Published in RCI Interface
Moisture Problems in Seam Fastened Metal Systems

Pre-engineered metal buildings with seam fastened steel panel roof assemblies are common in the industrial and commercial sector, in most regions of the United States. As many owners of these roof systems can attest, they often exhibit moisture related problems, typically after 10 or more years of service.

It has been Benchmark's experience that moisture problems are generally related to one or both of the following:

• Leakage from roof system or component deficiencies
• Condensation from an inappropriate or deteriorated vapor retarder/insulation assembly

Roof System Deficiencies

Horizontal panel seams, vertical panel seams, panel fasteners, metal panels and perimeter/penetration flashings are the five primary components of the seam fastened metal roof assembly. Deterioration of one or more of these components are common causes of roof leakage.

Horizontal Seams occur where the bottom edge of a roof panel overlaps the top edge of the down slope panel. Over time, sealants within these seams become brittle as the panels expand and contract.

Foot traffic, expansion, contraction and other forces deflect the panel seams, resulting in broken seals and open seams. Water driven by wind, negative building pressure or ice damming during freeze/thaw periods, may travel up slope and into the building through these voids.

Vertical Seams occur where the sides of adjacent panels overlap. Similar to horizontal seams, sealant deterioration and panel deflection also reduce vertical seam integrity and allow water entry in conjunction with wind, negative building pressure, or ice damming.

Panel Fasteners typically contain neoprene sealing washers to prevent water infiltration. Ultraviolet exposure and panel movement from expansion and contraction normally deteriorate these washers over time. In addition, the fastener holes elongate from panel movement, in some instances larger than the washers. Over-tightening of fasteners during installation immediately damages the washers, and increases their susceptibility to deterioration as they age. Loose, missing or rusted fasteners are also common deficiencies.

Steel Panels by nature, are susceptible to deterioration over the long term. Common deficiencies include rusting, kinking, puncturing or finish deterioration.

Perimeter and penetration flashings are common locations that exhibit leakage. The effects of panel movement are concentrated at these locations, thus accelerating deterioration of sealants, fasteners and other flashing components. Premature flashing problems can also be attributed to poor design and installation. Quality workmanship and use of appropriate flashing material are vital to long-term flashing and roof system performance.

Insulation/Vapor Retarder Deficiencies

For the insulating system to perform in a metal building, two components are required; a vapor retarder that is airtight and possesses an acceptable permeability rating, and insulation with sufficient R-Value to permit the underlying vapor retarder to remain above the dew point temperature during the coldest anticipated weather. Condensation and widespread dripping may occur if one or both of these criteria are not satisfied, especially in northern climates.

• Assuming the vapor retarder material's perm rating is acceptable, airtightness may be compromised by a number of defects. Puncturing during or after installation is the most common problem. Poorly installed seams, age induced deterioration of seams, or poorly detailed terminations at perimeters and penetrations may also be responsible.
• Insulation R-Value is significantly reduced when moisture enters the system. This moisture may result from leaks in the metal roof assembly, or condensation from a damaged vapor retarder. As the R-Value continues to decrease, so does the insulation's ability to maintain the vapor retarder at a temperature above the dew point. Condensation may result, depending upon interior relative humidity and exterior temperature. In several cases, drippage from condensation may be more detrimental than leakage from the metal roof.

Corrective Measures

Corrective measures vary widely in scope and cost depending on the severity of the problems. Possible solutions include routine maintenance of problem components, installation of an elastomeric coating system, application of a single-ply membrane recover, or complete replacement of the metal roof, insulation, and vapor retarder down to the structural framing system.