ᐅ Extensive green roof on a flat roof or with a 2% slope on a garage?

I am planning a 6m (20 ft) wide double garage with 52 sqm (560 sq ft) of extensive green roofing (Sedum, max 10cm (4 inches) substrate depth). The structural design supports 125 kg/m² (26 psf) for the green roof load. The roof profile above the garage doors should be horizontal. EPDM waterproofing with a thickness of 1.5mm (0.06 inches) is planned.

Now I am unsure which roof design to choose:

A) Flat roof with 2 central roof drains
+ Easy to build
+ Visually appealing
+ Uniform 10cm (4 inches) high parapet/roof edge all around
- Sedum might suffer from waterlogging
- Higher risk and damage potential if waterproofing fails
- Ice formation in winter
- Deposits accumulating in the drainage layer

B) Two sloping wedges towards the roof center with a 1% slope
Advantages and disadvantages similar to option A but less pronounced. Parapet height varies between 10-16cm (4-6 inches).

C) 2% slope with gutter drainage towards the garden side
+ Reliable drainage
+ Better for extensive green roofs
+ No water in the garage and possibility to install a rainwater barrel outside at the downpipe
- Uneven parapet height (10cm (4 inches) at the highest point of the slope, 22cm (9 inches) at the lowest)
- Less attractive appearance due to the garden-facing gutter

Which option would you recommend and why?

The safest option is a 2% slope toward the gutter.

It is also important that the drainage layer can retain water.

I have tried all variations, even up to a 20% slope so that it is visible from the ground.

The roof pitch is a matter of personal preference; 4 valleys with a slope toward the center would also be possible, allowing you to maintain the 2% slope toward the drain.

It has to please you for the next 3 to 50 years.

Definitely avoid placing it at the top; you don’t want a weak spot in the middle and at the lowest point on the roof at the same time. I would suggest designing the flat roof either as a shed roof (for the simplest water drainage) or as a gable roof (which overall requires less construction height). The downside of a gable roof is that you will need a second downspout and occasionally have one more gutter to clean.

If the gutter bothers you visually, you can also place it on the edge instead of in front of it. This is a bit more complex but makes the gutter less visible. However, in both cases, the downspout will be on the exterior of the facade and not hidden. You should carefully consider whether you want to prioritize function over appearance after thinking this through. These issues can follow you for a long time, and you may often wonder why you didn’t choose the other option.

Okay, great, thank you all. So it will be a 2% slope with a drainage channel. On three sides, there will be a horizontal parapet, and on one side it will be open to the garden with a gravel stop edge.

28cm (11 inches) beam height
2.5cm (1 inch) OSB thickness
12cm (5 inches) 2% slope over 6.3m (21 feet), i.e., highest point
10cm (4 inches) parapet at the highest point and 10cm (4 inches) of EPDM membrane turned up (parapet at the lowest point is 10 + 12 = 22cm (9 inches))
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Total roof height 52.5cm (21 inches)

For the green roof, I am considering the Optigrün complete flat roof package "Individual" for 0–5° roof pitch.
What is the best way to prevent the substrate and plants from slowly sliding downhill on the filter fleece?

A gravel catch strip in front of the gutter, the rest will hold on its own.

When it’s dry and windy, keep it moist until something grows.

Optionally, you can run the 10cm (4 inches) parapet height parallel to the roof slope towards the gutter. This might look better from the garden, as it won’t appear as high.

The parapet cladding on the side will then be tapered.

I didn’t want to run it parallel because we have our 5m (16 feet) Hörmann driveway gate on the side, and then the sloped parapet would look odd.