I am running my downspouts to buried pipe to be piped away from my house. The side I am working on has two downspouts. I am also adding about 5 small atrium drains. The underground pipe that will lead to daylight will be about 70 feet long. (This is not a straight run, there are curves as I am going parallel to the front of my house, turning, then going parallel to the side of my house.)

In that 70 feet, there will be approximately 17 inches of drop.

What size/how many underground pipes would you recommend?

That's not a lot of drop for the distance - if it was sanitary sewer
piping you'd need a 6" pipe - but it's not.

Worst case is your drain line will overflow with rainwater at the input
end and no big deal.

You can probably get by nicely with either 3" smooth drain pipe (like
ABS drain line) or 4" corrugated drain line.

Either way, put screens at the input ends to keep big crap from getting
into the pipe itself - easier to clean the removable screen than rod out
the pipe.

Also, be very careful in setting the grade and not getting a dip or
'belly' in the pipe that can be a breeding space for mosquito larvae.

Carla

On 10/21/2015 7:16 AM, stryped wrote:
I am running my downspouts to buried pipe to be piped away from my house. The side I am working on has two downspouts. I am also adding about 5 small atrium drains. The underground pipe that will lead to daylight will be about 70 feet long. (This is not a straight run, there are curves as I am going parallel to the front of my house, turning, then going parallel to the side of my house.)

In that 70 feet, there will be approximately 17 inches of drop.

What size/how many underground pipes would you recommend?