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Using IMP Roofs Over Decking

Know the IMP roof installation differences of decking versus open purlin/joists

Metl Span Feb19 1

Insulated metal panel (IMP) roofs are known for being low maintenance, energy efficient and durable. As with all roofing systems, the longevity and proper performance of the roof relies on proper design and installation. When roof IMPs are installed over decking, there are a few important differences to consider versus application over open purlins or joists.

Roof System Differences Between Pre-Engineered and Structural Steel

Pre-engineered buildings are designed with integral cross bracing in the roof, eliminating the need for decking. IMPs are then attached directly to the top flanges of roof purlins, where they can help provide stability against purlin roll under roof loads.

However, in structural steel buildings metal decking is commonly used in lieu of diaphragm bracing. The use of decking is a relatively economical way to brace the roof against building movement caused by various loads acting on the building (wind, snow, seismic, etc.). In short, decking not only adds rigidity to the roof, but provides stability to the walls as well.

In summary, roof IMPs should be considered as part of the building envelope but should not be used to provide lateral bracing when designing the structure. They should not be relied upon to provide roof diaphragm.

Establishing Environmental Control Layer Continuity

It is often better to use IMPs over decking systems due to their superior thermal resistance, low maintenance, low life cycle cost and architectural appeal. The key to proper design and performance is to maintain continuity of a building’s control layers. In other words, the designer should detail the roofing and wall intersections so that proper water, air/vapor and thermal protection is maintained. Creating envelope continuity is critical, especially at the roof perimeter (eave, rake and ridge). Flashing and sealants are used to seal the underside of the roof panel to the interior face of the insulated metal wall panel providing a continuous air/vapor barrier.

The roof panels should extend out beyond the structure to match the outer edges of the insulated metal wall panels. Gaps between the tops of the wall panels and the bottom of the roof panels should be filled with expandable foam, providing a continuous thermal barrier. Establishing proper control layers ensures the panel system will perform properly and eliminates the risk of condensation between the insulated panels and the decking system.

Attaching the IMPs to decking is also simple as the panel clips and fasteners in the panel side joints can be attached anywhere on the high cells (ribs) of the decking system.

Paul Collyer is vice president of business development at Metl-Span, Lewisville, Texas. To learn more, visit