I have three separate metal downspouts coming down through my tar/gravel roof and emptying into the patio. My question: would it be acceptable to cut each downspout off and join all three with some 3" ABS or PVC pipe and send all the water to the side of the house (a hill that descends into the street). Note that this is not the standard sloping roof with a rain gutter.

yes it would BUT whether 3" pipe is correct depends on the surface area of the roof.
post the square feet of the surface collecting rain water then using sizing charts then proper sizing can be determined. I hope CA gets some rain and them downspouts get some action! You may want to consider adding gravel or some other means to control erosion on the hillside where you want to dump the water.



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You can do anything you want to, as long as you find a pipe that will attach to your downspouts.

Thanks for the reply. Yes, I'll get a sense of the surface area and go from there. Perhaps 4" is better. And yes, I'm hoping this coming "rainy season" really means it. We need it desperately.

Yes, it'll take some creative elbows and adapters but it all seems doable. Just need the right width of pipe to handle the runoff of three. Thanks.

I also have a dutch gutter system in my house and over the years it has been really difficult to find anyone with experience in these systems.

Calculations vary by locale and formulas changes by jurisdictions, but the rule of thumb rough formula I use to calculate runoffs is the Rational Formula.

Q = CiA

This is really for large basin area runoff calculations but since we will always oversize it would work as a ball park estimate.

Q is flow rate calculated in cfs cubic feet per second.
C is the runoff coefficient based on the material, for example, gravels allows a lot of water to infiltrate to the ground and concrete pavement allows almost nil. Your roof, is going to all run off so use C value of 1.
i is rainfall intensity in inches per hour. In Florida for highway safety and visibility we use 4 inches per hour as absolute maximum rainfall intensity as it's safe for someone to drive, and that's very heavy rain.
A is the area in ACRE. This is the part of the roof that is contributing to that downspout. Calculate all areas draining to it and ignore other roof areas sloping away from it.

Once you figure out your flow rate in CFS, convert to GPM and you can easily figure out the pipe size needed. I would step up another size to compensate for possibly impeded flow from dropped leaves, dead birds, rotten fruits etc...

Wow, thanks so much. Lots of great info.



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