ᐅ Carport Drainage: Calculating Water Volume

Hello, can anyone tell me how to calculate the drainage for a carport?
We want to build a carport and need to inform the authorities about the drainage.
The carport will not be connected to the sewer system!
We plan to let the water seep into a hole behind the carport, but we don’t know what factors need to be considered for this.

Does anyone happen to have any ideas?

Thanks in advance

Just specify an infiltration trench. That might be sufficient for the building authority since you are not draining anything from the carport. Otherwise, you will need to calculate and demonstrate the appropriate size of the infiltration trench.

The question is, how is this calculated.

Flat roof? Search online for rainwater runoff calculation. This will give you the amount of water the soakaway needs to handle. With Graf, for example, you can request a calculation related to their products. In my opinion, that should be sufficient. Or does the building authority require specific conditions?

So Google gives me a thousand results, but nothing really clear. There must be a simple calculation for this, right?

Yes, it is a flat roof. 6 x 9m (20 x 30 ft).
What is "Graf"?

It doesn’t get any easier than this: "Calculate rainwater harvesting online – Roofer Müller"

Graf manufactures cisterns and soakaways. If you know the total amount of water you need to infiltrate, Graf will size the soakaway for you. Obviously, this is based on their products, but you can use it as a reference and should have a calculation that meets building authority requirements.