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Roof Drainage Calculations to BS EN 12056-3:2000
Gravity 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 for various return periods from 1 year to 500 years.
Note the previous standard gave rainfall intensity figures in mm per hour per square meter 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 use Eaves gutters and flat roofs
Cat 2   Return period of 1.5 x Design life of the building  suggested use Valley and parapet gutters for normal buildings
Cat 3  Return period of 4.5 x Design life of the building   suggested use Valley and parapet gutters 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 determined the 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 with a 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 calculation will 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 the tables. Although the depth of the gutter has most effect on the catchment area, if the gutter 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 valley
Parapet Gutter 

also know as a boundary wall gutter

Eaves Gutter eaves
Outlet Types - circular
The diagrams below shown the size and types of outlets shown in the tables, if other sizes or types are to be used then separate calculations will be required. For valley and parapet gutter both results for straight and tapered outlets are given. For eaves gutters only the results for straight outlets are shown.
Note: The roof area shown in the tables is based on outlets with no gratings or guards. The effect of introducing gratings or guards is to reduce the catchment area by up to half. 
For square outlets separate calculations will be required
Straight Outlets straight outlet
Tapered Outlets Tapered Outlets
BS EN 12056-3:2000 includes a calculation for the allowable capacity of the rainwater pipes. The standard recommends that the pipe should be designed at a maximum of 1/3rd full (Filling degree 0.33)
In our calculations we have chosen two pipe sizes and limited their capacity to 1/3rd full.
  • 100mm
  • 150mm
Note: Where there are offsets or horizontal runs of pipes, the a separate calculation will be required to determine the capacity of the pipe system from roof level to ground level.
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 allowable area 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 10 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 higher building, 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 outlets
  2. 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.
  5. If the allowable area is greater than the actual area then the gutter system is satisfactory.
  6. If not re-evaluate.
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