Contractor used exterior GlasRoc sheathing in a fire-rated shower wall. The inspector says no

In a post on Reddit's r/Homebuilding, the owner says they had asked for cement board or DensShield Fireguard behind the tile. The contractor put up GlasRoc Sheathing instead, and the inspector sided against the contractor.

The product data backs the inspector. CertainTeed makes GlasRoc Sheathing to ASTM C1177, a standard for glass-mat gypsum sheathing, and describes it as a panel for exterior walls and soffits. Its submittal sheet lists "Not recommended for use as a tile backer" among the installation limits. The company sells a different board, GlasRoc Tile Backer, made to ASTM C1178 for showers, tub surrounds and other interior wet areas, including a 5/8-inch Type X version for rated walls.

The International Residential Code points the same way. Section R702.4.2 requires tile backers on tub and shower walls to come from Table R702.4.2 and to be installed per the maker's instructions. The 2024 table names four standards: C1178 for glass-mat gypsum panels, C1278 for fiber-reinforced gypsum, C1288 for fiber-cement board and C1325 for cementitious backer units. Exterior sheathing made to C1177 is not on it.

The fire rating is a separate question. CertainTeed says its 5/8-inch GlasRoc Sheathing Type X is approved for "applicable" UL assemblies, so it counts only if the UL design for this wall lists it. Either way, the tile base still fails. The fix is to replace the panels with a listed backer, such as a C1178 Type X board, and to use a waterproofing membrane that meets ANSI A118.10, the standard CertainTeed names for its tile backer. The inspector's written correction notice is the owner's main document if the contractor refuses to pay for the tear-out.