Planters on Balconies and Roof Terraces: The Weight Guide

A filled 400mm cube planter weighs roughly 80–120kg; a filled 600mm cube can exceed 300kg. UK balconies are commonly designed for 1.5kN/m² (about 150kg per square metre) of imposed load, so large planters need thought — and for any significant roof terrace scheme, a structural engineer's confirmation is the responsible first step. Here's how to plan generous planting within safe limits.

What things actually weigh

• Saturated compost: roughly 1,000–1,400kg per cubic metre — water is the heavy part, and you must count the wet weight.
• 400mm cube, filled and wet: ~80–120kg.
• 600mm cube, filled and wet: ~250–350kg.
• 1000mm trough, filled and wet: 300kg+.

The steel itself is a minor share — a planter's weight is mostly soil and water, whatever material the vessel is.

Reducing weight without shrinking the look

• Bulk-fill the lower depth of deep planters with lightweight void filler — the technique in our planter filling guide can halve total weight.
• Use lightweight container compost rather than soil-based mixes.
• Choose shallow-rooted planting — grasses, herbs and perennials thrive in 300mm of medium; only trees need deep soil.

Placement matters as much as weight

Load capacity is highest at the building line (over walls and supports) and lowest mid-span. Put heavier planters against the wall edge of the balcony, never centred on an unsupported span — and spread multiple planters rather than clustering them.

When to involve an engineer

For a couple of 400mm planters on a modern balcony, the margins are normally comfortable. For anything larger — multiple big planters, troughs for screening, any water feature, or older buildings — a structural engineer's sign-off costs little against the consequences of guessing. Freeholders and management companies often require it anyway.

Frequently asked questions

Are steel planters worse than plastic for balconies?

The vessel is a small fraction of total weight — a filled plastic planter weighs almost the same as a filled steel one, without the stability and looks.

What does 1.5kN/m² mean in practice?

Roughly 150kg spread over each square metre — note it's a distributed figure, not a licence for a 150kg point load anywhere.

Which planters suit balconies best?

400mm cubes and cylinders — covered in our small-space planter guide.

Do I need drainage trays?

Yes on balconies — both to protect the deck surface and the neighbours below.

Plant up, safely

Browse balcony-friendly sizes in the Corten steel planter collection, made in Britain.