We have hybrid concrete tile roof that has concrete tiles attached to a dense foam backing. The roofing was installed about 15 yrs ago. In looking in the attic, I noticed there is not plywood or other sheathing under the roof tiles, just an underlayment. Can this type of roofing system be installed without plywood or some decking?
Its odd looking, but the batten support lets the tiles 'notch' into place. You'll see the same support for clay tiles, also.
Because of the 'hanging' aspect of the tile, most locales only allow tile roofs for pitches between 4/12 and 6/12 (rise over run). Lower pitches are possible, but then waterproof membranes are required. In those cases, sheathing support for the membrane would be needed.
You don't mention it, but not using sheathing is usually only done in warmer climates, where summer heat is the main problem.