The best overall “system” actually isn’t a system- it’s a watertight decking.
PVC interlocking decking creates a watertight deck that will keep the underside dry without the maintenance associated with “after-thought” add-on systems.
If you can, wait out the existing decking until it needs replacing and replace it with the PVC interlocking decking.
Depending on the height of the area under the deck and if the current non waterproof deck is in good shape, you could build a framework under the deck and attach galvanized roofing which would keep the water deflected and provide a reflective ceiling. This would make it easy to light the area.
The best overall “system” actually isn’t a system- it’s a watertight decking.
PVC interlocking decking creates a watertight deck that will keep the underside dry without the maintenance associated with “after-thought” add-on systems.
If you can, wait out the existing decking until it needs replacing and replace it with the PVC interlocking decking.
The existing is trex and only about 15 years old so I may not see the day it gets replaced. I suppose I could just redo it to get the rain shedding if the price isn't too much more than under decking.
How is the intersection with the walls handled? Is there a gutter or some way to divert the runoff to gutter drains? I have a large downspout on each side of the deck and was planning on dumping the runoff into those.
Got any links/examples? I will google around a bit for more info.
I am interested in the watertight PVC decking you speak of. I haven't seen that before. Can you provide a link to a product you're talking about?
I bought the panels from Lowes as well as the special screws with little rubber washers.:
https://www.lowes.com/pd/Tuftex-Actual-26-in-x-120-in-DeckDrain-Opaque-Tan-Under-Deck-Ceiling-Panel/3043409
My joists are perpendicular to the house wall, and the slope of my ceiling went down away from the house. I had to install small strips of treated lumber that went across, under the joists that got taller as I went away from the house to create the drain angle. I made my angle twice what was recommended to promote good drainage.
The ceiling panel was right at the bottom of the joist next to the house and was about three inches below the joist at the farthest point away from the house. I ran a gutter along the bottom edge of the ceiling that ran into a downspout.
I had replaced the deck boards with Trex about five years before I installed this ceiling underneath.
The ceiling worked great for about five years, then I started noticing a few dips in the deck surface.
I pulled back a few ceiling panels to find my joists were rotting, the joist nails were rusting out, there was mold growing on the joists, and roaches had built condominiums at the end of each joist. Not good!
I removed all of the ceiling panels, replaced a few joists, jacked up the others and added new joist hangers, and scrubbed off the mold. I told the roaches to get lost, and painted the joists with stuff that turns rotted wood into plastic.
I have about 6 feet of clearance under the deck, so it is just for storage, not living space.
If I had the vertical space under the deck, I would lower the ceiling about 6 inches or more to allow air to freely circulate between the deck and the ceiling.
I like the idea of replacing the Trex with a waterproof, interlocking decking material, but the Trex was so expensive. Maybe at the next house.
I did it w/ corrugated plastic panels to make the space under the deck into a storage shed.
https://www.lowes.com/pd/Tuftex-SeaCoaster-2-17-ft-x-12-ft-Corrugated-Pvc-Plastic-Roof-Panel/3010593
And plastic gutter and down spouts. Remember that you need a slope for the panels and also a slope for the gutter unless you want two downspouts.