Under-Deck Drainage System Selection (Above-Joist vs Below-Joist)

Why this matters

Under-deck drainage turns the space below a second-story deck into usable patio square footage. There are two architectures (above-joist and below-joist), four major proprietary systems, and a long list of detail screw-ups that turn the install into a swimming pool. Pick the wrong system for the deck height and roof pitch and the whole assembly fails on the first heavy rain.

The two architectures

Above-joist (between board and joist): The drainage trough sits on top of the joist, under the deck boards. Water passes through the gap between deck boards, hits the trough, and is channeled to a gutter at the deck rim. Joists stay dry.

• Products: Trex RainEscape, ZipUp Deck Drainage, DrySnap, Trex Elevations Drainage
• Pro: joists are protected, deck boards can be any material, retrofit-compatible after framing is up but before decking goes on
• Con: adds installation step before decking, requires precise pitch in the troughs

Below-joist (under the joist): The drainage panels are hung from the bottom of the joists, creating a ceiling that drains to a perimeter gutter. Joists get wet but the space below stays dry.

• Products: DrySpace by Trex, UnderDeck by Color Guard, Versadry, RainTight by TimberTech
• Pro: also functions as a finished ceiling, faster to install on existing decks, gives a flat ceiling appearance
• Con: joists remain in the wet zone (treat the lumber accordingly), retrofit ceiling-style products require careful slope to maintain drainage

Which to specify by deck condition

• New construction, two-story deck: above-joist preferred. Cleanest detail and joists stay dry for the life of the deck.
• Retrofit to existing deck with no current under-deck use: below-joist is the only choice without removing the decking.
• Customer wants a finished ceiling appearance and recessed lighting: below-joist with a ceiling product (DrySpace or UnderDeck) provides the framing for the lights.
• Deck height under 8 feet to grade: low-slope is hard to maintain; consider a ground-level patio instead.

Slope requirements

Every drainage trough needs minimum 1/4 inch per foot slope toward the gutter. Above-joist systems typically achieve this by setting the trough on a tapered ripping along the joist; below-joist systems use sloped hanger brackets. Insufficient slope causes ponding, mosquito breeding, and accelerated deck-board mold from the trapped humidity.

Gutter sizing and downspout placement

Use the same approach as a roof gutter. Calculate the deck area in square feet, multiply by the rainfall intensity for the location (consult NOAA Atlas 14 for 100-year, 5-minute rainfall), and size the gutter and downspouts to the SMACNA gutter tables. A 200-square-foot deck in a high-intensity rainfall region often needs 6-inch K-style or half-round gutter with a 3-by-4-inch downspout, not the standard 5-inch with 2-by-3-inch downspout that the homeowner assumes.

Joist treatment under above-joist systems

Even when the trough catches the water, condensation under the trough can wet the top of the joist. Two precautions:

• Use a butyl flashing tape (Vycor Deck Protector, FastenMaster FlashTape, Trex Protect) on the top of the joist before the trough is set
• Use stainless or hot-dip galvanized fasteners that can handle intermittent wet contact

Membrane vs trough vs panel

Three technology classes:

• Membrane systems (DrySnap, Trex RainEscape) use a flexible plastic membrane installed in long runs, sealed at the joist intersections with butyl tape. Lowest cost, most labor-intensive sealing.
• Rigid trough systems (Trex Elevations Drainage) use a rigid vinyl extrusion that snaps into a channel on the joist. Higher cost, faster install, more forgiving of bad slope.
• Below-joist panel systems (DrySpace, UnderDeck) use vinyl or aluminum panels hung from joist-mounted rails. Function as both drainage and ceiling.

What kills these systems

• Failure to seal the joist intersections with butyl tape on membrane systems
• Insufficient slope (less than 1/4 inch per foot)
• Undersized gutters that overflow into the under-deck space
• Penetrations for ceiling fans, lights, or speakers in below-joist panels without a re-sealed flashing detail
• Foot traffic on above-joist troughs during construction (the troughs are not load-rated; protect them with plywood during decking install)
• Pollen and leaf debris that clogs the gutter inlets; specify gutter guards

Maintenance expectation to communicate

Annual or semi-annual gutter cleaning, just like a house gutter. Membrane joints inspected every 5 years and resealed if the butyl tape has slipped. Below-joist panels can be unsnapped for inspection; some homeowners do this themselves.

References

• Trex RainEscape Installation Instructions, current revision (above-joist membrane system)
• Trex DrySpace Installation Instructions (below-joist panel ceiling system)
• TimberTech DrySpace and similar product documentation
• SMACNA Architectural Sheet Metal Manual, gutter and downspout sizing tables
• NOAA Atlas 14 Precipitation-Frequency Atlas of the United States, regional rainfall intensity
• North American Decking and Railing Association (NADRA) Best Practices, second-story deck drainage