The roof area that can be connected for rainwater harvesting depends on the local rainfall, as for example in the tropics significantly different values are to be expected than in temperate climate regions.
But there are also considerable differences within a climate zone. The diameter of the drainage pipes is decisive.
The architect plans the pipe diameter
When constructing a new building, the diameter of the drainage pipes is determined by the architect or planner. He takes into account the local climate conditions and the size of the connected roof area.
Determining the diameter yourself
If the rainwater filter is installed later, the diameter of the existing pipes is decisive. Depending on the installation position, it can be the (almost) horizontal sewer base pipe ("KG pipe") in the ground or the vertical downpipe.
For the sewer base pipe
KG pipes are standardised in the DN standard to match our rainwater filters. The outer diameter of the pipes is decisive for a precisely fitting connection. It is often indicated on the pipe or can be measured with a sliding gauge.
If the DN specification is not shown on the downpipe or pipe clamp, you can measure it yourself. The following article explains the easiest way to do this: