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Emergency Dra inage

Basic information on emer-

gency drainages

Arrangement of the emer-

gency overflows on the

flat roof

Autumn 2001, GIAG / Hun

Pluvia Roof Drainage System

Each flat roof or separate flat roof section and each gutter must

be equipped with appropriate emergency overflows, irrespective

of the number of outlets.

This requirement has been defined in the following standards and

regulations:

SN 592 000

VDI 3806

DIN 19 599

DIN EN 12056-3

DIN 1986-100

Guidelines for flat roofs issued by the German Roofers' Trade

Association

Avis Techniques CSTB

SSIV Guidelines for gutter dimensioning

Therefore, the drainage type (negative pressure system or con-

ventional roof drainage system), has no influence on the basicquestion whether emergency overflows should be provided.

Further reasons for the construction of emergency drainage sys-

tems, especially for negative pressure roof drainage systems, are

the following:

Dimensioning of the negative pressure system does not take into

account any country-specific calculation principles of the down-

stream non-pressure drainage.

Therefore, if the non-pressure drainage has smaller dimensions,

the discharge capacity might be reduced.

As regular cleaning of the city-sewer cannot be controlled,

obstructions might lead to rainwater backing up into the roof

drainage system.

Trapezoidal roof constructions are increasingly being designed

for smaller static loads. Therefore, the danger of collapse due to

record precipitations and of water penetrating into the building is

high.

When emergency overflows are positioned on a roof with a Pluvia

roof drainage system, the following measures must be taken into

account:

Emergency overflows must be positioned in such a way that they

do not obstruct the rainwater flow between the outlet and the

emergency overflow.

The weir must be placed 5 cm above the rainwater outlet, regard-

less of the type of emergency overflow used (this also applies to

roofs with vegetation). It must be ensured that there are no flat

roof edges or roof entryways, etc. below the emergency overflow

level.


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Emergency Dra inage



Dimensioning of emergency

overflows on the flat roof

The emergency overflow should always be constructed through

the roof balustrade, if possible. If the roof construction does not

allow an unobstructed emergency overflow through the

balustrade, emergency drainage can also be via another piping

system. This piping system should, however, not be directed to

the same city-seweras the Pluvia system. It is even better to

route this piping system above the ambient level.

External emergency overflows should have a square design; this

ensures a faster operating behaviour than a system with round

apertures.

Emergency overflows using another piping system may, howev-

er, also be round (with pipes). In this case, the inlet funnel should

preferably have a round design too.

An infinite number of country-specific guidelines and calculation

procedures is available for the dimensioning of emergency over-

flows. Regardless of these procedures and taking into accountrecord precipitations and the gravitational constant etc., Geberit

offers an easy way to calculate the dimensions of emergency over-

flows by using the company's empirical values.

Total amount of rainfall in litres / s x 25 cm2

Example:

Roof area with four outlets 8 l / s = 32 l / s x 25 cm2 = 800 cm2

In order to guarantee that this emergency overflow will respond as

quickly as possible to the respective amount of water and that the

roof will not become overloaded, the horizontal aperture of theemergency overflow should be large.

This means the height of the aperture should range from 10 to a

maximum of 15 cm. The overall size of the emergency overflow

aperture in the above example would then be 80 x 10 cm or

54 x 15 cm.

The total aperture size can, of course, also be divided into several

smaller apertures.

These dimensions can also be used for an internal emergency

drainage system (e.g. a pipe system). According to the above

example, approx. two PE pipes of 250 mm should be used to guar-

antee sufficient emergency drainage.


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Emergency Dra inage



Arrangement of emergency

overflows in the gutter

This dimensioning method ensures an efficient roof drainage even if

outlets are blocked or the city-sewer is obstructed. Record precip-

itations (~ rainfall intensity twice as high as normal ) can thus be

handled. Double rainfall intensity corresponds to a safety factor of

2.0. An even higher safety factor must be applied to buildings

requiring higher protection (hospitals, theatres,).

(For detailed information, see EN 12056-3 chap. 4.2.)

For gutter constructions, the appropriate arrangement of emer-

gency overflows is equally important. This especially applies to

internal gutters (shed-type gutters): For this gutter type, emergency

overflows can only be positioned at the two front ends of the gut-

ter.

For bracket mounted gutters, the emergency overflows can be

positioned near the outlets (lowest point of the corresponding gut-

ter section) as well as at the front ends.

To guarantee efficient emergency overflows in accordance with the

following dimensioning, the basic specifications on the use of the

Pluvia system for gutters must be considered:

Maximum distance of outlets 20 m,

Minimum gutter width 35 cm,

Gutter slope in the longitudinal direction 5 mm / m (0.5 %).

5

H

10-15 10-15var.

5

Internal emergency overflow (round)

Emergency overflow via balustrade (square)

max.10 m max. 20 m max. 20 m

0.5%0.5%

Arrangement of outlets in gutters


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Emergency Dra inage




overflows placed along-

side the gutter


overflows placed at the

front end of the gutter

For the dimensioning of emergency overflows for gutter construc-

tions, the two different arrangements must be considered.

For emergency overflows fixed alongside the gutter, dimensioning

is basically the same as for flat roof emergency overflows.

The only difference is that for gutter contructions the weir must be

placed 10 cm above the outlet.

Total amount of rainfall in litres / s x 25 cm2

The required aperture surface that has been calculated must be dis-

tributed equally among the number of outlets.

var.

10 cm

H10 - 15 cm

Emergency overflow for gutters, longitudinal arrangement

Example:

Gutter with four outlets of 6 l / s = 24 l / s x 25 cm2 = 600 cm2

For an aperture height of 10 cm the total aperture length is 60 cm.

Total aperture length 60 cm : 4 outlets = 15 cm; i.e. the ideal emer-

gency overflow aperture for each outlet is 15 x 10 cm.

Notwithstanding any existing calculation tables and the above-

mentioned basics on dimensioning, the following can be said when

calculating the front end emergency overflow aperture:

Both front ends of the gutter must be equipped with an emer-

gency overflow aperture.

These apertures must be mounted in such a way that the whole

gutter width is covered; the top must be kept open.

min. 35 cm

min. +

0.0 %

Cross-section of external gutters with minimum requirements


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Emergency dra inage



The correct dimensioning of the gutter in accordance with the

country-specific standards and guidelines is the prerequisite for

these emergency overflows to be able to meet the require-

ments.

The minimum requirements of a gutter can also be determined

using the chart below (if the volume flow is known).

G. width

5 - 10 cm

Cross-section of emergency overflow for gutters, front end arrangement

10

Gutter depth in cm

Volumeflow

VGutterinl/s

Gutter width in cm

4

8

12

16

20

24

28

32

36

40

44

48

52

56

60

15 20 25 30 35 40

40

50

35

75100

The weir must be mounted at least 5 cm and maximum 10 cm

above the following outlet aperture (depending on the distance

between the outlet and the front end or the longitudinal slope).

0.5%

max. 10 m

5 - 10 cm

Emergency overflow for gutters, front end arrangement


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Emergency Dra inage



Cleaning of the system and

maintenanceFlat roofs and gutters require regular maintenance. For example,

any contamination and vegetation must be removed and leaves

cleared away regularly in order to avoid having blocked drains.

This cleaning work should be undertaken more or less frequently

depending on the specific ambient conditions. The work should

include cleaning the Pluvia rainwater outlets as well as the emer-

gency overflows. In order to clean the Pluvia rainwater outlets, it is

necessary to briefly remove the cover and the inlet disk and also

clean the dirt off the inlet element underneath.

The frequency of cleaning has to be defined for the building by its

owner who can entrust this task to a suitable person such as the

caretaker.

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