While single-ply membrane roofing systems can be effective in certain applications, installing one over a metal building can introduce significant structural risk. According to Charlie Smith, McElroy Metal's National Recover Manager, the practice conflicts with fundamental metal building design principles and has been associated with roof collapses.
The Fundamental Design Mismatch
Smith explains, "A metal building is designed for a metal roof. It's not designed for a smooth-surface roof, like membrane roofing." This distinction is especially important when considering how water moves across the roof.
Metal roof panels are designed to carry water from the ridge to the eave in defined channels. "Every panel carries its share of the water from the ridge to the eave," Smith says. A smooth-surface membrane roof drains differently and may require a structure engineered for different loading and drainage conditions.
How the Structural Systems Differ
The incompatibility stems from differences between conventional low-slope roof framing and pre-engineered metal building roof framing.
| Design Factor | Conventional Low-Slope Roof | Metal Building Roof |
|---|---|---|
| Primary Roof Support | Bar joists are commonly manufactured with a built-in camber, or upward bow. | Straight purlins support the roof panels and may deflect more than bar joists. |
| Framing Continuity | Bar joists are generally not connected end-to-end, limiting the transfer of movement between bays. | Purlins are connected end-to-end, allowing deflection in one bay to influence adjacent bays. |
| Drainage Behavior | Bar joists flatten under design dead loads, helping create a surface suited to membrane-roof drainage. | Metal roof panels channel water along their ribs from the ridge toward the eave. |
The Danger of Ponding Water
Installing a membrane roof over a building designed for a metal roof can allow water to accumulate at the lowest point of the roof. As the ponded area spreads outward, the additional water weight can exceed the structure's original design assumptions. Five gallons of water weighs approximately 42 pounds, and that load can grow rapidly across a large roof area.
The risk is particularly serious on roofs with slopes below 1:12 and on buildings that rely on internal gutters, scuppers, or parapet walls for drainage.
How Progressive Deflection Can Lead to Collapse
One possible failure sequence begins with a clogged scupper that causes water to back up. As water ponds, the first purlin may deflect below the eave strut. Because the purlins are connected, downward movement in one bay can cause the purlin in the next bay to rise.
That movement can direct water from adjacent bays into the depressed area, increasing the load and accelerating the deflection. As more water collects, the cycle can continue until the roof structure fails.
What Contractors Should Consider
Membrane roofing systems have appropriate applications, but they should not be treated as a quick or economical recover option for every metal building. The existing roof structure, drainage path, slope, framing continuity, and anticipated loads should all be evaluated before selecting a recover system.
For metal buildings, a properly engineered metal roof retrofit or recover system can be designed around the building's structural and drainage requirements while extending the roof's service life.
Roof recover projects should be evaluated by qualified design professionals familiar with the existing building and applicable code requirements.
Smith summarizes the issue clearly: "A metal building is designed for a metal roof. It's not designed for a single-ply."
.png?width=440&height=293&name=How%20to%20Install%20Metal%20Roofing%20Over%20Existing%20Metal%20Roofs%20(1).png)
Comments on this article:
Scroll down to the bottom to submit a comment and join the conversation. Need help or have a question? Please contact us. Looking for a distributor or contractor? Please click here to get started.