Can I join multiple underground PVC drainage pipes to a common outlet?

Question

As described in a previous post I've discovered that I have corrugated drainage pipes, many of which are chock full of silt. Most of them are nearly level as well, which isn't super great. I'm considering ways to remedy the situation.

There are two sides of my house which could benefit from commonly connected pipe -- three downspouts total plus the basement sump drain could be connected to a single outlet and fed into my backyard which runs into a common stream. This would require a single trench with the least amount of damage to shrubbery or landscaping.

It's not the only plan I'm entertaining, but am curious if there's a reason I should avoid joining pipe like this.

Answer 1

There are a few things to consider when speculating on whether this will work:

• Is the outlet elevation adequately below the elevation of all inlets? It needs to be enough that high flow from one inlet doesn't result in backflow and spillage at another.
• Does one inlet have significantly more flow than the others, where it may tend to block flow or cause backflow? Sometimes moving water surprises a person with its force. Consider the size of the roof planes being handles by each.
• Is the single outlet of adequate diameter/volume to handle all three inlets? The pop-up valve is already a reduction in flow volume for one inlet, and in drain plumbing the pipe usually grows in size when branches join.
• Is it appropriate to put that much water in one exit location? Where will it flow and what will it saturate?
• What happens if the exit clogs and all three inlets overflow? Do you have basement water concerns?

The question isn't about can, but should, and we can't decide that for you.

Answer 2

Like joining any other drains, the main thing is to get bigger where you join them, and forever after until the outlet.

So if you have 3 4 inch drains, you'll want a 6" or 8" pipe to take the drainage of all 3 combined away, NOT to jam all 3 into one 4 inch pipe. Adding a sump pump (presumably 1.5-2" but under pressure, not just gravity) to the 3 drains, yeah, pick 8" pipe.