What an above-joist deck drainage system costs by deck size
Above-joist drainage is priced by the lineal foot of board, not by the square foot, because the board is the drainage system. A 5.5 inch board face plus the 3/16 inch gasket gap covers 5.6875 inches of deck, so one square foot of deck takes about 2.11 lineal feet of board. Here is what that looks like on four common deck sizes.
| Deck size | Square feet | Linear feet of board | Material cost |
|---|---|---|---|
| 12 x 16 | 192 | 405 | $3,241 |
| 16 x 20 | 320 | 675 | $5,401 |
| 16 x 24 | 384 | 810 | $6,482 |
| 20 x 24 | 480 | 1,013 | $8,102 |
Material cost only, at $8.00 per lineal foot. Linear feet are rounded to the nearest whole foot. Boards ship in fixed lengths, so your dealer will round up to whole boards and allow for cuts.
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Pricing shown is material cost only, is a typical figure, and can vary by color, length, accessories, and dealer. Labor is quoted separately. Request a written quote for an exact number.
What is included in that price
The $8.00 per lineal foot figure is a materials number, and it covers the whole drainage assembly, not just the board:
- Cellular PVC deck boards in the color and length you choose
- The Dexerdry TPE seal between every board, which is what makes the surface watertight
- Compatible fasteners for the installation
What the price does not cover:
- Labor, which your builder or dealer quotes separately
- Framing: joists, beams, posts, footings, and the ledger
- Railing
- Trim and fascia
- Permits and inspections
Because the drainage is part of the deck surface, there is no separate drainage product on the invoice. See how the two-part system goes together.
How above-joist and below-joist drainage costs differ
The two approaches do not just cost different amounts, they cost in different ways.
Above-joist. The drainage is built into the deck surface. One trade installs it in one pass while the deck is being built, so there is no second system to buy, no second crew to schedule, and no under-deck ceiling required. The joists stay dry from day one.
Below-joist. A separate trough, membrane, or gutter assembly is bought on top of the deck boards and installed as a second operation, working overhead in every joist bay after the framing is up. That is added material cost, plus added labor hours, plus added schedule.
There is also a long-term line item that never appears on the original invoice. Below-joist systems protect the space under the deck, but they leave the framing itself wet, because the water still passes between the boards before anything catches it. Joist rot, fastener corrosion, and ledger issues remain a live cost for as long as the deck stands. Compare the two approaches side by side or read how decking can keep the space under a deck dry without a ceiling.
Can it go on an existing deck?
Here is the honest answer. AmeriDex is installed as the deck surface, with the seal placed between every board as the boards go down, so it is a new-build or re-deck system rather than something that can be slipped in later.
During a re-deck, where the old boards come off and new boards go on, it can often be installed on your existing joists, provided the framing is sound and correctly spaced. That is a common and cost-effective path: you are already paying for new decking, and the drainage comes with it.
What it cannot do is go underneath a deck whose boards stay in place. If that is your situation, a below-joist trough or membrane retrofit is the category to look at, with the trade-off that the framing keeps getting wet. Not sure which one you are? Send your dimensions through the quote form and a dealer can assess whether your framing can carry a re-deck.