Substrates and mortar must match the specific waterproofing system.
For the PSC Pro WP membrane, the manufacturer’s Gen II Custom MUD kit instructions allow drywall and describe concrete board as preferred for its additional strength. This is a system-specific statement, not permission to install every waterproofing membrane over every wallboard.
The same instructions say to avoid pre-waterproofed substrates and additional topical or liquid waterproofing behind Pro WP because extra barriers can substantially slow the mortar’s cure. Do not apply Pro WP directly over bare wood.
PSC’s Pro GEN II instructions require a suitable premium modified latex/polymer thinset mixed from dry material. Premixed bucket or pail thinset is prohibited for this kit. Follow the membrane’s overlap, bond-coverage and curing instructions, plus the board manufacturer’s fastening and support requirements and applicable building code.
For another membrane or prefabricated board system, use that system’s own approved substrates and mortar requirements.
Sources: PSC Gen II Custom MUD kit instructions (wallboard section) and PSC Pro GEN II shower-kit instructions.
