Clean Freaks Co. Stucco soft-wash in Los Altos Hills.

Stucco is a porous cement-and-lime plaster floated over lath, and its sand-finished face is far softer than it looks. Turn a pressure wand on it at working force and the water drives behind the finish, opens the hairline map cracks every troweled wall carries, and scours the float texture into bald, mottled patches that never quite match again. On elastomeric-coated and acrylic-finish elevations the risk shifts to delamination — pressure lifts the coating at its weakest seam. The correct method is the opposite of force: a low-pressure application of dwell chemistry that kills the growth at the root, then a gentle rinse.

The Los Altos Hills climate is what makes that growth relentless. Sitting in the mid-Peninsula fog belt, these hillsides hold marine-layer moisture on their north and west elevations long after the sun clears the valley floor, and that standing damp feeds the black and green biological film — algae, mildew, and gloeocapsa — that settles into stucco’s porous surface. Add the estates’ mature coast live oaks, which shed leaf litter and drip tannins down walls and under eaves, and the staining becomes a layered problem: living growth over mineral and organic discoloration, each needing its own dwell time.

Then there is the ground itself. On graded hillside lots, whatever runs off a wall runs downhill — toward foundation plantings, storm inlets, and the shallow, sensitive root zones of native oaks that resent both flooding and the sodium in ordinary soft-wash mixes. Cleaning stucco here is as much a runoff-control exercise as a surface one.

1. The partner reads the wall first — mapping finish type (traditional cement plaster, acrylic, or elastomeric), hairline cracking, and the north- and west-facing zones where fog holds the heaviest biological growth — so pressure never exceeds what that specific finish tolerates.
2. Runoff is planned before a drop is applied: downslope oak root zones and foundation beds are pre-wetted and bermed or tarped, storm inlets protected, and the wash sequence set to carry rinse toward a controlled discharge rather than down the grade.
3. A low-pressure soft-wash solution is applied and given real dwell time to kill algae and mildew at the root and lift oak tannin staining, with a neutralizing rinse to protect surrounding plantings from residual sodium.
4. Elevations are worked top-down and rinsed at gentle volume so the sand-float texture and any color coat stay intact, then the drying wall is checked for shadowed regrowth zones the fog will otherwise reseed within weeks.

• Late spring, once the winter wet season closes — after months of rain and fog have fed a full bloom of algae on the shaded elevations, and warm dry weeks let treated stucco cure clean.
• Early fall, before the first storms return — clearing summer’s oak litter and dust off the walls so the finish enters the wet months without organic film waiting to darken.
• After a heavy oak-shed stretch — when tannin drip and leaf debris have streaked walls and under-eave returns, before the marks set into porous stucco.
• When north and west elevations green up first — the fog-belt tell that the marine layer is reseeding growth on the damp side of the house.
• Ahead of listing, a garden event, or seasonal arrival — when the estate’s street-facing elevations need to read crisp on short notice.