To select the size of the Classical rainwater system the following method should be used.
- Calculate the effective catchment area (x) to be drained from the following table.
| Table 1 – Calculation of area drained | |
|---|---|
| Type of Surface | Effective design area (m²) |
| 1 flat roof | plan area of relevant portion |
| 2 pitched roof @ 30° pitched roof @ 45° pitched roof @ 60° |
plan area of portion x 1.29 plan area of portion x 1.50 plan area of portion x 1.87 |
| 3 pitched roof over 70° or any wall | elevational area x 0.5 |
- Let the rainfall rate (r) = 75mm per hour.
- Your run off rate (q) = rx/3600 This gives the maximum litres of rainfall per second expected.
- Calculate the maximum run rate required by the gutter taking into consideration the position of the outlet in the run. For example:
- Find the capacity of the gutter from Table 2 below to match the run rate.
| Table 2 – Gutter Capacities | ||||||||
|---|---|---|---|---|---|---|---|---|
| Gutter Type and Size | ||||||||
| Capacity l/sec | Half Round | Beaded Half Round | Deep Half Round | Express | Ogee | Moulded No.46 | Notts Ogee | Box |
| 1.75 | – | – | 125 x 75 | – | – | – | – | – |
| 1.50 | – | – | – | 125 | – | – | – | – |
| 1.42 | – | – | – | – | 125 x 100 | – | – | – |
| 1.40 | 150 | – | – | – | – | – | – | – |
| 1.39 | – | – | – | – | – | – | – | 100 x 75 |
| 1.24 | – | – | 100 x 75 | – | – | – | – | – |
| 1.07 | – | – | – | – | – | – | 115 | – |
| 0.97 | – | 125 | – | – | – | – | – | – |
| 0.94 | 125 | – | – | – | – | – | – | – |
| 0.79 | 115 | 115 | – | – | – | – | – | – |
| 0.71 | – | – | – | – | 125 | – | – | – |
| 0.67 | – | – | – | – | – | 100 x 75 | – | – |
| 0.59 | – | 100 | – | – | 115 | – | – | – |
| 0.53 | 100 | – | – | – | – | – | – | – |
| 0.41 | – | – | – | – | 100 | – | – | – |
If you require your calculations checked, please contact our Technical Advisory Service.
- This figure may be reduced due to the length of the gutter run. To Find the true capacity of the gutter use the following calculations.
- Divide the length of the gutter to the outlet in mm (I) by the depth of the gutter in mm (d).
- Consult table 3 to get the next highest reduction factor.
- Multiply the reduction factor by the gutters stated capacity to get the true capacity.
| Table 3 – Reduction Factors | |
|---|---|
| l/d | Reduction Factor |
| 50 | 1.00 |
| 100 | 0.93 |
| 150 | 0.86 |
| 200 | 0.80 |
- When the true capacity of the gutter has been calculated, find the suitable size from table 2.
- The capacities of the rainwater pipes can be found in table 4.
| Table 4 – Pipe Diameters | |
|---|---|
| Pipe Diameter (mm) | Reduction Factor |
| 65 | 2.00 |
| 75 | 3.00 |
| 100 | 7.00 |
Capacity Testing
Sizes of the standard half round gutters have undergone hydraulic testing to confirm actual capacity capabilities and performance (other gutter capacities are calculated values). Tests were carried out independently at H.R. Wallingford.
The new rainwater sockets have also been assessed in conjunction with the gutters.