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How do I calculate a sufficient pipe diameter, for handling a given amount of impermeable pavement?

A general answer is best.


In my particular case I have approximately 675 square feet of tile roof and driveway which drain to the front of a garage. The garage door is the low spot. An additional 650sf of neighbor driveway more or less sheets into this same area. Area is San Francisco Bay Area, California USA.

From the low point I could run a pipe about 70 feet in length with a drop of 12 inches, to drain the water by gravity (1.4% slope on average with one cleanout). Or I could go 10 feet over and down at 4% slope to reach a sump in the home's basement. Or accept a drainage contractor's bid (total $30,000) and have this water pumped uphill to the curb about 2 feet.

Yellow: main catchment area.  Red: possible pipe route to intersect with neighbor system

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From the 2012 International Plumbing Code Section 1106, I get a pipe diameter of 3" based on table 1106.3 and this map:

SECTION 1106 SIZE OF CONDUCTORS, LEADERS AND STORM DRAINS

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Gravity is always better than pumps if it's an option. Pumps can fail for multiple reasons (lack of power, pump problems other than power, etc..) so not pumping uphill is my preference, given any reasonable option to do otherwise. Use 4" pipe - it costs little more than 3" once you're digging the trench anyway, and gives some reserve capacity... (for a more general consideration/recommendation on pipe size, do the math, and then bump the pipe up one size, at least, unless doing so is prohibitively expensive.)

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