Deck Slope Calculator – Measure Proper Deck Drainage

Last updated: 11 Apr 2025 | Author: Brij | Review By: Irshad

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By entering the deck length and slope ratio (like 1:48), it calculates the total slope drop, slope percentage, and angle, ensuring your deck avoids water pooling and lasts longer.

Deck Length:

feet

Slope Ratio (1:x):

Slope Calculation Results

Total Drop:

inches

Slope Percentage:

Slope Angle:

degrees

Note: A common deck slope is 1:80 to 1:40 (about 1/8" to 1/4" per foot) to ensure proper water drainage.

Recommended Deck Slope Ratio

For decks, especially to ensure proper drainage (so water doesn't pool), here's what's typically recommended:

Slope Type	Drop per Foot	Ratio (Run:Drop)	Total Drop over 100 ft
Minimum Slope	1/8" per ft	1:96	12.5 inches
Standard Slope	1/4" per ft	1:48	25 inches
Gentle Slope	~1/5" per ft	1:60	20 inches
Very gentle	~1/6" per ft	1:72	~16.6 inches
Extra gentle	1/10" per ft	1:120	10 inches

If you're building a deck, slope is important! A proper slope ensures water drains off the deck instead of pooling, preventing rot, mildew, and surface damage. This calculator helps you easily determine the total drop, slope percentage, and slope angle based on your deck's length and slope ratio.

What Is Deck Slope?

Deck slope is the slight incline built into the deck surface to allow water to run off efficiently. It's usually expressed in ratio form, like 1:48, meaning the deck drops 1 inch for every 48 inches (4 feet) of horizontal run.

Deck Slope Formulas

1️⃣ Total Drop (in inches)

To calculate the total drop (also called "slope height"):

Formula:

$\text{Total Drop} = \frac{\left( \text{Deck Length in feet} \times 12 \right)}{\text{Slope Ratio}}$

2️⃣ Slope Percentage

This tells you how steep the deck is, as a percentage:

Formula:

$\text{Slope Percentage} = \left( \frac{\text{Total Drop}}{\text{Deck Length in feet} \times 12} \right) \times 100\%$

3️⃣ Slope Angle (in degrees)

This is the angle your deck surface tilts at, in degrees:

Formula:

$\text{Slope Angle} = \tan^{-1} \left( \frac{\text{Total Drop}}{\text{Deck Length} \times 12} \right)$

Example: Deck Length = 100 feet, Slope Ratio = 1:48

Let’s walk through an example to better understand.

Deck Length = 100 feet
Slope Ratio = 1:48

1️⃣ Total Drop

$\text{Total Drop} = \frac{100 \times 12}{48} = \frac{1200}{48} = 25 \text{ inches}$

2️⃣ Slope Percentage

$\text{Slope \%} = \left( \frac{25}{1200} \right) \times 100 = 2.08\%$

3️⃣ Slope Angle

$\text{Slope Angle} = \tan^{-1} \left( \frac{25}{1200} \right) \approx \tan^{-1}(0.0208) \approx 1.19^\circ$

Conclusion

A deck slope of 1:48 is the standard for proper drainage. Using this calculator ensures your deck is functional and long-lasting. Whether you're building a new one or checking an existing structure, calculating slope accurately can save you from costly repairs in the future.

FAQs About Deck Slope

Q1: What is a deck slope?

A deck slope is the slight angle built into the surface of a deck to allow rainwater to drain off instead of pooling. It’s crucial for preventing wood rot, mold, and structural damage.

Q2: What is the recommended slope for a deck?

The typical recommended slope is 1/4 inch per foot (1:48 ratio). For light drainage, 1/8 inch per foot (1:96) may be used, but never less unless the deck is covered.

Q3: Is it okay to build a completely flat deck?

It’s not recommended. A flat deck can lead to standing water, which causes damage over time. Always incorporate a small slope unless the area is completely protected from water.

Q4: How do I calculate the slope of my deck?

You can calculate the slope using the formula:

Total Drop (in inches) = (Deck Length in feet × 12) / Slope Ratio

Or simply use the Deck Slope Calculator above to get all the results instantly.

Q5: Does slope affect deck appearance?

The slope is so slight (typically around 1–2 degrees) that it's not visually noticeable but functionally important. It keeps your deck safe and long-lasting.

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