There are two general choices when it comes to the substrate that is used beneath metal roofing. The choice is between installing the metal roofing panels over battens or over solid decking. This blog article will look at both options.
Some of the original installations of metal roofing in this country were over battens. Battens are also sometimes called “strapping,” “stringers,” or “purlins”. Basically they are small boards run horizontally across the trusses of a structure. They are typically spaced a specified distance apart one from the next and then the metal panels are installed over them. This sort of application may have originated in Europe though it was also the common way to install wood shingles which were commonplace in North America at that time. Battens allowed the wood shingles to dry out from both sides so that they did not hold moisture which would result in rot and deterioration.
What Should Go Under My Metal Roofing?
When aluminum roofing came on the scene, it was found that it, too, could be installed over battens. This was because of its high strength / low weight ratio. Installation over battens was cheaper than putting down dimensional lumber to create a smooth surface. Also, on steep roof surfaces, the battens provided a “ladder” of sorts for roof access. This was in the days before various scaffolding and other equipment was commonly available.
The other option for installing metal roofing calls for a “solid deck” to be in place and then the roofing is attached to it. A solid deck originally meant dimensional lumber installed tightly together. Today, it often means plywood or oriented-strand board (OSB).
As metal roofing systems were developed in this country, starting originally with hand-formed panels and then machine-fabricated corrugated sheets, the focus was on systems which could be installed over battens.
Over the years, a problem has developed with metal roofing installed over battens. That problem is the accumulation of condensation on the back side of the roofing panels. If this condensation accumulates too much, it quickly causes a dangerous situation for toxic mold and other problems in the attic space.
Many people wonder why this condensation was not an issue with the early metal roofs, nor is it usually an issue with larger buildings. There are a couple of reasons for that and they both stem back to what causes condensation. Condensation occurs when warm, moist air collects in a space and then has contact with a cool surface. Obviously, when temperatures drop at nighttime, the back side temperature of metal roofing drops as well. If warm, moist air contacts this, the “dewpoint” will be reached and condensation will develop.
In past years, buildings were not constructed nearly as airtight as they are today. They had more natural ventilation and airflow occurring through the building envelope. Today, of course, for energy efficiency, we strive to make buildings as tight as possible through the use of energy efficient windows and doors as well as various vapor and air barriers including housewraps. At the same time that buildings became more airtight, we also introduced more interior moisture sources. Things like indoor plumbing, cooking, houseplants, and ventless stoves all introduce a lot of moisture to the interior of a home. If that moisture is not adequately vented outward and it is instead offered the opportunity to come in contact with the cool underside of a metal roof, condensation and problems will occur.
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 this has happened, many metal roof systems have been developed which must be installed over solid decking as that has been the construction trend anyway. (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 strong attic ventilation and an interior vapor barrier compensate for metal roofing installed over battens and prevent condensation problems? That is always a possibility but most residences have not been designed nor built with anywhere close to the amount of ventilation that would be required for this.
The other common question surrounds why condensation is not still an issue when metal roofing is installed over solid decking. There are several factors involved in answering that question. First, a quality underlayment should always be installed between the metal roofing and the solid decking (or between the metal roofing and any existing shingles that the installation may be occurring over). This underlayment protects the decking or old shingles from any condensation which may occur on the back of the metal roofing. While 30-pound felt is still seen as an adequate underlayment for this, many contractors also now use the newer polymer-based “synthetic” underlayments for their ultimate durability. In the case of a non-ferrous metal like aluminum, the back side of the metal will be unaffected by condensation which may develop between it and the underlayment. Finally, many of today’s residential metal roofing systems are designed with an integral airspace between the metal and the underlayment surface. This airspace helps prevent trapping of moisture and encourages moisture to escape from the seams between the metal panels.
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.
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