Green roofs and roof terraces, and the loads they add

Green roofs and roof terraces, and the loads they add

Roof gardens and terraces are increasingly common on Sydney apartment buildings, and they are frequently added to buildings that were never designed to carry one.

The planting is the last consideration. What matters is weight, water and how the roof was originally designed.

The roof was designed for very little

A roof that is not intended for occupation carries its own weight, wind, and a small allowance for maintenance access. That allowance is a person with a tool bag, not a crowd.

Converting it to a trafficable terrace changes the imposed load to something several times higher, and if it becomes a communal area the figure rises again because a gathering of people is one of the heaviest loads a building normally carries.

That change alone often exceeds what an existing roof can take.

Saturated soil is the number people underestimate

Growing medium is specified by depth, and the structural figure is its weight when fully saturated, not when dry.

The difference between the two is large, and the saturated case is the design case, because it occurs during exactly the storm event that also brings wind load.

Depth drives everything. A shallow extensive system with sedums and grasses is a fraction of the weight of an intensive system with shrubs and small trees. That decision is structural before it is horticultural, and it should be made with the capacity of the roof in hand.

The other elements add up

Drainage layers, filter fabric, root barriers, protection boards and the waterproofing build-up all have weight.

Paving on pedestals for the trafficable areas adds more. Planters, particularly large ones, concentrate load into small footprints and are effectively point loads sitting on a slab designed for distributed ones.

Then there is everything that arrives afterwards. Furniture, a pergola, a barbecue, a spa. That last one is a genuine problem, and it turns up on roof terraces regularly without anyone checking.

Drainage decides whether it works

Water is the failure mode. A blocked outlet on a roof garden does not produce a visible puddle, because the water is inside the growing medium. The roof fills, the load climbs well above what was designed, and nothing looks wrong from above.

That is why overflow provision is essential rather than optional. Every drained area needs a secondary path that will discharge visibly if the primary one blocks, so the problem announces itself.

Outlets also need to be accessible for maintenance. An outlet buried under a metre of soil and a mature shrub will not be cleaned, and the system depends on it being cleaned.

Waterproofing under a garden

The membrane under a green roof is buried under everything above it and is effectively unrepairable without removing the garden.

It needs a root barrier, because roots find and exploit any weakness in a membrane over time. It needs to be flood tested before anything goes on top of it. And the detailing at upstands, penetrations and thresholds has to be right first time.

The cost of getting this wrong is not the membrane. It is stripping and reinstating the entire garden to reach it.

Wind, at height

A roof terrace on a tall building is an exposed environment. Planting, pergolas, umbrellas and screens all catch wind, and anything not fixed down is a hazard to the street below.

Balustrades on a roof terrace are also carrying a higher wind load than one at ground level, on top of the imposed load they already have to resist.

The assessment to do first

Establish what the existing roof can carry, from the original documentation verified on site. Then design the garden to that capacity, or strengthen deliberately.

We assess existing roofs for this work through structural engineering, and the answer usually shapes the landscape design rather than the other way around.