A retrofit sub-purlin or hat channel system is a structural support method used to upgrade an existing commercial metal roof without completely removing the current roof assembly. It adds new metal framing members—typically called sub-purlins, hat channels, or retrofit framing—over or alongside the existing roof structure. The new framing creates a stronger, more uniform surface for installing a new roofing system.
For commercial buildings with aging or structurally limited metal roofs, retrofit systems can provide a practical alternative to complete roof replacement.
How Does a Retrofit Sub-Purlin System Work?
A retrofit system typically involves installing galvanized or corrosion-resistant steel members over the existing roof. These members are strategically fastened to the existing structural framing, such as purlins or rafters, rather than relying solely on the existing metal roof panels.
Once installed, the retrofit framing creates a new plane above the existing roof. Insulation, decking, or a new roofing membrane can then be installed over this framework depending on the building’s requirements and the selected roofing assembly.
A hat channel gets its name from its cross-sectional shape, which resembles a hat. These lightweight steel channels are commonly used to provide additional support and create a level attachment surface. A sub-purlin performs a similar function but may be designed with greater structural capacity depending on the project.
Why Are Retrofit Systems Used on Commercial Roofs?
One of the biggest advantages is that a retrofit system can allow a building owner to upgrade an existing roof without tearing off the entire metal roof. This can reduce construction disruption and, depending on the existing conditions and project design, potentially reduce labor and disposal costs.
Retrofit framing can also help address problems associated with older metal roofs, including uneven surfaces, inadequate support for a new roofing system, and insufficient space for insulation.
A properly engineered retrofit assembly can provide:
- Additional structural support
- A more consistent surface for a new roof system
- Space for increased insulation
- Improved thermal performance
- Compatibility with certain low-slope commercial roofing systems
- Less disruption than a complete tear-off in some applications
- Extended service life for the building’s roof assembly
Can a Retrofit System Be Installed Over an Existing Metal Roof?
In many cases, yes, but the existing roof must be evaluated first. A retrofit system is not automatically appropriate for every commercial building.
A qualified roofing contractor or structural engineer should inspect the existing roof panels, purlins, fasteners, framing, corrosion, load capacity, drainage, and overall structural condition. The proposed retrofit assembly must also account for applicable building codes, wind uplift requirements, insulation requirements, and the weight of the new roofing system.
If the existing structure has significant corrosion or structural deterioration, additional repairs or a different roofing strategy may be necessary.
Retrofit Sub-Purlins vs. Hat Channels
The terms are sometimes used interchangeably, but they are not always identical. Hat channels are generally cold-formed steel members with a hat-shaped profile and are commonly used as secondary framing or attachment members. Sub-purlins are additional purlin-like structural members installed to provide support between or alongside existing structural components.
The appropriate member depends on the building’s design, the existing roof structure, the loads involved, and the new roofing system being installed.
Is a Retrofit Roof System Right for Your Building?
A retrofit sub-purlin or hat channel system can be an effective solution when an existing commercial metal roof needs additional support or when the owner wants to install a modern roofing assembly without a complete roof tear-off.
However, the correct approach depends on the existing roof’s condition and structural capacity. Shieldline Roofing can evaluate your commercial roof and help determine whether retrofit framing, roof restoration, coating, or replacement is the most appropriate solution.
For commercial property owners, the key is to have the roof and structural system evaluated before selecting a retrofit design. A properly engineered retrofit system can provide a durable foundation for a new commercial roofing assembly while minimizing unnecessary disruption to the building.
Related Questions
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- Can a liquid-applied system be used over a metal roof?
- What is a pre-engineered metal building and how is its roof different?
- Is a new metal roof quieter than an old one during rain?
- What are the environmental benefits of retrofitting a metal roof?
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