really techie problem with water runoff from roof

[Tomsand]

Help! I have a techie problem: I want to install a water butt to catch the water runoff from a large roof about 72sq meters. Currently there is only a single standard sized down pipe from the gutter which splashes on front patio area (not nice).

If I divert the down pipe into a 210 liter water butt and have a 20mm over flow hose connected to the butt how far down do I did to install the over flow hose so that butt does not become overwhelmed?

There is no easy way to divert down pipe into drain so I have chosen to use the water butt.

The details:
Area of roof 72 square meters
Down pipe diameter 70mm
Water butt 210 liters
Drain hose diameter 20mm
Length of drain hose run to road (mostly flat) 10 meters

I don't want the water butt to overflow, but I know draining out of a 10m, mostly flat, 20mm diameter hose will probably be slow.

 

[Bosswhite]

Dont think you are allowed to just run the overflow water onto a main road, what would happen if it freezes,
You need a soakaway to take the water from the down pipe.

 

[dcawkwell]

Don't they sell down pipe diverters I've seen them so they fill
a water butt. What's the problem?

The large roof is going the to fill that water butt after the first rain or two.
So you can assume the water butt is always going to be full with british weather.

20mm hose will most likely handle most rains. Anything heavy and it will be over the top of the water butt in no time.

 

[Tomsand]

I have seen the diverters, but as I say the down pipe doesn't go into a drain and empties onto patio area and ends up looking a bit like a mini pool. I only have a 20mm hose to take the water away from house. Last time I tried just running the hose onto the down pipe the water backed up and overflowed the gutters!

So I hit upon the idea up using a water butt to be like a holding tank to deal with the large volume of water that happens in a rain storm. Then the 20mm hose could deal with water in it's own sweet time. However I don't know how much rain to expect in a worst case scenario. I don't want to site the overflow too high on the water butt so that it doesn't drain away fast enough and allows the butt to over flow

So I'd like to know:

What's the max amount of water I could expect in say 1 days worth of rain from my roof area?
What's the rate of water flow I could expect from a 10m length of 20mm pipe per hour?

I could site the overflow hose three quarters of the way up the water butt, or playing safe halfway up the water butt.