Q: I live in an 8-year-old home in the Diamond Heights area of San Francisco. When it was built, a lot of excavation was done. The foundation had a lot of rebar, but below it was a lot of gravel. I was told that the gravel was there to improve drainage.
During heavy rains, water collects under the house and seeps up through the floor in the front of the house. I can see moisture under the granite entry (discoloration of the stone), and some of the moisture has crept up some 2-by-4s and onto the drywall.
Is there any way to create a drainage system? This happens only with really heavy rains. Or do I have to find a way to stop water from getting under the house?
A: El Nino strikes again. Every few years, the ocean current warms up, affecting our weather. Normal winters bring moderate amounts of rain to the San Francisco Bay Area, but El Nino winters can mean torrential downpours.
We’re sure most readers remember when homes have been undermined and slid down the hills of Marin County and the East Bay. And it was national news in 1995 when a sinkhole swallowed a mansion in San Francisco’s Seacliff enclave.
Thankfully, the rain is behind us for this season and El Nino hasn’t visited us in quite some time. So now is a good time to start working to correct your drainage problem.
Site selection and control of rainwater runoff are critical when building in the Bay Area hills. We fear that the heavy rains have brought attention to a potentially huge and very costly problem that can only get worse.
You mention that there is a lot of gravel under your retaining wall for drainage. Is there a drainpipe that diverts water from the base of the wall to the street? If not, this may be the source of your problem.
Check for pipes entering the gutter. If there is no pipe and the water is pooling at the base of the wall, it’s likely that as the ground becomes saturated, water could leach under your house, through the foundation to the interior.
A possible solution is a French drain, which is a trench dug around the house. A drainage pipe is placed at the bottom, and the trench is back-filled with gravel. Water flowing toward the house seeps down through the gravel into the drainage pipe and out to the street.
Although lack of a proper drainage system is the likely source of your problem, there are other potential causes. It’s even possible that water is leaching into the ground farther up the hill and taking a subterranean route under the gravel and drain, only to surface again on your flat lot.
You need an expert. We recommend you consult a soils engineer who is familiar with your area to help you find out what is going on and to figure out how to fix it. Good luck.
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