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Green Roof Load & Cost Calculator
Compare the added weight and installation cost of a lightweight extensive roof against a deeper intensive one.
Extensive vs. intensive green roofs
An extensive green roof uses a thin growing medium (typically 60–150mm) with hardy, low-maintenance plants like sedum — lightweight and comparatively affordable. An intensive green roof uses much deeper soil (300mm or more) that can support shrubs, small trees and even walkable garden space, but is dramatically heavier and more expensive to build and maintain.
Why saturated weight is the number that matters structurally
A green roof's growing medium acts like a sponge — its weight when fully saturated with water after heavy rain, not its dry weight, is the figure a structural engineer needs to check against the roof's load capacity. This calculator uses typical saturated weight-per-area figures for each roof type so the load estimate reflects the worst-case, wettest condition rather than an optimistic dry-weather number.
Frequently asked questions
What's a typical structural capacity for a normal roof?
Most conventional residential and light commercial roofs are designed for a live + dead load capacity in the rough range of 100–150 kg/m² beyond their own structure — enough for an extensive green roof in many cases, but usually well short of what an intensive green roof needs. This varies enormously by building age, structure type and local building code, which is exactly why a structural engineer's sign-off is essential rather than optional.
Why does the calculator insist on a structural engineer?
The added load from a saturated green roof is applied continuously across the whole roof area and stays there for the system's lifetime — unlike a temporary load. Exceeding a roof's actual capacity can cause serious structural damage or failure, so a qualified structural engineer must verify the existing structure (or a strengthened one) before installation, no matter how the numbers here look.
Do the cost figures include ongoing maintenance?
No — these are typical installation cost figures per square metre covering the growing medium, drainage layer, waterproofing membrane upgrade and planting. Intensive roofs in particular carry meaningfully higher ongoing maintenance costs (irrigation, pruning, replanting) that aren't included in this one-off installation estimate.
Can I mix extensive and intensive areas on one roof?
Yes, and it's common in practice — for example a walkable intensive garden zone near a door with extensive planting across the rest of the roof. Run this calculator once per zone with its own area to get a load and cost figure for each, then add the results together for the whole roof.