Under deck drainage systems: types, cost and what to buy

Three ways to keep the space under a deck dry

Every system does the same job: it intercepts the water that falls through the board gaps and steers it, on a slope, to a gutter at the low edge of the deck. They differ in where the waterproof layer sits, and that decides whether you can fit one to an existing deck.

Over-joist membranes are the cheapest per square foot and protect the frame best, because the joists stay dry as well as the patio. Under-joist panels are what most people retrofit, because nothing above has to change and the result looks like a soffit.

Worked example: a 12x16 second-storey deck

Take a 16 ft by 12 ft deck off a second floor: 192 sq ft, joists running 12 ft out from the house at 16 inches on centre. The deck joist calculator gives 13 joists, which leaves 12 bays to drain.

The fall is the number people forget. With 2x10 joists there is plenty of depth to slope panels 3 inches over 12 ft and still hide the gutter; with 2x6 joists on a long run it gets tight, and a between-joist trough or an over-joist membrane is the better choice.

What it costs

Typical 2026 prices, before any ceiling lights or fans, applied to the same 192 sq ft deck:

For scale, the deck cost calculator puts a 12x16 composite deck at $8,294-$10,598 at standard height, so a DIY kit adds roughly 5-15% to the build and an installed system roughly 15-45%. If you are pricing a raised deck from scratch, read second-storey deck cost first, because the drainage decision changes the order of work.

What drainage does not replace

• Ledger flashing. The ledger joint against the house is a separate leak path; flash it as the deck ledger board guide shows and count the tape with the deck flashing calculator.
• Joist tape. Under-joist panels keep the patio dry, not the tops of the joists, where water sits under every board. Tape them with the joist tape calculator quantities before the boards go on.
• A sound frame. Drainage hides the joists from view, so do a deck safety inspection before you close them in on an older deck.

Installing an under-joist system on an existing deck

1. Check the frame from below: soft wood, rusted hangers or a ledger without flashing get fixed first.
2. Fit the collector gutter along the low side (usually the outer beam), sloped about 1/8 in per ft toward the downspout end.
3. Snap a line on every joist face from the house end down to the gutter, giving the 1/4 in per ft fall.
4. Screw the hanger strips or rails to the joists along those lines.
5. Clip the panels in bay by bay, lapping each one into the gutter, and seal the house end so water cannot run back to the wall.
6. Hang the downspout and lead it away from the house footings, then run a hose on the deck to check every bay drains.

Any lights or a ceiling fan below need damp-location fixtures, and their wiring should be run before the panels close the bays.

Which one to choose

• Building a new raised deck: an over-joist membrane, installed after the joists are taped and before the decking. It protects the frame and costs least.
• Existing deck you want a dry patio under: under-joist panels. No boards come up and you get a finished ceiling.
• Shallow joists or a tight budget: between-joist troughs, sized to the bay width.
• Re-decking anyway: lift the boards, fit a membrane, and lay the new decking over it. The deck resurfacing cost calculator prices the re-deck.