With a typical roof shedding hundreds of gallons of water in a major rainstorm, an important and potentially expensive issue arises in many San Francisco remodels: where does it all go?

Rainwater diversion can be a cost-effective and sustainable solution. In this post, we will walk you through the definition of rainwater diversion, its benefits, and the permitting issues and methods necessary for execution.


Rainwater diversion is any method for managing storm water that keeps it from entering the sewer system. Water can be channeled into the surface landscape or a catchment system for re-use in other forms. (We will talk about catchment in Part 2 of this series).


Unlike nearby municipalities where storm water runs into the bay, San Francisco homeowners are required to connect their storm drains to the sewer main. But just as water runs down to the sewer main, gases also rise from it. The transfer looks like this:


Rainfall > Roof gutters > Downspouts > Sewer main


Sewer main > Downspouts > Roof gutters > Atmosphere

As long as the gutters are above the line of all windows, there isn’t a problem. However, room additions sometimes place operable windows in the line of fire. This is exactly what happened in our recent project at Sea View Terrace in San Francisco.


Because we built out the attic and added dormers above the line of the existing roof gutters, we had a problem on our hands. The sewer main’s gasses would rise through the downspouts and straight into the dormer windows. Rainwater diversion was the most cost-effective option in this case, saving the homeowner thousands of dollars in alternative construction methods.


There’s an easy formula for calculating the volume of rainwater your roof will shed.

With 1 inch of rainfall, 100 square feet of roof will collect 100 gallons of water.

As our case study has a 1,800 square foot roof, a 1-inch rainstorm will generate 1,800 gallons of water. So where does all this water go?

In an urban environment, one can easily see why the plumbing department is concerned about this much water being shed onto the ground. There are potential risks of flooding one’s property, a neighbor’s property, or causing sinkholes and major erosion. In order to meet the code, an engineered solution must be devised and approved to effectively divert this volume of rainwater.


In the case of our recent project at Sea View Terrace in San Francisco, we had 4 options for fixing the problem of rising sewer gases below an operable window.

1.  Put a P-trap and trap primer on every downspout (functioning like the pipe under your kitchen sink). This leaves a Rube Goldbergian contraption on multiple locations of the building’s exterior, which is very unsightly as seen in the image to the right.

2.  Run internal roof drains to eliminate unattractive external pipes. This option is only cost-effective in large remodels that change the roof pattern.

3.  Connect all down spouts to a dedicated sewer line and sewer lateral with a single vent pipe. In our Sea View Terrace residence, the city sewer lateral was 15 feet below the sidewalk, which meant for costly permitting and excavation.

4.  Divert all rainwater into dispersion swales or drywells, or collect them into cisterns for re-use.

In our Sea View Terrace case, a drywell as described in Option #4 was the most appropriate for the design of the home, budget, and coincidentally the most environmentally acceptable solution. The engineering, permitting, and installation costs were approximately 50% cheaper than a downspout sewer connection for this home, saving the homeowner significant dollars.


The city plumbing department does not want to be responsible for defining such systems, but it will approve an installed system designed by a geotechnical engineer (or soils engineer). Engineering Design & Construction was brought onto the project to calculate the rate of water absorption in the neighborhood, and based on this calculation, design a drywell to accommodate the storm water shedding form the Sea View Terrace roof.


A drywell is a cylindrical pit drilled into the ground to a diameter and depth defined by a soils engineer. A plastic tube is inserted, filled with gravel, and capped for safety. Storm water from the roof runs through underground pipes into the drywell and slowly drains into the ground.

With our rainwater diversion into a drywell, 1,800 gallons of storm water efficiently move from the rooftop all the way back into the landscape without risks of flooding or erosion. The homeowner saved significantly as compared to alternative construction methods and passed the San Francisco building codes with flying colors.

Stay tuned for Part II where we will discuss another method for handling storm water runoff: rainwater catchment.

Design Team: Andrew Mann Architecture, Scott Lewis Landscape Architecture, Engineering Design & Construction.

Jeff King is president and founder of Jeff King & Company, a San Francisco general building contractor specializing in residential remodels. Since starting his business in 1994, Jeff’s work has has been featured in the San Francisco Chronicle, EcoHome, Kitchen & Bath Ideas and more. Jeff King & Company was named one of Remodeling Magazine’s Big 50 in 2008 and has won awards from the National Association of the Remodeling Industry (NARI), Build It Green, and the Chrysalis Regional Award. Jeff King holds a Master of Fine Arts from UC Berkeley and is a B.I.G. Certified Green Building Professional.