Calculating Septic Drain Field Size in Ontario: The Definitive Guide for Beginners

Understanding how to calculate septic drain field size in Ontario is crucial for ensuring a properly functioning septic system. This comprehensive guide will delve into the intricacies of this calculation, providing homeowners and professionals with the necessary knowledge to determine the appropriate size for their specific needs.

Factors Influencing Drain Field Size

Numerous factors play a significant role in determining septic drain field size, including:

• Daily Wastewater Flow: The amount of wastewater generated by the household, typically estimated based on the number of bedrooms and occupants.
• Soil Characteristics: The type, texture, and permeability of the soil where the drain field will be installed.
• Depth to Groundwater: The distance between the bottom of the drain field and the water table.
• Slope of the Land: The incline or decline of the land where the drain field will be located.
• Local Regulations: Specific requirements and guidelines set forth by local authorities.

Step-by-Step Calculation

Step 1: Determine Daily Wastewater Flow

Estimate the daily wastewater flow based on the number of bedrooms in the house:

• 1-3 bedrooms: 250 gallons per day
• 4-5 bedrooms: 350 gallons per day
• 6+ bedrooms: 500 gallons per day

Step 2: Assess Soil Conditions

Conduct a soil evaluation to determine the soil type, texture, and permeability. This information can be obtained from a licensed soil tester or through local soil maps.

Step 3: Calculate Absorption Rate

Determine the soil’s absorption rate, which indicates how quickly water can drain through the soil. This rate is typically expressed in inches per hour.

Step 4: Determine Drain Field Area

Using the formula below, calculate the minimum required drain field area:

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Drain Field Area = (Daily Wastewater Flow × 1.5) / (Absorption Rate × 0.5)
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Step 5: Adjust for Slope and Groundwater Depth

Make adjustments to the drain field area based on the slope of the land and the depth to groundwater:

• Slope: Add 10% to the area for every 1% of slope.
• Groundwater Depth: Reduce the area by 25% if the depth to groundwater is less than 3 feet.

Example Calculation

For a 3-bedroom house with a daily wastewater flow of 250 gallons per day, installed in soil with an absorption rate of 1 inch per hour, the calculation would be as follows:

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Drain Field Area = (250 × 1.5) / (1 × 0.5) = 750 square feet
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Additional Considerations

• Trenches vs. Beds: Drain fields can be installed in trenches or beds. Trenches are typically deeper and narrower, while beds are shallower and wider.
• Gravel Fill: The drain field is filled with gravel, which provides support for the pipes and facilitates drainage.
• Distribution Box: A distribution box evenly distributes wastewater from the septic tank to the drain field.
• Inspection and Maintenance: Regular inspection and maintenance are essential to ensure the proper functioning of the septic system.

Common Questions and Answers

1. How often should I pump my septic tank?

Every 3-5 years, depending on the size of the tank and the number of occupants.

2. Can I install a drain field in clay soil?

Yes, but it may require a larger drain field area or alternative treatment systems.

3. What are the signs of a failing septic system?

• Sewage backups
• Slow drains
• Foul odors
• Lush vegetation around the drain field

4. How can I prevent septic system failure?

• Use water-efficient appliances
• Don’t flush non-biodegradable materials
• Have the system inspected and pumped regularly

5. What is the difference between a septic tank and a drain field?

The septic tank holds and treats wastewater, while the drain field disperses the treated wastewater into the ground.

6. How long does a septic system typically last?

20-30 years with proper maintenance.

7. Can I build a structure over my drain field?

No, it is important to keep the drain field accessible for maintenance and repairs.