Tips for multi-level deck drainage that nobody talks about

I went through this exact struggle last summer building a floating deck next to my detached garage. I used Bison pedestals because my slab was sloped about three inches across ten feet, and trying to shim that with scrap wood would have been a nightmare. You are spot on about the air gap; I left a half-inch space between the deck and the siding to prevent splashing from rotting the bottom plate of the garage. For the planters, I actually used EPDM pond liner scraps I had left over from a water feature project. It’s thick enough to handle shovel edges when I’m replanting my perennials and keeps that corrugated metal from sweating directly onto the pressure-treated 2x6s. I used G-Tape for the joists because the acrylic adhesive holds up better in my humid climate than some of the asphalt-based rolls. It took an extra two hours to apply everything, but seeing the water bead up on those joist tops after a heavy storm makes the effort feel justified. Moving the frame because of setback rules is no joke though—I had to shave two inches off my rim joist because the city inspector got picky about the distance to the property line.

I’ve found that using Simpson Strong-Tie ZMAX connectors is the only way to go when you’re that close to the soil. Standard hardware just flakes off in a couple of years when the air gap is tight and moisture builds up.

For the planters, grab some Henry Blueskin VP100. It’s a self-adhering membrane that’s way easier to work with than loose liners when you're trying to seal around corners and corrugated bits. It keeps the structural wood bone dry even if the soil is soaked for weeks.

While everyone focuses on membranes, I’ve noticed that most people completely neglect the end-grain treatment on their pressure-treated substructure, wich is statistically where the fungal decay will actually initiate once you've compromised the factory-infused chemical envelope by cross-cutting your joists to length. I always keep a small tin of copper naphthenate nearby to brush onto every single cut surface, particularly where the joists meet the rim board, because even with high-quality flashing tape, capillary action can still draw moisture into those unprotected fibers. Also, ensure you use 316-grade stainless steel fasteners for those corrugated metal planters to prevent galvanic corrosion where the disparate metals interact in a high-moisture environment.

I learned the hard way to skip the cheap landscape fabric under a ground-level build. I switched to a heavy-duty non-woven geotextile and topped it with three inches of clean 3/4-inch crushed stone before the frame went down. It stopped the weeds from pushing trough and acutally helped the drainage enought that the area stays dry even after a downpour. Also, if you're doing built-in planters, I found that drilling 1-inch drainage holes in the bottom and lining the floor with an inch of river rock before the soil goes in prevents that 'soupy bottom' that eventually eats through any liner.