I have a downspout that is fed by about a 300 square foot roof.
Presently the downspout is directed away from the house by a 10 foot pipe.
I would love to get rid of that pipe and drain the water into a French drain.
I’ve watched tons of video on how to build a French drain but it never tells me how big the drain should be and other details vary.
Here is a rough plan for what I think I should do:
Dig a trench about 2 foot deep and 2 feet wide and 10 feet long.
Fill it about 12 inches with coarse gravel.
Lay a perforated drainage pipe and connect to my downspout.
A lot depends on how absorbent the ground is. If you are on clay, the water will sit in the drain for ages. If you are on sand, it won’t need much.
You might do better with a soakaway and here is a link to a detailed explanation, including size calculations. https://www.pavingexpert.com/drain08
I have no connection whatsoever to this site (other than as a satisfied user), but I highly recommend it as a resource for any hard landscaping project.
Too late for the edit, but you would do well to scan through the information on land drainage generally - he doesn’t think much of calling a gravel-filled trench a french drain. https://www.pavingexpert.com/drain03
Check with your local government’s engineering department. There may be official construction standards regarding drainage and soil type.
The legal standards may not be relevant to your situation -ie. a single pipe within your own property. (They are usually written to avoid drainage problems with neighboring properties. )
But within those standards there may well be some diagrams showing the engineering details for the type of soil in your area, the size of pipe, depth of trench, etc…
And if your pipe is within a certain distance of your neighbor’s property line, then there may actually be a legal requirement to get an official building permit .
If you are on sand, what you propose is likely fine. Here, we have a lot of clay. Instead of perforated pipe, we bury solid PVC leading to a catch basin, sunk in a gravel pit, with a surface drain.