You can picture coffee under the upstairs deck while rain patters overhead. Then one drip lands on the chair. A finished ceiling can make this a lovely place to sit, but the water route above it needs to work first, and remain reachable afterward.
Decide where the water will travel
Ask the installer to show the collection surface, slope, seams, downspout, and discharge point. Follow that route all the way into the garden. A system that moves the drip onto the basement entrance has merely relocated the problem.
Trex RainEscape's installation resources illustrate an above-joist system installed as part of the deck assembly. Retrofitting that kind of system can involve lifting boards. Compare actual product details rather than assuming every under-deck product installs from below.
Keep the ceiling repairable
Coordinate lighting, wiring, and access panels before the ceiling closes. Ask the drainage installer which joints and outlets may need attention, then put the ceiling openings where they help with those tasks. A panel directly below a fitting is little use if a framing member still blocks your hand. Check the access with the trades while it's open, and keep a photo showing the ceiling layout against the drainage above.
Compare quotes at the same boundary
- Is removal and reinstatement of existing decking included?
- Are wall junctions and posts shown in the drainage detail?
- Where does the collected water discharge?
- Who handles electrical work and verifies suitable fixtures?
- Can the ceiling be opened without destroying the drainage layer?
Wind-driven rain can still reach an open-sided outdoor space. Keep that distinction in the furniture choice and storage plan. This is a sheltered outdoor room, unless a separately designed enclosure says otherwise. It does not become indoor storage because the ceiling looks finished.