A rain garden is a shallow, planted depression that collects and infiltrates stormwater before it can sheet across hard surfaces or enter a municipal storm sewer. On a typical Canadian residential lot, it intercepts runoff from a roof, driveway, or patio and holds it long enough for the soil to absorb the water over the following 24 to 48 hours. Getting the dimensions right is the single most important step — a garden that is too small fills and overflows during moderate rain events, while one that is oversized risks waterlogging plants that cannot tolerate extended inundation.
Determine Your Contributing Drainage Area
Start by measuring all impervious surfaces that will drain toward the proposed garden. This typically includes:
- Roof sections directed by downspouts
- Sections of driveway or walkway that slope toward the garden
- Adjacent lawn areas where the grade moves water in the same direction
For a standard Canadian detached home, a single downspout commonly drains 50 to 100 square metres of rooftop. Measure the portion of the roof plane — not the footprint — that drains to each downspout. Multiply length by width for rectangular sections; for L-shaped or hip roof sections, break them into rectangles.
Test Your Soil Permeability
Permeability determines how quickly water drains through the soil and whether the basin will empty within the required 48-hour window. A basic percolation test takes about 30 minutes and requires no special tools.
- Dig a hole 300 mm deep and 300 mm wide at the proposed garden location.
- Fill it with water and allow it to drain completely. This pre-saturates the soil.
- Fill it again to the 300 mm mark. Measure the water level drop over 60 minutes using a ruler or stick marked at the surface.
- A drop of 12 mm or more per hour is generally acceptable. Slower drainage indicates clay-heavy soil and may require an amended planting medium or a smaller garden with overflow control.
Sandy loam soils typical of southern British Columbia and parts of Ontario will drain readily. Clay soils in much of the Prairies and the St. Lawrence lowlands may require 100 to 150 mm of compost-amended backfill in the basin bottom to improve permeability without creating a perched water table.
Account for Slope
The site slope affects both where water collects naturally and how the basin edges must be graded. Rain gardens function best on slopes between 2 and 12 percent. Below 2 percent, water enters too slowly and the basin may not drain fully; above 12 percent, berms become difficult to maintain and runoff velocity can erode the inlet.
Measure slope using a level, a straight board 1.2 metres long, and a tape measure. Place one end of the board at the uphill point and level it. Measure the gap between the downhill end and the ground surface. A 6 mm gap over 1.2 metres equals a 0.5 percent slope; 60 mm over 1.2 metres is 5 percent.
On slopes greater than 6 percent, position the garden below a level spreading area or build a small level spreader — a trench filled with clean gravel perpendicular to the slope — to slow water before it enters the basin.
Calculate Basin Dimensions
Once you know the contributing area, soil permeability, and slope, the basin size follows a straightforward formula. The goal is to hold the runoff from a 25 mm rainfall event — the common design storm used by many Canadian municipalities for residential lot calculations.
Example calculation for a 75 m² roof section draining to a single downspout:
- Design storm volume: 75 m² × 0.025 m = 1.875 m³ (approximately 1,875 litres)
- With a basin depth of 200 mm (0.2 m), required surface area = 1.875 ÷ 0.2 = 9.4 m²
- A 3 m × 3.2 m basin (9.6 m²) meets this requirement with minor rounding
Aim for a maximum ponding depth of 150 to 250 mm. Shallower basins drain faster and are less likely to stress plants during dry periods; deeper basins can handle larger storm events but require plants with greater flood tolerance.
Choose the Right Location
Location affects both function and long-term maintenance. The garden must be at least:
- 3 metres from any building foundation
- 3 metres from property lines (confirm local setback requirements with your municipality)
- 15 metres from any well or septic system
- Downhill from the downspout outlet or receiving area
Avoid locations directly over utility lines. Contact your municipality or call 1-800-400-2255 (Ontario One Call) or the equivalent in your province before any excavation deeper than 300 mm.
Grading the Basin and Overflow Route
The basin should have a flat bottom to promote even infiltration. Side slopes should be gentle — typically 3:1 horizontal to vertical or shallower — to prevent slumping and allow mowing or edging around the perimeter.
An overflow outlet is not optional. During heavy events that exceed the design storm, water needs a controlled path away from the foundation. Grade a low point in the downslope berm or install a perforated overflow pipe set at the maximum ponding depth. The overflow should direct excess water toward a lawn or vegetated area, not toward a neighbouring property or a sump window well.
References
Ontario Ministry of the Environment — Stormwater Management Planning and Design Manual
City of Toronto — Rain Gardens (Lot Level Source Controls)
