ᐅ Extensive green roof on a flat roof or with a 2% slope on a garage?
Created on: 14 Sep 2024 00:35
H
helipus
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?
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?
So, the roofer just went full speed ahead and is trying hard to convince me to go with a flat roof with zero slope and two drain boxes. He is absolutely convinced that otherwise, over the years, sediments and debris will gradually move down the slope. He also thinks that a parapet on all four sides without a gutter simply looks much nicer, as do we. He is having a hard time offering anything other than 0% slope. I think he’s trying to persuade me, especially since the installation is much easier.
G
Gerddieter16 Sep 2024 22:13I’m not sure if this helps you, but I can share what the site manager, landscaper, and building consultant told me.
It’s also about our garage roof, which will have an intensive green roof with 50 cm (20 inches) of substrate/soil for grass and shrubs.
I wanted a slope because of moisture, etc.
Everyone said no; the garage roof is already waterproof, and the water on the roof should not constantly drain away completely, otherwise nothing will be able to grow. Now there is no slope at all, a parapet all around (on one side not visible since it is very low), and the drainage is arranged so that about 5 cm (2 inches) of water is retained for the plants...
Best regards, GD
It’s also about our garage roof, which will have an intensive green roof with 50 cm (20 inches) of substrate/soil for grass and shrubs.
I wanted a slope because of moisture, etc.
Everyone said no; the garage roof is already waterproof, and the water on the roof should not constantly drain away completely, otherwise nothing will be able to grow. Now there is no slope at all, a parapet all around (on one side not visible since it is very low), and the drainage is arranged so that about 5 cm (2 inches) of water is retained for the plants...
Best regards, GD
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