I'm about to pour a new patio and I'm planning to run my downspouts underneath it, directing water into a popup emitter. The setup involves routing water from the gutters through two downspout points until it reaches the edge of the cement, where it will transition into a 3" corrugated pipe connected to some buried catch basins in my yard. I'm wondering whether to go with a 3" or 4" PVC schedule 40 pipe for this project. My house is roughly 1800 sq ft with a single ridge roof, so I don't expect an enormous water flow. I've heard from some sources that 3" should be sufficient for residential drainage, especially since tests have shown 3" corrugated pipes can be quite effective. However, it seems like in my area (NE Ohio), the standard practice leans towards 4" pipes. What do you all think?
3 Answers
I'd recommend going with a 4" pipe personally, especially if your downspouts are the standard 2x3 size. A larger pipe could ensure better flow and help prevent clogs, which might save you trouble down the line.
As an engineer who designs plumbing systems, I can tell you that a 3" pipe can handle up to 55 gallons per minute at a slope of 1/8" per foot, while a 4" pipe can handle 115 gallons per minute. If you have two downspouts managing drainage from about half your roof, a 3" pipe can effectively handle significant rainfall, even rates of almost 12 inches per hour! This means a 3" pipe is usually more than enough unless you have serious clogging issues with leaves or debris. In that case, a larger 4" pipe might be a better choice to prevent blockages.
I felt the same way about using 3"; I just eyeballed it and figured it'd handle water well. 12" of rain sounds unreal! Glad to know my hunch was good.
Honestly, nobody ever regrets going bigger with pipes for rain drainage. You won't hear anyone say they wish they'd made it smaller! It's always safer to err on the side of larger diameter pipes if you're worried about flow.
My downspouts are the same size, and I've always felt that bigger is better with pipes like these.