Flat roof in a new build: Structure and cost of our green roof

For us, it was clear from the start: We want a flat roof. In addition to our preference for clean lines, this also allows us to avoid sloped ceilings on the upper floor. The roofing work on the two roof areas has now begun. The main roof has an area of 216 m² and the garage roof 47 m². Because the development plan requires extensive greening, we are planting around 178 m² of the roof area.

A flat roof comes with some special features: from the multi-layered structure and the parapet to emergency overflows and cable penetrations.

Main Roof: Structure and Waterproofing in Detail

The structure of a flat roof must fulfill several functions to ensure it remains watertight, provides thermal insulation, and allows water to drain reliably. Here is our roof structure:

Flat Roof: Layer Structure

The first layer goes on the bare reinforced concrete slab:

• Bituminous Primer: Improves the adhesion of the vapor barrier as a primer.
• Vapor Barrier (Vedagard AL-E): Does what its name suggests and prevents moisture from the building from rising into the insulation layers and condensing there.
• Base Insulation: For the first insulation layer, 8 cm EPS rigid foam boards are used.
• Tapered Insulation: This layer prevents standing water on the roof by creating a 2% slope towards the rainwater drains. The tapered insulation is between 2 cm and 22 cm thick and averages 20 cm. 
• Bituminous Waterproofing (Vedastar SU Safety): Two layers of this waterproofing are welded on. For a green roof, it must be root-resistant to prevent damage from plant roots.
• Protection Fleece: Provides additional protection for the waterproofing against roots.
• Extensive Green Roof: The sediment layer follows the fleece, serving as the substrate for the plants.

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We are not greening the entire roof area: We are laying a gravel strip adjacent to the parapet (the roof edge finish), around ventilation outlets, roof drains, and cable penetrations. This protects the connections and allows for later maintenance work without damaging the plants.

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The Parapet: Structure of the Roof Edge

The parapet refers to the wall at the roof edge. It directs rainwater into the roof drains and prevents, for example, gravel from falling off the roof. Ours is made of KS-masonry and was built on the reinforced concrete slab. To prevent thermal bridges, it must be insulated on all sides:

EPS rigid foam boards insulate the parapet on the roof side. On the house wall side, the insulation consists of an external thermal insulation composite system (WLG035). Its top also has insulation that is tapered towards the roof. Rain that falls on the parapet runs onto the roof surface and then into the rain pipes via the roof drains.

An OSB formwork, which is also welded with the bituminous waterproofing, serves as the final layer. Protection against the weather is provided by a titanium zinc covering that completely encloses the top of the parapet.

Drains, Emergency Overflows, and Cable Entries in the Flat Roof

For the roof to remain watertight even during heavy rain, for the downpipes not to freeze in winter, and for the photovoltaic system to be able to conduct electricity into the house, drains, feedthroughs, and conduits are necessary. Here is an overview:

Roof Feedthroughs for Drainage and Ventilation

• Roof Drains: The main roof has one roof drain that channels rainwater through an interior downpipe in the garage into the sewage system (separate system). On the opposite side, a parapet drain directs rainwater via a rain pipe.
• Emergency Overflows: In the event that both main drains are blocked – for example by leaves or extreme weather – there is one emergency overflow on both the north and south sides. This directs the excess water directly into the ground before too much water can accumulate on the roof.
• Ventilation Outlets: The roof has three outlets connected to the sewage downpipes. These ventilate the sewage system so that no negative pressure is created when water drains. Without this ventilation, the toilet siphons would be sucked dry when flushing, which would lead to bad smells from the sewage system.

Downpipe Heating Cable: Frost Protection for Green Roofs

A green roof with a slight slope drains water more slowly. Therefore, the water in the downpipe can freeze and block it in very low temperatures during winter. To prevent this, we are installing a downpipe heating system. This switches on automatically at a predetermined temperature and keeps the pipe free of ice.

Cable Feedthroughs for Photovoltaic System

We have a DN100 Flesodur feedthrough on the roof for the cables of our photovoltaic system. It connects the photovoltaic system to the technical room in the basement via a conduit. The cables for the outdoor unit of the air conditioning system also run through this cable feedthrough.

Garage Roof: Structure, Material, and Greening

The reinforced concrete slab of the garage was already constructed with the necessary slope. And since the garage remains unheated and uninsulated, neither a vapor barrier nor insulation is required. This reduces the structure to the essentials: The bituminous primer is directly followed by the two-layer bituminous waterproofing. As with the main roof, we are opting for extensive greening, but only in areas not overhung by the main roof.

The garage parapet is similar to the main roof's. However, it is significantly lower (approx. 12.5 cm high) and has no insulation.

Cost Factor Flat Roof: What Our Green Roof Costs

A waterproof, insulated green roof has its price. Here is what the complete roof structure for the 263 m² roof area costs.

General Work

Flat Roof House

Flat Roof Garage