The short answer
A hillside drainage system is four things working together: gutters and downspouts that discharge well away from the building, surface swales, subsurface perforated pipe in gravel, and catch basins. All four have a service life, all four fail silently, and the first symptom is usually water inside.
Most Lafayette homeowners on sloped lots do not know what drainage they have. That is not a criticism — it is buried, it was installed before they bought the house, and it only matters two or three days a year.
But it is what stands between a hillside and your foundation during an atmospheric river, and every component of it has a finite life.
What is actually protecting your house
| Component | What it does | How it fails |
|---|---|---|
| Surface grading | Directs water away from the building at the surface | Landscaping renewal reverses the fall; settlement creates low spots |
| Swales | Shallow channels that intercept and route runoff around the building | Get filled in, planted over or flattened during landscaping |
| Area drains and catch basins | Collect surface water into a piped system | Grates block with leaf litter; basins silt up level with the grate |
| French drains | Perforated pipe in gravel that collects subsurface water | Fines migrate in and silt the pipe; filter fabric clogs; roots crush it |
| Retaining wall drainage | Weep holes and drain pipe relieving pressure behind the wall | Weep holes block; backfill drain silts; pressure builds |
| Downspout discharge | Carries roof water away from the foundation | Discharges too close; buried lines crack or disconnect |
| Sump pump | Removes water that collects in a low point | Fails, or loses power exactly when it is needed |
Note the pattern in that right-hand column: nothing fails loudly. Everything fails silently, over years, and presents on the one night that tests it.
Why Lafayette hillsides in particular
Three local factors compound.
Expansive clay soil. Much of the county sits on clay that swells when wet and shrinks when dry. It absorbs very little water once saturated, so surplus rainfall sheets across the surface rather than soaking in — and the seasonal movement works joints in buried pipe open.
Atmospheric river intensity. Several inches of rain in a few hours onto already-saturated ground is a fundamentally different load from steady winter drizzle. Drainage sized for historical patterns gets overwhelmed.
Oak leaf litter. Relentless, and it mats when wet. It is the single most common cause of a blocked grate or gutter in this town, and every winter intrusion we clean up has a blocked drain somewhere in its history.
The best diagnostic there is, and it is free
Walk your property during heavy rain. Not after it. During.
Put on a coat and boots, take a torch if it is dark, and spend ten minutes watching. You will learn more than any dry-weather inspection can tell you:
- Where water actually enters the property, and from where.
- Which area drains are taking water and which are ponding.
- Whether the retaining wall weep holes are running — the ones that work will be.
- Where water pools against the building rather than moving past it.
- Where downspouts discharge and whether it is far enough away.
- Whether any water is arriving from a neighboring property, a road or a trail.
Photograph it. Those photographs are useful for a drainage contractor, and if the water is coming from public land or a neighboring property, they matter for a different reason entirely.
The fixes, cheapest first
- Clear everything. Gutters, valleys, area drain grates, catch basins, crawl space vents. Free, and it solves a surprising proportion of problems.
- Extend downspout discharges six to ten feet from the foundation. A rigid extension costs very little and is one of the highest-return fixes available.
- Test each drain with a bucket of water. If it does not go somewhere, it is not working.
- Regrade the strip immediately around the building so it falls away. Often just a matter of moving soil and mulch.
- Clear or replace blocked weep holes in retaining walls.
- Jet or replace silted subsurface drains. A drainage contractor with a jetter can often recover a French drain rather than replacing it.
- Install an interception swale or new French drain uphill of the building where surface water arrives in volume.
- Add a sump with a battery backup where a low point genuinely collects water.
What happens when it does get in
On a raised-foundation Lafayette house, the water usually goes into the crawl space through a foundation vent, and nobody notices for weeks. The first symptom is a musty smell upstairs, because air moves upward out of a crawl space into the living space. See flooded basement and crawl space cleanup.
On a stepped or split-level property, it comes under a lower-level door or against a below-grade wall. Either way it is surface water, which means Category 3 handling and — importantly — no coverage under a standard homeowners policy. See flood cleanup and flood insurance vs homeowners insurance.
That last point is why drainage spending is usually the better investment. Insurance pays after; drainage prevents. See the Acalanes Ridge and Burton Valley pages for how this plays out neighborhood by neighborhood.
Frequently asked
How do I know if my hillside drainage is working?
Walk the property during heavy rain rather than after it. In ten minutes you will see which area drains are taking water and which are ponding, whether retaining wall weep holes are running, where water pools against the building, and where it is entering the property. On a dry day, pour a bucket into each drain grate and confirm the water goes somewhere, and check for efflorescence on retaining walls.
Why did my drainage work for twenty years and then stop?
Everything in a drainage system fails silently over time. Perforated French drain pipe silts up as fines migrate in, filter fabric clogs, roots crush pipe, catch basins fill level with the grate, retaining wall weep holes block, and landscaping renewal quietly reverses the surface fall. None of it shows a symptom until the one storm a year that genuinely loads the system.
What is the cheapest thing I can do to reduce hillside water risk?
Clear everything, then extend the downspouts. Clearing gutters, roof valleys, area drain grates, catch basins and crawl space vents is free and solves a surprising proportion of problems, particularly with oak leaf litter. Extending downspout discharges six to ten feet from the foundation costs very little and is among the highest-return fixes available on any sloped lot.
Is hillside water intrusion covered by insurance?
No. Surface water — runoff from a slope, water entering through a crawl space vent or under a lower-level door — is excluded from standard California homeowners policies and requires separate flood insurance. That is a substantial part of why drainage spending is usually the better investment for a hillside property: insurance pays after the event, drainage prevents it.