roof drainage calculation

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Roof Drainage Calculations

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Roof Drainage Calculations to BS EN 12056-3:2000Gravity drainage systems inside buildings -Part 3: Roof drainage layout and calculation

Rainfall Intensity

BS EN 12056-3:2000 gives rainfall intensity in litres per second per square meter for

a 2 minute storm event. For the UK, the maps in the standard show the intensity forvarious return periods from 1 year to 500 years.

Note the previous standard gave rainfall intensity figures in mm per hour per squaremeter for 2 minute storm event. To convert to mm/hr multiply the l/sec by 3600.

e.g. 0.048 l/sec is equal to 172.8mm/hr or 75mm/hr is equal to 0.021l/sec.

The standard also gives four categories of design rainfall intensity

Cat 1 Return period of 1 year suggested useEaves gutt ers and flatroofs

Cat 2 Return period of 1.5 x Designlife of the building

suggested useValley and parapetgutters for normal buildings

Cat 3 Return period of 4.5 x Designlife of the building

suggested useValley and parapetgutters for higher risk buildings

Cat 4 Maximum probable rainfall suggested use highest risk buildings

As an aid we have chosen a selection of towns throughout the UK, and determinedthe design rainfall intensity for each town as follows:

• Eaves Gutters and Flat Roofs - Return period of 1 year

• Valley and Parapet Gutter - Return period of 50 years (Equivalent to Cat 2 witha design life of approximately 30 years)

For other design rainfall intensities separate calculations will be required.

Where your project is not near one on the towns chosen, then separate calculationwill again be required.

If there is a town or towns that you would like to be included please let us know

Gutter Types

The gutter shapes and sizes shown below have been used in the preparation of thetables. Although the depth of the gutter has most effect on the catchment area, if thegutter size or shape varies significantly from that shown below then a separate

calculation will be required.The calculated capacity is based on the gutter being:

• Nominally Level

• Freely discharging (maximum flow capacity)

Valley Gutter

Parapet Gutter

also know as a boundary wallgutter

Eaves Gutter

Outlet Types - circular

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The diagrams below shown the size and types of outlets shown in the tables, if othersizes or types are to be used then separate calculations will be required. For valleyand parapet gutter both results for straight and tapered outlets are given. For eavesgutters only the results for straight outlets are shown.

Note: The roof area shown in the tables is based on outlets with no gratings orguards. The effect of introducing gratings or guards is to reduce the catchment areaby up to half.

For square outlets separate calculations will be required

Straight Outlets

Tapered Outlets

Pipes

BS EN 12056-3:2000 includes a calculation for the allowable capacity of therainwater pipes. The standard recommends that the pipe should be designed at amaximum of 1/3rd full (Filling degree 0.33)

In our calculations we have chosen two pipe sizes and limited their capacity to 1/3rdfull.

• 100mm

• 150mm

Note: Where there are offsets or horizontal runs of pipes, the a separate calculationwill be required to determine the capacity of the pipe system from roof level to groundlevel.

Flat Roofs

For flat roofs the design head of water has been taken as 35mm

The catchment area is given for outlets with and without gravel guards.

Catchment Area

The catchment area given in the tables is based on the least of:

• Gutter Capacity

• Outlet Capacity

• Pipe Capacity

Flat Roofs

The catchment area can be taken as the allowable area drained.

Pitched roofs

The catchment area has to be multiplied by the pitch factor to obtain the allowablearea drained.

Valley Gutter (only valid where the roof pitches and slope lengths are equal.)

Pitch 0° 5° 10° 15° 20° 25°

Factor 1.0 1.01 1.02 1.04 1.06 1.10

Parapet or Eaves Gutter

Pitch 0° 5° 10º 15° 20° 25°

Factor 1.0 0.96 0.93 0.91 0.90 0.89

Please Note: Where a wall or walls drain on to the roof, for example from a higherbuilding, then more detailed analysis of the catchment area is required.

Use of Tables

To use the tables:

1. Determine the depth of gutter required, based on the distance between outlets2. Calculate the allowable area drained by multiplying the catchment area by the

pitch factor.3. Calculate the actual area drained from the equations given below.4. Compare the actual area drained with the allowable area drained.


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5. If the allowable area is greater than the actual area then the gutter system issatisfactory.

6. If not re-evaluate.

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