Not every green roof sits atop a tower. On a small urban site, the candidates are often a shed roof, a garage, a low extension, or the ground plane of an enclosed courtyard. The same principles that govern large installations still apply at this scale, just with tighter margins for error.
Extensive versus intensive
Green roofs are commonly grouped into two families. An extensive roof uses a shallow growing layer and low, drought-tolerant plants such as sedum; it is lighter and needs less upkeep. An intensive roof carries a deeper substrate able to support shrubs or small trees, behaving more like a garden and weighing far more. For most small courtyards and outbuildings, the extensive approach is the realistic one.
| Type | Growing depth | Typical planting | Maintenance |
|---|---|---|---|
| Extensive | Shallow | Sedum, hardy perennials | Low |
| Intensive | Deep | Shrubs, small trees | High |
Structure comes first
A saturated green roof is heavy, and the load grows with snow on top of it in a Canadian winter. Whether the structure can carry an extensive layer plus snow is a question for a structural engineer, not a guess. An existing shed built for shingles alone may need reinforcement before it can hold a planted assembly.
Design load includes the green roof saturated with water and the snow that sits on it. In much of Canada, snow load is the factor that decides whether a small roof can take any planting at all.
The layers under the plants
A working green roof is a stack, not just soil on a membrane. From the top down, a typical extensive build includes:
- Vegetation layer
- Growing medium (engineered, lightweight)
- Filter fabric
- Drainage layer
- Protection and root barrier
- Waterproof membrane on the roof deck
The waterproofing and root barrier matter most, because a leak under an established roof is expensive to reach.
Runoff and the urban benefit
One reason cities encourage green roofs is stormwater: the growing layer absorbs rainfall and releases it slowly, easing pressure on drains during heavy storms. The City of Toronto, for example, has had a green roof requirement for certain new buildings through its municipal green roof bylaw. Smaller private projects are not usually covered, but the same water-holding behaviour applies at any scale.
Northern exposure and plant survival
Roof surfaces are exposed to full sun, drying wind and winter cold with little shelter. Extensive roofs lean on sedum and other hardy, shallow-rooted plants for exactly that reason. In a shaded courtyard, the challenge inverts toward low light and slower drying, which favours different, shade-tolerant species.
Further reading
The general green roof article covers system types and benefits in more depth. For load planning, local building authorities and a structural engineer remain the authoritative sources.
Related reading: balcony vertical gardens and living walls on facades.