding Water
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 Collapse Mechanism
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.
Industry Recognition
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."
About McElroy Metal
Since 1963, McElroy Metal has served the construction industry with quality products and excellent customer service. The employee-owned components manufacturer is headquartered in Bossier City, La., and has 14 manufacturing facilities across the United States. Quality, service and performance have been the cornerstone of McElroy Metal’s business philosophy and have contributed to the success of the company through the years. As a preferred service provider, these values will continue to be at the forefront of McElroy Metal’s model along with a strong focus on the customer.
Topics: Recover and Retrofit, Metal Buildings
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