Suitable cement boards for roof sheathing fire protection – GreenBuildingAdvisor

Problem: Looking either for a source of GP DensDeck in the Denver, CO region, or a suitable alternative.

We are re-roofing a house in a WUI zone. The old roof (asphalt shingle) will be stripped. I propose that we lay down a surface of GP DensDeck over the existing sheathing, then the metal roof supplier’s recommended underlayment, and finally the metal roof.

We are unable  to find a supplier of GP DensDeck in the region. What might be some suitable alternatives?

Would a simple interior cement backer board such as USG Durock or James Hardie’s backer board work? I’d consider upgrading to a fully adhered roofing underlayment and wrapping it down the edges of the backer board for additional protection to the board. Of course there will also be the usual drip edge flashings for protection.