Roof Terminology Part 3: Deck, Battens & Underlayment

Roof Terminology Part 3: Deck, Battens & Underlayment

All roofing products require a proper surface for installation. In this section, we’ll explain why what lies beneath a proper metal roofing installation is important.

Roof Deck

One of the frequent benefits of a metal roof is that it often be installed over existing shingles. When applicable, this option offers additional ecological benefits to our North Carolina environment.

Regardless of whether your old shingles are left in place, what is beneath them? What is the lumber that comprises your roof deck? This is important to know. Virtually any metal roofing product intended for residential use can be installed over what is called solid decking—typically a plywood, OSB (Oriented Strand Board), or closely spaced dimensional lumber surface placed over and attached to your roof rafters.


Some metal roofing products though, can also be installed over battens or lathe boards. Sometimes called “strapping,” these are spaced apart boards fastened to and spanning horizontally across your home’s rafters. There are a few products that must be batten-mounted, even if the battens must be installed over a solid deck. And there are some tile and shake profile products which, while they normally require solid decking, can also be installed over existing wood shingles on spaced sheathing.

Making sure that the product you’re choosing is appropriate for your application is critical. The final authority on how the metal roofing should attach to the roof should be your roofing manufacturer. Generally, the more heavily formed a product is, the easier it will be to install over existing shingles but that is not a hard and fast rule. It is always best to consult with the manufacturer. Quality manufacturers are ready and willing to answer your questions and will ask for dimensions, photos and any other information you can give them.


Underlayment serves as an extra layer of protection against water penetration and protects both the panel and the interior of your home during installation and for years to come.  If your metal roof is to be installed over solid decking or over existing shingles, a layer of quality underlayment between the two is essential.  Underlayment is also a good idea when the roof is installed over battens though a natural tendency for the underlayment to drop between the battens can be a problem.

For many years, the standard underlayment beneath a metal roof was 30-pound felt. In some cases, local building codes may require a more fire resistant underlayment or may even require the use of self-adhering “ice and water shield” underlayment over the perimeter and valleys of the roof. Always make sure that you’re adhering to the more stringent of either building code requirements or manufacturer specifications. If a self-adhering underlayment is used, it must be a smooth surface product rather than a granulated surface product so that the rough surface does not scratch the back of the metal panels.  A scratched panel could leave your roof vulnerable to rust and eventual cracks and holes from the abrasiveness.

Recent years have seen the development of many new underlayments generically called “synthetics”. These polymer-based products are longer lasting and easier to install than 30-pound felt.  They also serve as a “slip sheet” for standing seam products that can be prone to sticking to traditional roofing felt in hot weather.  When this happens, expansion of the metal can tear or wrinkle the felt underlayment.

At McCarthy Metal Roofing, we’re pleased to provide quality installations of Classic Metal Roofing Systems in the Triangle Area of North Carolina, and beyond.  We encourage you to learn more about the many benefits of metal roofing.