Steps to Calculate Suitable Pot Sizes for Your Rooftop

When it comes to rooftop gardens, choosing the right pot sizes is crucial. You need to find the perfect balance between the needs of your plants, the structural limitations of your balcony or rooftop & the feeling of being well integrated into the context.

A relatively simple installation (and easy to calculate) with simple rectangular pots placed on top of the balcony surface. Installation by Roberto Landello.

Follow these steps to work out if a pot is a suitable size for your balcony:

1. Calculate the Pot Area:

For a rectangular pot, the area is simply length × width. Our example pot is (2m × 1m): Pot Area = 2m × 1m = 2m²

For a round pot, the area is calculated using the formula for the area of a circle: Area = π × radius² So if your round pot has a diameter of 1m, the radius would be 0.5m: Pot Area = 3.1416 × (0.5m)² = 3.1416 × 0.25m² = 0.785m²

2. Calculate the Total Weight of the Pot + Growing Medium + Plants:

• Pot Weight: Our example pot weights 180kg. Check the weight of the pot, which is usually listed by the manufacturer. If not, use a scale to weigh the pot at the store.

• Growing Medium:

Pot Volume: 2m × 1m × 1m = 2m³ (We'll use the rectangular pot for the example)

Growing Medium Weight: 2m³ × 1120kg/m³ = 2240kg

• Plant Weight: Assuming approximate 30kg/m². It will be heavier with trees and lighter with only groundcover but works on average.

2m² × 30kg/m² = 60kg

3. Calculate the Total Weight

Add the pot weight, growing medium, and plant weight: Total Weight = 180kg (pot) + 2240kg (growing medium) + 60kg (plants) = 2480kg

4. Calculate the Weight per Square Meter

Since the pot covers 2m², divide the total weight by the area to get the weight per square meter: Weight per m² = 2480kg ÷ 2m² = 1240kg/m² For most balconies, except those made especially to hold an 'intensive' green roof, this weight will be far too heavy & weight compromises will have to be made. (see below)

Formula

1. Calculate Pot Area: Area = Length × Width

2. Total Weight: Total Weight = Pot Weight + (Pot Volume × Saturated Density of Growing Medium) + (Area × Plant Weight per m²)

3. Weight per m²: Weight per m² = Total Weight ÷ Pot Area

Does it work? Checking and Adjusting the Pot Size

Once you've calculated the weight per square meter for your pot, it's important to check if it falls within your balcony's load limit. Here's what to do next:

Determine roof load capacity

• Check your building's maximum load capacity per square meter (kg/m²). This information may be found in your building's structural documents or be obtained from the building management.

If the weight per m² is too high:

• Adjust the Pot Size: Consider using a smaller pot to reduce the total volume of growing media, water, and plants.

• Reduce Soil Volume: If you want to keep a larger pot for aesthetic or root development reasons, you can reduce the amount of growing media by creating voids with drainage cells or filler materials. This decreases the overall weight while still allowing the plant to thrive.

If the weight per m² is adequately low:

• Go Bigger with the Pot: If you find that your pot is well below the balcony's load limit, consider upsizing the pot. Larger containers provide more room for root development, which leads to healthier, more resilient plants.

• Optimize for Healthier Plants: Bigger pots hold more growing media and water, meaning the plants will have better access to nutrients and moisture, and you won't need to water as often.

By adjusting your pot size and soil volume, you can ensure that your garden design stays within safe weight limits while also providing the best possible environment for your plants.

Want to keep reading? Check out the next post in this series: Creating Your Growing Media (More than just potting mix).

Want expert help? Check out my Balcony Design and Construction Services page.