Flachdachentwässerung | Lösungen & Bauwissen für Profis

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Knowledge for building services planning

Flat roofs have numerous advantages, but they must overcome an enormous structural challenge. When planning and executing, safety, fire protection and therefore the question of roof drainage are top priorities. ACO offers functional solutions that are specially tailored to flat roofs and ensure optimal rainwater drainage. Gravity drainage systems are suitable for smaller areas. On large roofs of around 150 m² or more, drainage systems with negative pressure are preferred. ACO offers products made of cast iron and stainless steel. There are also special solutions for green roofs, parapets and emergency drainage. ACO supports you in planning the right flat roof drainage and hydraulic calculation of pressure flow systems with the latest, standard-compliant design software and a high level of practical knowledge.

Standards and guidelines for flat roof drainage
Gravity and vacuum drainage, emergency drainage
Calculation of flat roof drainage
Dimension and type of drains
Discharge restrictions
Rainwater retention with green roofs
Fire protection
Products from ACO

The use of flat roofs is becoming increasingly versatile: they serve not only as parking spaces, but also as roof terraces,
sports or play areas, and even parks. Green roofs in particular make an important contribution to balancing
sealed surfaces. Due to these diverse uses, the requirements and regulations vary considerably. There is therefore no universal standard for the use of roof areas; instead, various specific guidelines and regulations must be observed. The safe design of flat roof drainage is the responsibility of the architects and building services engineers and must be carried out in a manner appropriate to the respective roof construction.

Design of flat roof drainage

The drainage calculation is based on the specifications of DIN 1986-100‘Drainage systems for buildings and properties’,
which defines the general requirements for the planning, execution and maintenance of drainage systems. In addition,
DIN EN 12056-3‘Gravity drainage systems within buildings’ is also used.

Planning, execution and maintenance of flat roof drainage

Specific requirements and technical specifications for the waterproofing of roofs and similar components are laid down in DIN 18531 ‘Waterproofing of roofs, balconies, loggias and pergolas’. This standard is particularly relevant for the planning and implementation of waterproof outdoor areas and roof structures. In addition, DIN 18532 ‘Waterproofing of concrete traffic areas’ should also be mentioned here. This deals with so-called top decks, i.e. flat roofs that are accessible to vehicles. Architects and planners also refer to the flat roof guidelines, which are issued as technical rules for waterproofing by the Central Association of German Roofers (ZVDH) as recommendations. Information on maintenance can be found in DIN 1986-3.

Dimensioning of the drainage system

Another decisive factor for the dimensioning of drainage systems is the local rainfall, which is determined in accordance with ‘KOSTRA-DWD-2020’. This data, provided by the German Weather Service (DWD), provides information about the expected precipitation amounts in the respective region and is therefore decisive for the design of drainage capacities. In addition, the local regulations of the municipality in which the construction project is located must be observed. These regulations often take into account region-specific conditions and requirements, which may be based on KOSTRA data, to ensure that the drainage systems are sufficiently dimensioned to cope with the expected rainfall amounts.

Selection of the drainage system	►	Relevant for planning	►	Requirements for the product
Gravity drainage Vacuum drainage	DIN EN 12056-3 DIN 1986-100	Tested in accordance with DIN EN 1253-2, Ü mark No CE marking, type plates or declaration of performance required

All ACO flat roof drains comply with the relevant standards, are manufactured in accordance with these standards and thus guarantee the highest standards of safety and quality.

Flat roof drains collect rainwater from flat roofs, parking decks, green roofs and terraces. The rainwater is usually drained away from there via drainage pipes. There are two basic types of flat roof drainage systems: gravity drainage and vacuum drainage. → Decision diagram

Vacuum drainage is a system for draining rainwater from roof surfaces, which is characterised by the creation of negative pressure in the collection pipes. In accordance with DIN 1986-100:2016-12, the drainage flows from individual roof drains are fed through connection pipes below the roof structure to a central downpipe. The resulting negative pressure enables rapid drainage of the roof surface by drawing the water through the system at high flow velocity. One advantage of vacuum drainage is the possibility of laying the pipes below the roof without a gradient, which allows for flexible room design and use. The full filling of the pressure pipe system also achieves a high flow velocity, which promotes self-cleaning of the pipe system and reduces maintenance costs.

Vacuum drainage works with special flat roof drains which, in contrast to gravity drainage, are operated with pipes that are filled to capacity (filling ratio h/d 1.0). In order to achieve this full filling, the entry of air (air suction) must be prevented when rainwater enters the pipe. As soon as the design rainfall required for operation is reached, the system operates with fully filled pipes in the pressure flow range, which means that the connected roof area is drained quickly and safely. However, this also means that as long as only light rainfall occurs, the systems operate as gravity drainage if the backwater required for operation cannot be reached.

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In the event of low rainfall, drainage is based on the principle of gravity.

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After heavy rain sets in, the pipe system begins to fill completely with water.

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Once the pipe is completely filled, a suction effect is created. Rainwater flowing in is sucked through the pipe system with no air trapped inside.

Gravity drainage is a drainage system based on the principle of gravity to drain rainwater from flat roofs, terraces and similar surfaces. In this system, the water flows freely, without negative pressure, through the flat roof drains and the connected pipes. The efficiency of gravity drainage depends largely on the correct dimensioning of the pipe diameters, which are calculated in accordance with the standards DIN EN 12056-3 and DIN EN 1986-100.

A characteristic feature of gravity drainage systems is the large number of drain bodies connected to a downpipe via a collecting pipe or directly to the downpipes, which requires a sufficient gradient and a corresponding number of connections to the underground pipes to ensure effective water drainage. For gravity drainage systems in accordance with DIN EN 12056-3 (installed inside buildings), the filling ratio (h/d) must not exceed 0.7 (horizontal pipe) or 0.33 (vertical pipe), as only up to this value can sufficient ventilation of the pipes and thus safe drainage of rainwater be guaranteed.

Roof drains on flat roofs in accordance with DIN EN 1253-2 must be dimensioned for regular operation (standard operation) up to specified accumulation heights.
Any precipitation exceeding the calculated rainfall can lead to accumulation on the roof surfaces. However, additional accumulation on the flat roof can lead to damage to the roof structures or to uncontrolled flooding of the areas below. Therefore, these additional amounts must usually be drained via specially designed drainage systems, known as emergency drainage systems.
Emergency drainage systems are mandatory for lightweight roofs, and for roofs with green roofs, static verification must be provided for the emergency drainage system, taking into account the target water depth. (DIN 1986-100)

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Emergency drainage systems are an essential component of modern rainwater drainage systems, especially when it comes to coping with extreme weather events such as once-in-a-century rainfall. Emergency drainage must not be connected to the sewer system. According to DIN 1986-100, it must be directed to a property area that can be flooded freely and without damage. Discharge would place an additional burden on the sewers.

In order to protect their sewer systems from hydraulic overload, local authorities can also impose discharge restrictions on roof drainage. Whether these discharge restrictions must be taken into account in planning can be found in the sewage regulations. Every low point must have an emergency drain in addition to the standard drain. Emergency drainage must be planned for flat roofs; it can only be dispensed with if there is static proof of the load-bearing capacity of the roof area and if it is used for rainwater retention as planned. Emergency drainage can be provided by emergency drains or emergency overflows.

Emergency drainage can be implemented by means of additional emergency drains or parapet drains, which are designed to activate when a critical water level is reached and drain off the excess water. Waiving emergency drainage is only justifiable for solid roofs with static proof. These can be, for example, roofs with planned rainwater retention. However, these usually have a drain with an integrated overflow system to minimise the risk of overloading the roof structure.

When planning drainage for flat roofs, there are a few basic considerations to take into account to ensure that rainwater is effectively drained away. The number and type of drains required depend on the size of the roof area, the roof construction and local rainfall conditions. Different roof materials and structures, such as concrete, gravel or green roofs, influence how much water actually drains away. This is taken into account by the drainage coefficient. Rainfall, i.e. the amount of rain expected in a particular region, also plays an important role in the dimensioning of drainage systems.

Gravity drainage: Calculation of rainwater volumes

When planning drainage systems for flat roofs, it is crucial to calculate the exact amount of rainwater (QD for standard drainage and QD,Not for emergency drainage). These calculations are based on equations (1) and (2) and can be applied to all exposed surfaces such as roofs and roof terraces.

(1) Q_D= r(5,5) ⋅ Cs ⋅ A ⋅ 1/10.000 (in l/s)
(2) Q_D,Not= (r(5,100) − r(5,5) ∙ Cs) ∙ A/10.000 (in l/s)

→ Calculation example

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Standard drainage

The calculated rainfall intensity r, expressed in l/(s ∙ ha), is a statistical value provided by the German Weather Service that indicates the expected amount of rainfall per second and hectare. According to DIN 1986-100, a rainfall duration of 5 minutes is used for the design, with a return period of 5 years for standard drainage and 100 years for emergency drainage. The current rainfall values can be found in Kostra-DWD-2020.

The peak runoff coefficient Cs takes into account the proportion of rainwater retained by the roof structure and the proportion that must be drained away by the drainage system. For gravel surfaces, this value is 0.8, which means that 20% of the water is retained and the standard drainage system can be dimensioned accordingly smaller.

Emergency drainage

Emergency drainage is dimensioned larger than standard drainage in order to account for the difference between standard and emergency drainage. The peak runoff coefficient is first multiplied by the rainfall intensity for standard drainage, and this product is then subtracted from the rainfall intensity for emergency drainage. The area A of the flat roof, expressed in m², is decisive for the calculation and must be assumed to be the area projected in the floor plan. The rainfall intensity, peak runoff coefficient and area factors are divided by 10,000. The divisor is derived from the conversion factor from ha to m² and is 10,000. Gravity drainage is usually sufficient for residential and office buildings. Siphonic drainage systems are specially designed for situations with large amounts of rainwater and extensive roof areas.

Vacuum drainage: Planning support from our application engineers

The project planning team at Application Technology Roofing will support you with your roof drainage, including calculation, planning and dimensioning of the waste water pipes.

Consulting on the design of flat roof drainage systems

Design of vacuum drainage

Project planning for piping and drainage requirements

→ Contact us

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When planning the drainage of a flat roof, the size and type of drains must be carefully selected. This decision depends on the amount of rainwater that needs to be drained from the roof, as well as the number of drains required and the nature of the roof itself. To determine how many drains are needed, divide the amount of rainwater (Q) by the drainage capacity of a single drain (QDA), which is specified by the manufacturer. This capacity indicates how much water the drain can handle and is often higher than the minimum standard specified in DIN EN 1253-2.

It is important to note that every low point on the roof, i.e. every point where water could collect, must have at least one regular drain or one emergency drain. In roof valleys, the lines where two roof surfaces meet, the drains should not be more than 20 metres apart. The choice of drains, including the top part and the attachment piece, depends on the roof construction. The drains are usually supplied with a press seal or adhesive flange for attaching the sealing membranes. A width of at least 100 mm is required for adhesive flanges. According to DIN 18531, the distance between drains and walls or other upright components should be at least 30 cm for membrane-type waterproofing. When using liquid sealants, this distance should be at least 10 cm, but this does not apply to parapet drains.

Local authorities are increasingly imposing discharge restrictions in order to reduce the load on the sewer system and minimise the risk of flooding. Some reasons for these restrictions are:

Climate change: Climate change is leading to an increase in extreme weather events with heavy rainfall. This can exceed the capacity of drainage systems and lead to flooding.
Sealed surfaces: Sealed surfaces in cities due to increasing development prevent rainwater from seeping into the ground. Instead, more water flows directly into the sewer system, which increases the load on it.
Environmental protection: Delaying the discharge of rainwater into the sewer system reduces the risk of mixed water overflows into natural water bodies. This protects the environment from pollution by untreated wastewater.
Sustainable water management: Discharge restrictions promote the use of sustainable water management measures, such as rainwater harvesting or the installation of green roofs, which help to reduce direct runoff and increase biodiversity.

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Green roofs are an effective solution for storing rainwater and reducing direct water runoff. They not only contribute to delayed rainwater runoff, but also improve biodiversity and the microclimate, especially in urban areas. However, additional aspects must be taken into account when planning a green roof. These include the additional load caused by the vegetation and the need for root-resistant roof waterproofing. In the case of tall or particularly wind-prone buildings, wind suction load and drift resistance must also be taken into account.

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Greening also affects roof drainage. The peak runoff coefficient, which takes the roof structure into account, and an effective drainage layer are crucial for the correct drainage of rainwater. Rainwater management refers to the sustainable use of rainwater. Roof drainage can make a valuable contribution here, but the objectives must first be defined. For example, should the roof structure in combination with the roof drainage system delay the drainage of heavy rainfall, or should the rainwater be retained on the roof to bridge dry periods? In order to implement these requirements in practice, drainage systems with retention attachments have proven effective. Their throttle hole allows part of the rainwater to be drained with a time delay or to remain on the roof. In combination with discharge restrictions, green roofs, in conjunction with coordinated drainage, offer a sustainable solution for rainwater management that brings both ecological and urban planning advantages.

To protect the roof drains from root penetration, DIN 1986-100 recommends a 50 cm wide gravel strip with a grain size of 16/32, as specified in the flat roof guideline.

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When planning drainage for flat roofs that transition into surrounding roof terraces on staggered storeys, the question often arises as to whether rainwater from a flat roof may be directed onto an adjacent terrace. In general, this is possible, especially if the water flows from a smaller to a larger area, which complies with DIN 1986-100 and is permitted in exceptional cases. In areas where the water hits, reinforcement of the waterproofing or roof covering may be necessary. According to DIN 18531, additional water volumes from higher roof areas must be taken into account in the planning. However, cascade drainage carries risks, such as water entering the building, especially if the maintenance of the roof drains is neglected and there are barrier-free transitions to the roof terrace. From a planning and regulatory perspective, it is more appropriate to drain rainwater into pipes or flat channels instead of directing it to an area below. The specifications for the slope of flat roofs can be found in the Flat Roof Guideline and DIN 18531. A minimum slope of 2% is recommended to prevent puddles from forming due to structural deflection or unevenness. A slope of 5% or more can be assumed to result in a puddle-free surface. Roofs in application class K1 can be designed without a slope, provided that the waterproofing meets the higher requirements of application class K2. For roofs in application class K2, in addition to the minimum slope, a slope of 1% in the valleys is recommended.

Flat roof drains are important components of the roof structure, enabling water to drain from the roof into the interior of the building. However, in the event of a fire, there is a risk that toxic fumes and smoke could escape upwards through these drains and enter neighbouring buildings located higher up, for example through windows or doors.

To minimise this risk, national building regulations stipulate the use of special fire protection flat roof drains, especially if the distance between a roof drain and a wall is less than five metres. These fire protection drains are specially designed to prevent the spread of fire and smoke, but they are not standard construction products and therefore require general building approval (abZ) and general type approval (aBG).

In certain situations, such as when flat roofs serve as escape routes, it may be necessary to install fire protection drains to ensure safety, regardless of the distance to the wall. These drains are equipped with fire protection devices that prevent the spread of fire and smoke. It is important that the fire resistance class of the roof drain is at least equal to or higher than that of the roof ceiling in order to ensure optimum protection.

ACO flat roof drainage Fire protection for flat roofs — If flat roof drains are located closer than five metres to rising walls with windows or other openings, these flat roof drainage must be designed as fire protection drainage.

State building regulations

State building regulations require the use of flat roof drains with appropriate fire protection measures if the distance between roof drains and an ascending wall (with openings or without fire resistance) is less than five metres in these areas.

Fire resistance class

In addition to the fire protection inserts, the drains themselves must also have a fire resistance class (R). In addition to the fire resistance class (R), the same classification also exists for building ceilings, which begins with the letter ‘F’ instead of the letter ‘R’. The fire resistance of F90 for a building ceiling is therefore 90 minutes.

Feuerwiderstandsdauer	Bauaufsichtliche Bennenung	Feuerwiderstandsklasse (R) für Rohrabschottungen	Feuerwiderstandklasse (F) für raumabschließende Bauteile
30 Minuten	feuerhemmend	R30	F30
60 Minuten	hochfeuerhemmend	R60	F60
90 Minuten	feuerbeständig	R90	F90
120 Minuten	hochfeuerbeständig	R120	F120