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Due to the infrequency of extremely heavy rainfall in the United Kingdom, it is almost impossible to design a cost efficient rainwater system to give complete immunity from flooding and overflow.
Because of this it is generally regarded as normal practice to design rainwater systems using a rainwater intensity level of 75mm per hour. To calculate the gutter requirement, multiply the E.R.A. (m2) by a factor of 0.0208 (75mm/hour divided by 3600 seconds/hour) to give the flow rate in litres/second (l/s).
Once a flow rate has been determined use tables 1 & 2 (below) to select the required gutter profile.
Note: If a gutter angle is introduced into the rainwater gutter run, the effective gutter capacity will be affected and an adjustment must be made to the maximum roof area which each rainwater system is capable of draining (see Code of Practice BS EN 12056-3:2000 for further details).
To calculate the revised maximum roof area which can be drained when the gutter run contains an angle the following factors must be applied to the figures in tables 1 and 2 below.
Table 1 - Gutter Flow Capacity based on level fall
|
|
Flow capacity
(l/s) |
Maximum Roof Area
(m2) |
Maximum Roof Area
(m2) |
Outlet fixed at |
End |
Centre |
End |
Centre |
75mm - Half Round |
0.390 |
0.680 |
18 |
32 |
112mm - Half Round |
0.912 |
1.833 |
44 |
88 |
112mm - Square |
1.083 |
2.167 |
52 |
104 |
Polyflow |
1.800 |
3.600 |
86 |
172 |
Ogee |
1.833 |
3.667 |
88 |
176 |
Sovereign |
2.100 |
4.200 |
101 |
202 |
150min - Half Round |
2.300 |
4.600 |
110 |
220 |
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Table 2 - Gutter Flow Capacity based on 1:600 fall
|
|
Flow capacity
(l/s) |
Maximum Roof Area
(m2) |
Maximum Roof Area
(m2) |
Outlet fixed at |
End |
Centre |
End |
Centre |
75mm - Half Round |
0.520 |
0.900 |
24 |
42 |
112mm - Half Round |
1.300 |
2.600 |
62 |
124 |
112mm - Square |
1.517 |
3.033 |
72 |
144 |
Polyflow |
2.500 |
5.000 |
120 |
240 |
Ogee |
2.567 |
5.133 |
123 |
246 |
Sovereign |
2.950 |
5.900 |
142 |
284 |
150min - Half Round |
3.233 |
6.467 |
154 |
308 |
Table 3 - Colour Availability
Polypipe Gutter |
75mm Half Round |
112mm Half Round |
112mm Square |
117x
75mm Polyflow |
130x
70mm Ogee |
117x
75mm Sovereign |
150mm Half Round |
Black |
No |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Brown |
No |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Grey |
Yes |
Yes |
Yes |
Yes |
No |
No |
Yes |
White |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
No |
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More about roof drainage design |
Untitled Document
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