Stone-coated steel was developed in New Zealand over 50-years ago and even today this market as well as others in the Pacific Rim use very little solid sheathing (Plywood or OSB sheathing) across the roof deck. This, unique construction practice enabled stone-coated steel panels to gain strong market share as they were easily mounted on “Battens” (usually 2'X2' wood strips) that spanned the rafters and provided front and back fastening points for each stone-coated steel panel. As these types of products were introduced to North America the 'batten' application was modified to accommodate applications over existing roofs such as Asphalt or wood shingles and even wood shakes. In these installations a batten grid consisting of '1X4 Counter-battens' (running vertically as a support batten) and '2X2 battens horizontally across for the stone-coated steel panels to be fastened to. Metro developed “Direct-to-Roof-Deck” installation methods using its existing designs and after extensive testing concluded the 'batten-less' application over solid sheathed roof decks provided exceptional wind uplift performance, just what the market needed to ensure protection against Florida and other gulf state's hurricane prone areas.
On the other hand if your home is located, say in Texas around the Dallas area not only is hail a concern (All Metro products are Class-4 Impact rated) but also usually the roofs are very steep and here is where a batten system can provide a benefit to the Installation process as the batten grid forms a ladder across the roof area to make the install process a little easier.
Whether a roof is installed on battens or batten-less really depends on the climate & roof design. To explain why climate has a bearing on the application method; let's assume the home is in south Florida (West Palm Beach), this market is considered by the building code as a severe wind region Known as HVHZ (High Velocity Hurricane Zone) and in some areas roofs need to withstand winds of up to 150-mph. In this case Metro has proven in numerous scientific tests at independent Laboratories (Underwriters Laboratories, UL) that the highest performing wind uplift system is a Batten-Less install method which uses two fasteners in an 'X' pattern.
Either method can be used to meet all building code requirements across North America and both methods (Batten or Batten-Less) can meet the new California Title-24 Energy code exception, due to providing a minimum 3/4" air-space above the roof deck sheathing which ensures the roof meets the new California Title-24 Energy Code requirements providing a truly green roof for the 21st century.