"R907.3 Re-covering versus replacement. New roof coverings shall not be installed without first removing existing roof coverings where any of the following conditions occur: 1. Where the existing roof or roof covering is water-soaked or has deteriorated to the point that the existing roof or roof covering is not adequate as a base for additional roofing.
2. Where the existing roof covering is wood shake, slate, clay, cement or asbestos cement tile.
3. Where the existing roof has two or more applications of any type of roof covering.
4. For asphalt shingles, when the building is located in an area subject to moderate or severe hail exposure according to Figure R903.5.
Complete and separate roofing systems, such as standing-seam metal roof systems, that are designed to transmit the roof loads directly to the building’s structural system and that do not rely on existing roofs and roof coverings for support, shall not require the removal of existing roof coverings.
Installation of metal panel, metal shingle, and concrete and clay tile roof coverings over existing wood shake roofs shall be permitted when the application is in accordance with Section R907.4.
The application of new protective coating over existing spray polyurethane foam roofing systems shall be permitted without tear-off of existing roof coverings."
Well, (1: major premise) if the manufacturer of the material is the one which specifies how to install the material, and (2: supporting premise) the manufacturer of the material always specifies what kind of material must be installed under the shingles being installed, and (3: supporting premise) the manufacturer never specifies that shingles are an appropriate underlayment for shingles, and (4: conclusion) never would be the answer, based on the manufacturer’s specifications (other rules and regulations notwithstanding, and typically deferring to manufacturer’s specifications). :mrgreen: