A Strategy for Moisture Prevention
Posted: September 3, 2021 | Project Management
The growth and overall popularity of metal used in architectural applications has increased steadily over the last three decades likely because the options for colors, sizes, textures, etc. are virtually limitless. However, one unfortunate by-product of this enormous growth is a lack of understanding of what metal products are watertight, and those that are well … not.
Of course, metal roofs are only as watertight as their seams and details. Structural standing seam metal roofs are really the only systems that can be relied upon to be watertight at the panel seam in most applications. Structural standing seam metal roofs, in general, are characterized by 2" or higher seams, mechanical field seaming of the panels, factory-applied, high quality seam sealants, and usage of a clip attachment that allows thermal movement. These are all characteristics that allow these panels to span open framing and stay watertight. It’s important to remember that architectural standing seam roofing usually relies on an underlayment for its waterproofing.
Here is a brief moisture prevention strategy for standing seam metal roofing:
• Use full length panels as much as possible. Keep the number of panel laps to a minimum. If panel laps must be made, use approved sealants with compression fasteners that don’t back out.
• Eliminate exposed fasteners where possible. The right question to ask is "if the screw or rivet fails, will my roof leak?" If the answer is yes, consider a different detail; exposed fasteners rarely last the life of a modern metal roof system.
• Use a roof panel that fits with the slope requirements. Most manufacturer warranties are limited to ½:12 minimum. Very few offer warranties as low as the building code minimum of ¼:12 slope. This is for good reason. The seams and details ...at these very low slopes.... must be especially water tight to keep water out.
• Use proper metal and foam closures with approved sealants. Unfortunately, metal details are often lacking in waterproofing specifics when projects are bid. Manufacturers offer the closure assemblies needed to properly seal ridge, rake, valley, and eave details. Manufacturer’s shop drawings should always be used to determine the proper components and their application.
• Sealants should never be exposed. No matter how good the sealant (silicone, urethane, or a hybrid), it will never last the life of the metal exposed to the sun’s drying UV rays.
• Insulation above deck: Ventilate the underside of the SSMR. Condensation on the underside of a metal panel is a concern in any climate, so the roof assembly should always provide a way to prevent or manage accumulated moisture. Stand off clips allow for air flow below the roof panel. This space coupled with a proper high-temperature metal underlayment provides a means to manage incidental moisture which may occur here.
• Cold attic spaces: Ventilate an attic space properly. Air flow should have a direct path to ventilate a roof properly. The risk is a potential for condensation. Passive ventilation should be done one of two ways.
1. Vent the eave (soffit) and the ridge to provide air flow the entire length of the roof. International Building Code provides requirements for the amount of ventilation per square foot of roof area.
2. Vent though the gable walls on both ends of the roof. A combination of both, and even use of powered vents in combination with these, can short circuit the air flow and cause a problem.
• Plan for temporary waterproofing during installation. The greatest chance for a leak in a metal roof is when it is being installed. Night tie-ins need to be thought out and planned in advance of construction.
Most points in this strategy can be applied to metal wall construction as well. Metal panel rainscreen applications are very popular, and are where we have seen some of the most exciting use of metal products. These applications focus on managing moisture in a drainage cavity as a way of preventing intrusion into the building.
Whether a roof or a wall application, moisture prevention in metal construction is best accomplished with good preparation and a team approach to the project. Manufacturers are an important member of this team and should be relied on thoroughly throughout the project.
About the author: John L. Pierson, P.E., has more than 20 years experience in the construction industry and is highly regarded for his thoughtful, creative and efficient approach to building enclosure solutions. John's expertise includes construction material testing, product development and management as well as construction management of multimillion dollar design-build projects. In his current role as Director of Engineering for Garland Co., John supervises a staff of engineers who perform a full range of services to provide high-performance options and solutions to a variety of building envelope projects. John is a frequent presenter of seminars and AIA-accredited classes on building enclosure technology and code compliance. John is active in several industry associations, including American Society of Civil Engineers, Building Enclosure Council, and Air Barrier Association of America. www.garlandco.com