The core engineering difference
Every deck drainage system has the same goal: a dry space under the deck. The two ways to get there are fundamentally different.
- Above-joist drainage places the watertight layer at the walking surface of the deck. Water never reaches the framing. The system is integrated into the deck boards as the deck is built. Read the full above-joist drainage explainer.
- Below-joist drainage places the watertight layer beneath the framing as a tray, panel, or membrane assembly. Water runs through the deck boards, soaks the joists, and is collected below. The system is hung underneath an existing or new deck after the boards are in place.
The first approach keeps the structure dry. The second approach keeps the patio space below dry while the framing gets wet on every storm. That single distinction drives most of the comparisons that follow.
Side-by-side comparison
| Attribute | Above-Joist (Integrated) | Below-Joist (Retrofit) |
|---|---|---|
| Where the water stops | At the deck surface | Below the joists |
| Joists stay dry | Yes | No |
| Application | New construction only | New or retrofit |
| Trades involved | One: deck builder | Two: deck builder plus drainage installer |
| Visible from below | No, framing is dry and can be finished any way | Yes, tray or panel ceiling is part of the look |
| Long-term maintenance | Surface cleaning only | Tray cleanout, debris removal, periodic flashing inspection |
| Risk of joist rot over time | Removed by design | Same as a standard wet-framed deck |
When above-joist wins
If the deck is being built from scratch, an integrated above-joist drainage system is the right call. The framing stays dry for the life of the structure. The install is a single trade. The under-deck ceiling can be finished any way the homeowner wants because there is nothing hanging from the joists. And the surface that does the waterproofing is also the surface that carries the deck warranty, so the homeowner is not splitting coverage between a board manufacturer and a separate drainage manufacturer.
When below-joist still makes sense
A below-joist retrofit system is the right answer when the deck already exists and the homeowner does not want to tear up the surface to rebuild it. It will keep the patio space below dry on rainy days. The trade-off is that the framing will continue to get wet on every storm, which is the same condition any standard deck has had since the day it was built. The retrofit system simply adds a dry zone underneath.
How AmeriDex fits in
AmeriDex is the integrated above-joist deck drainage system. Cellular PVC deck boards lock onto the Dexerdry TPE seal so 100% of the rain is diverted off the deck and the framing stays dry. It is engineered for new construction with a conventional 16 inches on-center joist layout, no special sub-framing, and a 25-year residential limited warranty. Walk through how the system works step by step, request free samples in all seven colors, or submit your project for a free quote.