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A 20-degree roof slope rises about 4.4 inches for every 12 inches of horizontal run. In common roofing terms, that is approximately a 4.4:12 slope, often rounded to about 4½:12.
A 20-degree slope is also about a 36% grade. It is a moderate roof slope. It is clearly steeper than a low-slope roof, but much less steep than roofs with angles of 35 to 45 degrees.
20-Degree Roof Slope Conversion
Here are the common ways to describe a 20-degree roof:
| Measurement | 20-Degree Roof |
|---|---|
| Angle from horizontal | 20° |
| Rise per 12 inches of run | About 4.4 inches |
| Roof slope | About 4.4:12 |
| Rise-to-run ratio | About 1:2.75 |
| Percent slope | About 36.4% |
The numbers describe the same roof in different ways.
For example, if you move 12 inches horizontally toward the ridge, the roof surface rises about 4.4 inches vertically.
How to Convert 20 Degrees to Roof Slope
Roof slope is based on the relationship between vertical rise and horizontal run.
The calculation is:
Rise ÷ Run = tan(angle)
For a 20-degree roof:
tan(20°) ≈ 0.364
Using the standard 12-inch horizontal run:
0.364 × 12 ≈ 4.37 inches
So:
20° ≈ 4.37:12
For normal planning, calling it roughly 4.4:12 is usually accurate enough.
What Does a 4.4:12 Roof Look Like?
A 4.4:12 roof has a noticeable slope without being especially steep.
For comparison:
| Roof Angle | Approximate Slope |
|---|---|
| 10° | 2.1:12 |
| 15° | 3.2:12 |
| 20° | 4.4:12 |
| 25° | 5.6:12 |
| 30° | 6.9:12 |
| 35° | 8.4:12 |
| 45° | 12:12 |
These are approximate conversions because an angle does not always produce a neat whole-number rise.
How Much Does a 20-Degree Roof Rise?
The total rise depends on the horizontal distance from the roof edge to the ridge.
Since a 20-degree roof rises about 0.364 units for every unit of horizontal run:
Rise = horizontal run × 0.364
For example:
- A 4-foot run rises about 1.46 feet.
- A 6-foot run rises about 2.18 feet.
- An 8-foot run rises about 2.91 feet.
- A 10-foot run rises about 3.64 feet.
For a simple symmetrical gable roof, remember that the horizontal run is usually half the building width, not the full width.
A building that is 20 feet wide, for example, normally has about a 10-foot horizontal run from an outside wall to the center ridge. At 20 degrees, that would produce roughly 3.64 feet of rise, before accounting for the exact roof design.
Is a 20-Degree Roof Good for Collecting Rainwater?
A 20-degree roof can work well for rainwater collection. The slope is more than enough for gravity to move runoff toward the gutters when the roof and drainage system are designed correctly.
Roof angle is usually less important to rainwater yield than:
- Horizontal roof catchment area
- Rainfall amount
- Roofing material
- Gutter size and placement
- Downspout capacity
- Debris buildup
- Overflow design
Evaluating selecting the right roof is most effective at collecting rainwater helps coordinate roof discharge and the drainage capacity on site.
When estimating how much rainwater a roof can collect, use the roof's horizontal footprint or projected catchment area, not the larger sloping surface area.
For example, making a roof steeper does not mean the same building suddenly catches much more rain. Rain falls onto the horizontal area beneath the roof.
Does Roof Slope Affect Rainwater Quality?
Slope can affect how quickly water and debris leave the roof, but it does not determine whether collected rainwater is safe to use.
Roof runoff may contain leaves, dust, bird droppings, insects, roofing residues, and other contaminants. A rainwater system may therefore use features such as gutter screens, debris exclusion, a first-flush device, and suitable filtration depending on the intended use.
A first flush diverts some of the initial runoff from a rainfall event because that water can carry a concentrated amount of material washed from the roof.
Water used for drinking needs much more careful treatment than water used for garden irrigation. Roof runoff should not be considered potable, or safe for drinking, simply because it looks clear or passes through one filter.
Measure the Roof Before Assuming the Angle
If you are trying to identify an existing roof, measuring the rise and run is often easier than trying to measure its angle directly.
For example, a measured roof slope of:
4⅜ inches rise per 12 inches run
is very close to 20 degrees.
Small differences are normal. A roof described as 4:12 is about 18.4 degrees, while a 4.5:12 roof is about 20.6 degrees.
This difference can matter when you are checking minimum slope requirements for roofing materials, flashing, mounting hardware, or other roof components. Follow the requirements for the specific material or system rather than rounding the roof angle when compatibility matters.
Roof Safety Matters More as Slope Increases
A 20-degree roof may not look especially steep from the ground, but it can still be dangerous to walk on. Roofing material can also become very slippery when it is wet, dusty, mossy, or covered with loose debris.
For rainwater-system work, many jobs can be done from ground level, including cleaning accessible filters, checking storage tanks, and servicing downspout components.
Work involving roof access, high gutters, anchors, structural changes, or difficult-to-reach components should be handled with suitable fall protection and, when appropriate, by a qualified professional.
Frequently Asked Questions
What roof pitch is 20 degrees?
A 20-degree roof is approximately 4.4:12. That means it rises about 4.4 inches for every 12 inches of horizontal run.
Is a 4:12 roof the same as 20 degrees?
No. A 4:12 roof is about 18.4 degrees. A true 20-degree roof is slightly steeper at about 4.37:12.
What degree angle is a 4.5:12 roof?
A 4.5:12 roof is approximately 20.6 degrees, making it very close to a 20-degree roof.
What percentage slope is 20 degrees?
A 20-degree slope is about 36.4%. Percent slope is calculated by dividing rise by horizontal run and multiplying by 100.
How much does a 20-degree roof rise over 12 feet?
Over 12 feet of horizontal run, a 20-degree roof rises about 4.37 feet. Actual building dimensions may differ because roof overhangs and framing affect the run.
Is a 20-degree roof considered steep?
It is generally a moderate slope rather than an especially steep roof. However, it can still be unsafe to walk on, particularly when wet or covered with debris.
Does a 20-degree roof work for rainwater harvesting?
Yes. A 20-degree roof can drain rainwater effectively toward gutters. Collection performance also depends on the roof area, gutters, downspouts, debris control, storage capacity, and maintenance.
