Above the joists
An above-joist system is coordinated before the deck surface closes access. Product-specific details need to work with the house connection, fasteners and openings.
This position should be identified in the proposal, alongside the deck structure and finish ceiling where one is included. Do not assume every branded system uses the same assembly.
Beneath the deck
A retrofit system can collect water below an existing walking surface where clearance and condition permit. Ask what remains exposed above the collection panels and how the installation affects structural inspection.
Closing a ceiling should not hide components that need repair. Specify access to drains, connections and any electrical equipment.
Beyond the collection area
Gutters and downspouts need a planned discharge. Leaves and debris introduce cleaning needs, and open sides still allow weather to enter the sitting area.
Match the expectations to the actual system and site. A sheltered outdoor area is a different project from a sealed or conditioned room.