Why Measure Roof Size for a Rain Garden?

Rain gardens are designed to capture and absorb rainwater from impervious surfaces like roofs and driveways. Knowing the size of your roof helps estimate the volume of water your rain garden needs to handle. This ensures it’s the right size and location to ensure maximum efficiency in capturing and filtering your roof runoff while still support your landscaping goals.

Step 1: Open Google Earth

1. Open Google Earth web version on your computer. 
2. Enter your home address in the search bar and zoom in until your property fills the screen.

Step 2: Measure Your Roof Area (that drains into the downspout feeding the rain garden)

1. Select the “Ruler” tool in the toolbar (look for the icon of a ruler).
2. Choose the “Polygon” option to measure an area.
3. Click on each corner of your roof area that drains into an individual downspout. Outline only that area for its shape. 
4. For irregular roof designs, divide the roof into smaller sections and measure each individually and then add up the areas that drain into individual downspouts that will be entering the rain garden. 
5. Close the polygon by clicking on the starting point.
6. The measurement will display the total area in square meters or feet, depending on your settings. Record this value.

   Image: Catchment area calculation through google map

Step 3: Calculate Rainwater Runoff

Rainwater runoff is influenced by the size of your roof and the amount of rainfall in your area. Use this formula:

Runoff (liters) = Roof Area (square meters) × 5 Year Average Rainfall (mm) × Runoff Coefficient

• Roof Area: The value you measured in Google Earth.
• Rainfall:  5 Year Average rainfall for Hamilton, Ontario (around 736.24mm annually).
• Runoff Coefficient: Use 0.9 for impermeable roofs (e.g., asphalt shingles).

Example:

• Roof area: 50 m²
• Rainfall: 736.24 mm
• Runoff coefficient: 0.9

Runoff = 50 × 736.24 × 0.9 = 33,130.8  liters annually.

Note: 1 mm of rainfall over 1 m² of area is equivalent to 1 litre of water, allowing direct conversion from mm·m² to litres.

Step 4: Assess Downspout Locations

1. Identify where your downspouts are located.
2. Determine which portion of the roof drains into each downspout. You can repeat the Google Earth measurement process for these sections if your roof is segmented.
3. The runoff from each downspout will be proportional to the roof area it serves.

Step 5: Plan Your Rain Garden

Firstly, choose a location where the rain garden is positioned directly in line with the downspout so that runoff is efficiently directed into the system, and ensure the site has adequate space based on soil conditions that support adequate infiltration. In addition, design the system with a clear overflow outlet to safely manage excess water during heavy rainfall events, preventing pooling, erosion, or unintended flooding of adjacent areas.

Now that you know how much water flows from your downspout, it is important to understand your soil type when sizing a rain garden, as different soils have different drainage and infiltration rates. You can get a general idea of your soil type through a simple ribbon test, which can help identify whether your soil is mainly clay, loam, or sand. For a more accurate understanding of how quickly water moves through your soil, you can also perform a soil infiltration test. The results of these tests can help you determine whether you have free-draining sandy soil or slower-draining clay soil, which can affect the size of your rain garden.

A general rule is that the rain garden should be about 10-20% of the roof area draining into it. However, it can vary depending on the type of soil you have. 

1. Clay soils drain slowly →  Recommended % of Catchment Area = 30-40%
2. Sandy soils drain quickly → Recommended % of Catchment Area= 20%

Recommended Project Area Formula

Recommended Project Area (m²) = Catchment Area × (Recommended % ÷ 100)

Example: Clay Soil Rain Garden

Step 1: Determine Catchment Area

• Roof area draining to the rain garden: 50 m²

Step 2: Calculate Recommended Rain Garden Size

For clay soils, use 40–50% of the catchment area.

Formula:

Rain Garden Area (m²) = Catchment Area × (Recommended % ÷ 100)

Example:

• Minimum size: 50 × 0.40 = 20 m²
• Maximum size: 50 × 0.50 = 25 m²

Recommended rain garden size: 20–25 m²

Example: Sandy Soil Rain Garden

Sandy soils drain quickly, so a smaller rain garden is generally sufficient.

Step 1: Determine Catchment Area

• Roof area draining to the rain garden: 50 m²

Step 2: Calculate Recommended Rain Garden Size

For sandy soils, use 10–20% of the catchment area.

Formula:

Rain Garden Area (m²) = Catchment Area × (Recommended % ÷ 100)

Example:

1. Minimum size: 50 × 0.10 = 5 m²
2. Maximum size: 50 × 0.20 = 10 m²

Recommended rain garden size: 5–10 m²

Volume Diverted by Rain Garden

Over time, properly designed and engineered in-ground Low Impact Development (LID) features such as rain gardens, bio-swales, and dry swales are capable of managing and gradually infiltrating the runoff volume generated from the contributing catchment area. These systems are designed to reduce peak flow rates, promote groundwater recharge, and provide controlled detention, provided that appropriate overflow outlets are incorporated to safely convey excess water during extreme storm events and prevent system failure.

Using suitable native plants can also help a rain garden work more effectively. Native plants can improve soil structure, support water infiltration, provide habitat for wildlife, and make the garden more resilient over time. A properly designed overflow outlet is also important to safely direct excess water during larger storm events and prevent pooling or flooding.

In Conclusion, calculating your roof size using Google Earth is a simple and effective way to understand your home’s stormwater runoff. With this information, you can design a rain garden that manages water sustainably, protects local waterways, and enhances your property’s beauty. Ready to start? Grab your ruler tool and get measuring!