The first real storm of the season is the worst time to find out your drainage doesn’t work. Gutters overflow, a downspout dumps water right against the foundation, and the low corner of the yard turns into a pond.
The good news: most of what goes wrong in a Bay Area winter can be spotted in an afternoon with a ladder, a garden hose, and a flashlight. This pre-winter drainage checklist walks you through what to check, what you can handle yourself, and what’s worth getting fixed before the ground gets soaked.
Why Fall Is the Window for Drainage Work
In the Bay Area, almost all of our rain falls in a few months. Unincorporated San Mateo County treats October 1 through April 30 as the wet season for grading purposes. Once storms start lining up, soil stays saturated, trenches fill with water, and anything you dig turns to mud.
Our clay-heavy soils make it worse. Clay drains slowly and swells when it gets wet, so water that sits next to a foundation has nowhere to go. Our post on Bay Area soil types and drainage problems explains why. Fixing drainage while the ground is still dry is faster, cleaner, and easier on your yard.
The Pre-Winter Drainage Checklist
Work from the roof down. Water lands on the roof first, so that’s where most problems start.
1. Clean the Gutters (After the Leaves Drop)
A clogged gutter overflows right at the drip line, which is usually right next to the foundation. Clear out leaves, check that gutters slope toward the downspouts, and look for sagging sections. If you have big trees overhead, plan on doing this more than once as they shed through the fall.
2. Trace Every Downspout to Where It Ends
This one is easy to miss. Walk the house and follow each downspout. Does it dump onto a splash block right at the wall? Into a planter bed? Onto a slope?
For reference, the California Residential Code says that where expansive or collapsible soils are known to exist, roof drainage must discharge at least 5 feet from the foundation walls or into an approved drainage system. On Peninsula clay, treat 5 feet as a minimum.
If your downspouts connect to buried pipe, run a garden hose into each one for a few minutes. Water should come out the other end, at a pop-up emitter, curb outlet, or daylight outlet. If it backs up and spills at the top, the line is clogged or crushed.
3. Check the Grade Against the Foundation
The ground right around your house should slope away from it. The California Residential Code calls for the grade to fall at least 6 inches within the first 10 feet from the foundation. Over time, backfill settles and planter beds and mulch build up against the siding.
Lay a straight board or a long level on the ground perpendicular to the wall. If it tips toward the house, or if water stood against the wall last winter, that area needs regrading. Our guide to property grading covers what that involves.
4. Clear Area Drains and Catch Basins
Lift the grates on patio drains, driveway drains, and yard catch basins. Scoop out the leaves, silt, and gravel that collected over the summer. Then run a hose into each one and make sure the water actually leaves. A basin that fills up and sits there has a blocked outlet pipe.
5. Find Your French Drain Outlet and Cleanouts
A French drain is only as good as its outlet. Find where yours discharges and clear any soil, roots, or debris around the end of the pipe. If there are cleanout caps, open them and flush the line with a hose. A drain that stays bone dry through a hard rain may be clogged with silt. If you’re not sure how yours is supposed to work, start with how French drains work and when you need one.
6. Walk the Swales, Ditches, and Slopes
On hillside lots, surface water is usually moved by swales, V-ditches, and culverts. Clear them of leaves and soil, and check for spots where last year’s runoff cut a channel. Bare soil on a slope is the first thing to wash out, so cover it before the rain with mulch, straw wattles, or erosion control fabric.
7. Look at Retaining Walls and Weep Holes
Retaining walls need a way to let water out from behind them. Make sure weep holes and drain outlets at the base aren’t packed with soil or covered by plantings. Bowing, leaning, or new cracks are a sign that water is building up behind the wall, and that’s worth having looked at before winter, not after.
8. Check the Crawlspace and Sump Pump
Grab a flashlight and look under the house for water lines on the foundation, damp soil, or standing water from last season. If you have a sump pump, pour a bucket of water into the pit and make sure it kicks on and discharges away from the house.
9. Keep the Street Inlet Clear
If your drainage discharges to the curb, it’s only as good as the storm drain it ends up in. Leaves raked or blown into the gutter pile up at the inlet and can back water up onto your driveway. Keep your stretch of curb clear through the fall.
10. Have a Storm Plan
- Sandbags: Many Bay Area cities stock sandbags during the wet season. The City of San Mateo, for example, runs self-serve stations for residents and property owners, with a limit of 10 bags per vehicle. Check your own city’s site before a storm is in the forecast.
- Rain barrels: San Mateo County residents can get a rebate on a rain barrel through BAWSCA and Flows To Bay. For July 2026 through June 2027, it ranges from up to $100 to up to $200 depending on barrel size, with an extra local rebate in some cities.
- Photos: During the first big rain, put on a jacket and take photos of where water goes. That’s the best information you can give a contractor.
What You Can Do Yourself vs. When to Call a Contractor
Cleaning gutters, clearing grates, flushing lines, and pulling mulch back from the siding are all good homeowner jobs. It’s time to bring in a contractor when:
- A buried downspout or drain line won’t flush, or you find crushed or disconnected pipe.
- The ground slopes toward the house and needs real regrading, not just a few bags of soil.
- Water got into the crawlspace, garage, or basement last winter.
- A slope is eroding or a retaining wall is moving.
- You need a new French drain, catch basin, or underground downspout system.
Two things to keep in mind. Any digging, even for a drain line, starts with a call to 811. California law requires homeowners and contractors to notify Underground Service Alert two working days before digging so buried utilities get marked. And larger earthwork that needs a grading permit can be restricted in winter. In unincorporated San Mateo County, grading during the October 1 to April 30 wet season requires an approved exception, and work is limited to dry days. Check with your city or the County if your project is bigger than a simple drain line.
Not sure which list you’re on? Our post on 5 signs your property needs drainage work can help you tell a nuisance from a real problem.
Frequently Asked Questions
When should I do pre-winter drainage work in the Bay Area?
As early in the fall as you can, ideally before storms start lining up. Dry soil is easier to dig and compact.
How far from my house should downspouts discharge?
Where expansive or collapsible soils are known to exist, the California Residential Code requires roof drainage to discharge at least 5 feet from the foundation or into an approved drainage system. On Bay Area clay, treat 5 feet as the minimum.
Can drainage work still be done once the rain starts?
Often, yes. Many drainage repairs can be scheduled between storms. Wet soil does slow things down, and larger grading projects may face wet-season limits, so the sooner you plan, the more options you have.
How much does drainage work cost?
It depends on the site: the soil, how far water has to travel, access, and what’s in the way. A walk-through of your property is the only way to get a real number.
Get Your Drainage Ready Before the Big Storms
Harris Excavation is a licensed General Engineering Contractor (CSLB #1117960) based in South San Francisco. We install and repair residential drainage across San Mateo and Santa Clara Counties, from underground downspout lines and catch basins to French drains and regrading.
If your checklist turned up something you’d rather not tackle yourself, send us a quote request. Tell us what you found, add photos if you have them, and we’ll set up a time to walk the property with you.
