Deck Drainage Calculator
Two distinct drainage scenarios: ABOVE-DECK (water draining through board gaps + slope to grade per IRC R507.2.4) and UNDER-DECK (finished dry space below a raised deck using Trex RainEscape, TimberTech DrySpace, Zip-UP UnderDeck, DeckDrain channels, or DIY EPDM membrane). Above-deck mode: computes drainage capacity from board gap × deck area vs required GPM at peak rainfall (~3.5″/hr design), slope compliance, annual runoff volume. Under-deck mode: 5 system options compared at the same scope with installed cost, lifespan, warranty, drainage capacity. Includes gutter ($8.50/lf), ceiling finish ($3.50/sqft for systems without built-in ceiling), full roof system add-on ($4.20/sqft), ceiling fan ($425), and LED lighting kit ($285 per 100 sqft).
Inputs
Deck dimensions
Trex RainEscape
16 × 14 ft (224 sqft) · under-deck
Cost breakdown
| Trex RainEscape system ($9.5/sqft × 224 sqft) | $2,128 |
| Perimeter gutter ($8.50/lf × 60 ft) | $510 |
| LED lighting kit | $855 |
| Labor (35% of materials) | $1,223 |
| Project total (high, +10% contingency) | $5,322 |
System comparison (same scope)
Recommendations
- ▸ DIY EPDM membrane + gutter is cheapest at $4,810 (saves $512 vs current). Trade-off: 12-yr lifespan vs 25-yr.
IRC + manufacturer references
- • IRC 2021 R507.2.4 — Decks shall provide positive drainage away from house ledger (1/8″ per ft minimum)
- • IRC 2021 R408.2 — Vapor barrier required when enclosing under-deck space
- • IBC 2021 1610.1 — Drainage capacity for outdoor surfaces
- • Manufacturer install guide — Trex RainEscape, TimberTech DrySpace, Zip-UP UnderDeck for ICC-ES ESR-listed installation
- • Local code — under-deck drainage may require separate plumbing permit if connecting to storm system
How to use
Three steps. Permit-ready output.
- 01
Pick mode: above-deck or under-deck
ABOVE-DECK: water drains between board gaps + flows by slope to grade. This is the default for most decks. UNDER-DECK: install a membrane/gutter system below joists to create dry finished space underneath — common for raised decks where the under-deck area becomes usable (patio, storage). Most decks use above-deck only; raised decks (>5 ft above grade) increasingly add under-deck systems for living space.
- 02
Above-deck: enter board gap + slope + rainfall
Board gap typically 3/16″ (0.1875″) for composite/PVC, 1/8″ (0.125″) for wood. Slope IRC R507.2.4 minimum 1/8″ per foot away from house. Annual rainfall by region (US avg 42″). Calculator computes drainage capacity (gpm), compliance check, annual runoff volume to plan dispersal (rain garden, dry well, storm system).
- 03
Under-deck: pick system + roof type
5 systems: Trex RainEscape ($9.50/sqft, 25-yr warranty, polypropylene troughs), TimberTech DrySpace ($8.80/sqft, snap-together panels), Zip-UP UnderDeck ($11.50/sqft, premium aluminum + built-in lighting), DeckDrain channels ($7.20/sqft, bare drainage no ceiling), DIY EPDM membrane ($4.50/sqft, 5-yr warranty). Roof type: gutter only (no ceiling), ceiling system (finished underneath), full roof (+ insulation for conditioned space).
- 04
Under-deck add-ons
Ceiling fan ($425 with outlet) — recommended for hot climates to circulate air. LED lighting kit ($285 per 100 sqft) — recessed lights + transformer + dimmer, almost always worth it. Some systems (Zip-UP) include lighting channels in the panels — no separate kit needed. Most homeowners regret skipping lighting at install time.
- 05
Read drainage capacity + compliance
Above mode: GPM provided vs required at peak rainfall; slope compliance per IRC; annual runoff. Under mode: cost breakdown by component, system comparison panel ranks 5 options cheapest-to-most-expensive. Warnings on tight board gaps, insufficient slope, DIY membrane warranty risk, ceiling visibility issues.
How we calculate
The math, fully transparent.
The Deck Drainage Calculator supports two distinct drainage scenarios: ABOVE-DECK (water draining through board gaps + slope to grade per IRC R507.2.4) and UNDER-DECK (finished dry space below a raised deck using Trex RainEscape, TimberTech DrySpace, Zip-UP UnderDeck, DeckDrain channels, or DIY EPDM membrane). Above-deck mode: computes drainage capacity from board gap × deck area vs required GPM at peak rainfall (~3.5″/hr design), slope compliance, annual runoff volume. Under-deck mode: 5 system options compared at the same scope with installed cost, lifespan, warranty, drainage capacity. Includes gutter ($8.50/lf), ceiling finish ($3.50/sqft for systems without built-in ceiling), full roof system add-on ($4.20/sqft), ceiling fan ($425), and LED lighting kit ($285 per 100 sqft).
IRC references
- IRC 2021 R507.2.4 — Decks shall provide positive drainage away from house ledger (1/8″ per ft minimum)
- IRC 2021 R408.2 — Vapor barrier required when enclosing under-deck space
- IBC 2021 1610.1 — Drainage capacity for outdoor surfaces
- Manufacturer install guide — Trex RainEscape, TimberTech DrySpace, Zip-UP UnderDeck for ICC-ES ESR-listed install
- Local code — under-deck drainage may require separate plumbing permit if connecting to storm system
2026-Q1 retail pricing — Trex RainEscape $9.50/sqft (25-yr warranty), TimberTech DrySpace $8.80/sqft (25-yr), Zip-UP UnderDeck $11.50/sqft (25-yr), DeckDrain channels $7.20/sqft, DIY EPDM $4.50/sqft. Aluminum K-style gutter $8.50/lf. Ceiling finish $3.50/sqft. Full roof system $4.20/sqft. Ceiling fan $425. LED kit $285 per 100 sqft. Labor 35% of materials. 10% contingency.
Each gap between board rows is gapWidth × deck_length. Total open drainage area = row count × this. Empirical vertical flow rate ~90 gpm per sqft of gap area for 1/8-3/16″ gaps. Required GPM = deck_sqft × peak_rainfall_in/hr × 0.62 gal/sqft-in / 60 min. Pass if actual ≥ required.
Deck must slope away from house at min 1/8″ per ft (1% grade). Pitch 3/16-1/4″ per ft is better for faster water shedding. Less than 1/16″ per ft = essentially flat = puddles + accelerated rot/mold. Frame the joists with 1/8″ drop per foot away from ledger.
Approximation assuming 100% runoff (no absorption from deck — concrete-equivalent runoff coefficient). Real value 85-95% depending on deck material. For 224 sqft deck × 42″ annual rainfall = ~5,800 gal/yr. Plan for dispersal: rain garden 0.5-1× deck area, dry well, or storm system connection.
System pricing: Trex RainEscape $9.50/sqft, TimberTech DrySpace $8.80, Zip-UP $11.50, DeckDrain $7.20, DIY membrane $4.50. Gutter aluminum K-style $8.50/lf around perimeter. Ceiling separate $3.50/sqft if system doesn't include (DeckDrain, DIY membrane). Full roof add-on $4.20/sqft for insulation. Labor 35% of materials. 10% contingency.
Trex RainEscape 4.0 in/hr, TimberTech DrySpace 4.5, Zip-UP 5.0, DeckDrain 3.5, DIY 3.0. Compare to your region's peak rainfall. NE region ~3.0 in/hr, S/Gulf coast ~4.5 in/hr. Systems should exceed peak by 25% margin. Trex passes nationally; DIY membrane may fail in Gulf storms.
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