| Run | Width | Slope | Drop | Rain | Peak Flow |
|---|---|---|---|---|---|
| 12 ft | 10 ft | 2.08% (0.25 in/ft) | 3.00 in (76.2 mm) | 1.0 in/hr for 1 hr | 21.2 L/min (5.6 gpm) |
| 4 m | 3 m | 1.50% | 60.0 mm | 20 mm/hr for 2 hr | 20.0 L/min (5.3 gpm) |
- Slope fraction = slope% ÷ 100
- Drop = run length × slope fraction
- Angle = arctan(slope fraction)
- Deck area = run × width
- Rain depth = intensity × duration
- Runoff volume (L) = area(m²) × rain depth(mm)
- Peak flow (L/min) = area(m²) × intensity(mm/hr) ÷ 60
- Select your units and enter deck run length and width.
- Choose a slope format: percent, ratio, or inches per foot.
- Pick exposure guidance, then set drainage direction notes.
- Optional: add rain intensity and duration for runoff estimates.
- Optional: add drains and capacity to check peak flow.
- Press Calculate to see results above the form.
- Use Download CSV or Download PDF for records.
Why slope drives drainage
A deck sheds water only when gravity has a clear path. Slope converts the surface into a shallow plane so runoff moves instead of lingering in low spots. Even small drops matter: a 1% slope creates 10 mm drop per meter, guiding water toward edges, channels, or a center drain. Always confirm the fall direction avoids doors and foundation edges, and prevent reverse slope at thresholds.
Choosing a practical target
For exposed boards, many builders start near 2% because it balances quick shedding and comfortable footing. Covered surfaces can work with less because direct rainfall is reduced, but cleaning water still needs a route. The calculator converts percent, ratio, and inches-per-foot so you can match site habits and material tolerances. With composite or membranes, check manufacturer limits on deflection and standing water.
Runoff volume and peak flow
Rainfall depth equals intensity times duration. When that depth is applied to deck area, you get an estimated runoff volume. Peak flow uses the same intensity without duration, showing the maximum liters per minute the surface may deliver. This is useful when sizing drain paths or evaluating whether scuppers may overflow. Estimates assume most water becomes runoff, so add safety margin for wind-driven rain.
Drain points and capacity checks
Drains work best when placed at the lowest elevation line, with framing heights set to preserve slope across joists. If you enter drain count and capacity, the tool compares total capacity to peak flow. A shortfall suggests adding another outlet, widening channels, or increasing pipe size to reduce backup risk. Keep access points serviceable for cleaning and future inspection.
Field verification and maintenance
After setting heights, confirm slope using a level, laser, or string line, then recheck after fastening boards. Keep gaps clear, maintain outlet screens, and clean channel drains before storm season. If ponding appears, identify the low point and correct it with localized shimming or surface refinishing. Document your drop and outlet locations for simple logs and records.
FAQs
1) What slope range is typical for outdoor decks?
Many outdoor decks use about 1% to 2% to encourage shedding while remaining comfortable to walk on. Steeper slopes drain faster but can feel noticeable and may affect furniture stability.
2) How do I choose between percent, ratio, and inches per foot?
Use the format your crew measures on site. Percent is universal, ratio matches some drawings, and inches per foot fits common carpenter layout. The calculator converts them so the resulting drop stays consistent.
3) Does board direction affect drainage?
It can. Orienting boards parallel to flow can speed drainage, while perpendicular boards may slow flow across seams. Maintain consistent gaps and avoid raised edges that interrupt the runoff path.
4) What rainfall value should I enter?
Enter a conservative local intensity for design checks, such as a heavy storm rate used for planning. If you are unsure, test multiple values to see how peak flow changes and size drains with a buffer.
5) Why can ponding happen even with a calculated slope?
Framing settlement, uneven joists, board cupping, or debris can create local low points. Recheck elevations during installation, keep drainage routes clear, and correct isolated dips before they stain or grow algae.
6) Is the drain capacity check a guarantee?
No. It is a screening estimate based on idealized runoff and stated drain capacity. Real performance depends on inlet design, clogging, pipe losses, and outlet conditions. Treat results as planning guidance and verify on site.