Drainage

[fourbybud]

Not sure if this is the correct place to post but here it is.
I have just built a rather large building{36'X56'} that has one downspout per side, 2 total. I need to run drain tile 250' to the road. I am going to use pvc, not black corrugated. My question is what diameter should I use. 4",6", or one 4" for each side?

Thanks

[EJD Lawnpride]

Are you running one line or two? If one than I'd go with the 6"

[fourbybud]

I plan on running 1 line to the road, after I join them together. Thinking 4" off both downspouts then 6" from where they join to the road.

[txgrassguy]

The primary question is what kind of fall, or down slope, to the road exists.
Given the dimensions of the road, assuming one inch per hour of sustained rain fall you are attempting to drain approximately 1,200 gallons of water.
The main problem with 4" line is capacity - corrugated pipe is slightly less than sewer line, both of which are substantially cheaper than schedule 40.
Assume nominal capacity of a 4" of approximately 280 gallons per minute at a 1% slope, ie 1" down per 100 linear feet of pipe run. What will happen is that for approximately 40 minutes the pipe will drain then the gutters will back up and over flow.
So what you have to answer is a local climate issue, what is historical averages per minute of sustained rainfall in your area?
At or below 1"/hour and you will be alright given a minimum of a 1" fall to the main drain outlet.