When it comes to the material used beneath metal roofing, there are generally two main options. The choice is between installing the metal roofing panels over battens or over solid decking. This article will look at both options.
Early metal roofing installations in this country often used battens, also known as “strapping,” “stringers,” or “purlins.” These are small boards placed horizontally across the trusses of a structure, spaced at specific intervals, with metal panels installed on top. This method may have originated in Europe, but it was also commonly used for wood shingles in North America. Battens helped wood shingles dry from both sides, preventing moisture buildup, rot, and deterioration.
What Should Go Under My Metal Roofing?
When aluminum roofing was introduced, it was discovered that it could also be installed over battens due to its strength and lightweight properties. This method was more cost-effective than using dimensional lumber for a smooth surface. On steep roofs, battens also served as a makeshift ladder for easier access, before modern scaffolding became common.
The other installation option is attaching the metal roofing to a “solid deck,” which used to mean tightly fitted dimensional lumber but now typically involves plywood or oriented-strand board (OSB).
As metal roofing systems evolved from hand-formed panels to machine-made corrugated sheets, they were designed to be installed over battens. However, over time, condensation became an issue with this method. Moisture can build up on the underside of the metal panels, leading to problems like toxic mold in the attic.
Many people wonder why condensation wasn’t an issue with older metal roofs or larger buildings. Condensation happens when warm, moist air meets a cool surface, like the underside of a metal roof at night. If the air cools enough to reach the dewpoint, moisture forms. In modern, tightly sealed homes, this trapped air can cause condensation, so proper ventilation and insulation are essential to prevent moisture damage and mold growth.
In the past, buildings had more natural airflow and weren’t as airtight as they are today. Modern homes are built to be more energy-efficient with tight seals and less ventilation, trapping more moisture inside. Indoor activities like cooking, using plumbing, and even houseplants add moisture. If this moisture isn’t properly vented and instead reaches the cool underside of the metal roof, condensation will form, leading to potential issues.
Even today in large buildings such as warehouses, factories, or barns, metal roofing can be successfully installed over battens without condensation worries. This is because these buildings have less moisture created in them (usually) and they also typically have a great deal of natural airflow with frequent opening of doors, windows, roof hatches, and other things.
However, because of the condensation concerns, recent years have shown manufacturers of metal roofing shy away from recommending installations over battens and instead recommend solid decking. This has particularly been the case with smaller buildings or residential applications. As a result, many metal roofing systems have been designed to be installed over solid decking, aligning with modern construction practices. (It should be noted also that some metal roof systems have designs which require that they must be secured to battens, not solid decking. However, to avoid condensation concerns with those products, the battens can be installed over solid decking instead of direct to the roof trusses.)
Can effective attic ventilation and an interior vapor barrier mitigate condensation issues with metal roofing installed over battens? While this is a possibility, most homes aren’t designed or constructed with sufficient ventilation to support this solution.
Another common question is why condensation isn’t a problem when metal roofing is installed over solid decking. Several factors contribute to this. First, a quality underlayment should always be placed between the metal roofing and the solid decking (or any existing shingles). This underlayment protects the decking or shingles from any condensation that might form on the underside of the metal roof. While 30-pound felt is still considered adequate, many contractors now prefer using more durable polymer-based synthetic underlayment. For non-ferrous metals like aluminum, condensation that develops between the metal and the underlayment won’t affect the metal itself. Additionally, many modern residential metal roofing systems feature an airspace between the metal and the underlayment, which helps prevent moisture buildup and allows any trapped moisture to escape through the seams of the metal panels.
In conclusion, whereas many older metal roofs, or metal roofs on larger buildings, have been successfully installed over battens rather than solid decking, it is the clear preference for residential applications that metal roofing always be installed over solid decking to avoid dangerous and damaging condensation issues.