Hillside and Expansive Clay Lot Drainage for East Bay Homes

Hillside and Clay Lot Guide

What the 2023 storms showed East Bay hillside owners

On the night of January 18, 2023, after a string of atmospheric river storms, a house on Cedar Lane in Orinda started sliding down the steep hill behind it. The city red-tagged the home, and three days later the house next door was red-tagged as well. Both families had to move out. City staff told KPIX they could not yet comment on a cause, and we will not guess at one.

What those storms made plain is how much water a hillside lot takes on in a wet winter, and how little of it should come from your own roof. Gutters gather the whole roof’s rain into a few downspouts. Empty those at the top of a slope, beside a retaining wall or into the fill under a deck, and a gentle rain becomes a few concentrated streams aimed at the ground you most need dry.

Down on the valley floor the worry is different. A geotechnical exploration prepared for the City of San Ramon’s City Center site reported moderate to “very high expansion potential” in the native and fill soils there. Expansive clay swells when wet and shrinks as it dries, which is what heaves and cracks slabs. The report called for downspout water to run through solid pipe to the street or another approved outlet. That was a civic project, not your yard, but the lesson carries: water dropped at the foundation stays there.

We do gutters and drainage, nothing else, and will tell you when a question belongs with a geotechnical engineer.

Two kinds of lots

Hillside lot or clay lot: where roof water does the damage

Hillside lots. In Orinda, Lafayette, the Berkeley Hills, Diablo and the upper streets of Alamo and Danville, plenty of homes sit on a cut pad with a slope falling away below and a deck or retaining wall out back. Roof water let go on the downhill side soaks into that slope and into the fill beneath it. You see the same layout around Springbrook Road in Acalanes Ridge, near Briones Regional Park in Reliez Valley and off Tice Valley Boulevard in Castle Hill.

Expansive clay lots. In San Ramon, Dougherty Valley, Dublin, Pleasanton and Livermore, the lots are flatter but the ground underneath is often clay. It drains slowly, holds water against the slab for weeks after a storm, then shrinks back in the dry months. A downspout emptying beside the garage keeps that cycle running right where the house feels it.

Plenty of homes in Blackhawk, Shell Ridge and Walnut Creek have both. Either way, the fix at the roof is the same: catch it all, carry it in a closed pipe, release it where the ground can take it.

Gutters, downspouts, drain lines

Three steps that keep roof water off the slope and away from the slab

1. Size the gutter to the roof. Steep roofs push water into the trough fast. A gutter that spills over its front edge in a hard rain with clear outlets is too small, and the overflow lands on the foundation line. We form 5 inch, 6 inch and 7 inch seamless gutters on our truck. Our 6 inch profile carries roughly 40 percent more water than 5 inch and pairs with larger 3 by 4 downspouts. Sometimes one more downspout is the better answer.

2. Route every downspout on purpose. Walk each downspout to where it empties. On a hillside lot, none should release on the downhill side of the house, onto a slope face, behind a retaining wall or under a deck. On a clay lot, none should empty at the slab. Moving a downspout to the uphill or street side often shortens the buried line. Extensions reaching at least 5 feet out are a fair stopgap.

3. Carry it underground in solid pipe. Our underground gutter drainage ties downspouts into buried lines that release at daylight, into a drywell or at a storm connection where your city allows it. On a slope, we choose the outlet with you, never a pipe that just ends at the top of the hill. If your property already has a drainage plan or a geotechnical report, we work to it. BroGuard mesh keeps leaves out of a pipe you cannot see.