Deck Drainage Calculator

Optional but recommended for raised decks where the under-deck area is usable space. A standard deck without under-deck system drains through board gaps + slope to grade — works fine but the area underneath is wet, drippy, and unfinished. With an under-deck system, you get a finished dry patio space — adds significant home value and outdoor living square footage. Cost-benefit: ~$2,000-4,500 for a 200 sqft deck system, returns 60-75% at resale per RemodelingCost vs Value.

Both are top-tier systems, similar performance (4.0 vs 4.5 in/hr drainage). Trex RainEscape ($9.50/sqft) uses polypropylene troughs + downspouts under joists — works with ANY decking brand, more parts to install. TimberTech DrySpace ($8.80/sqft) uses snap-together vinyl panels that double as the ceiling — fewer parts, simpler install, but only fits TimberTech 12-16″ joist spacing. Both 25-yr warranty. Pick TimberTech if you want a clean modern panel look + your deck is composite. Pick Trex if you have an existing deck (different brand) or 24″ joist spacing.

IRC R507.2.4 requires minimum 1/8″ per foot (about 1% grade) AWAY from the house. Better practice: 3/16″ to 1/4″ per foot. This is achieved by framing the joists with a slight drop per foot away from the ledger. For a 14 ft deck spanning out from the house: ledger end is highest, far end is 14 × 1/8 = 1.75″ lower (or 14 × 1/4 = 3.5″ for 1/4″ per ft). The slope doesn't need to be visible to occupants but must be visible to water.

Standard: 1/8″ (0.125″) for wood, 3/16″ (0.1875″) for composite/PVC. Composite/PVC gets larger gap because boards expand more with temperature (5-6e-5 in/in/°F vs wood 2.5e-6). Too tight (< 1/16″): water doesn't drain, debris clogs, mold/mildew forms. Too wide (> 1/4″): looks unfinished, drops phones and pet paws. Match the gap to your decking material — manufacturers publish exact spec (Trex 3/16″ at install temp 60-80°F, AZEK same). Use the Expansion Gap Calculator on this site for thermal expansion math.

DIY EPDM membrane is doable for handy homeowners ($4.50/sqft material). Trex RainEscape + TimberTech DrySpace + Zip-UP are RTA systems that DIY-savvy homeowners can install with the manufacturer's video instructions — usually a 2-3 day weekend project. NOT DIY: Dec-K-ing or DeckDrain channel systems require professional install due to precise leveling. Pro install adds ~35% labor cost across all systems but ensures warranty validity + proper waterproofing. For a $4K material job, that's $1,400 labor = pay the pro unless you have construction experience.

Per sqft installed: DIY EPDM membrane $4.50, DeckDrain channels $7.20, TimberTech DrySpace $8.80, Trex RainEscape $9.50, Zip-UP UnderDeck $11.50. Add gutter $8.50/lf perimeter, lighting kit $285 per 100 sqft, ceiling fan $425. For a 224 sqft deck with Trex RainEscape + gutter + lighting: ~$2,400-3,200 installed. With Zip-UP premium system + full roof + fan + lighting: $4,800-6,500 installed. Includes 35% labor pct + 10% contingency.

Properly installed systems catch 95-98% of water. Some seepage at joist edges + perimeter is normal but minor. Major leaks indicate install error — most commonly: (1) inadequate slope on troughs/panels (must drain to gutter, 1/8″ per ft min), (2) failed sealant at joist-trough connection, (3) clogged gutter from leaves. Routine maintenance: clean gutters 2× per year, inspect sealant annually, replace any UV-damaged trough sections. With maintenance, top systems last 25-30 years.

Opposite — properly installed systems EXTEND joist life by 10-15 years because the joists stay drier. Joists in standard above-deck-only drainage are exposed to all rainfall + splash. With under-deck system, joists are above the membrane and stay dry. Verify install: the membrane should slope away from joists (water flows DOWNHILL into troughs), and joist tops should still receive ventilation (vent gaps at joist hangers). Wet joists from a FAILED under-deck system are worse than no system — fix leaks immediately.

Yes — most systems are designed for retrofit. Trex RainEscape, TimberTech DrySpace, Zip-UP all install from underneath without removing deck boards. DeckDrain channels also retrofit but require precise leveling. DIY EPDM membrane is retrofittable but messier. Process: clear under-deck area, install troughs/panels between joists, install perimeter gutter, install ceiling system, add lighting. Most installs take 2-4 days. Verify joist condition before retrofit — rotted joists must be sistered first.