Clean Freaks Co. Stone patio cleaning in Los Altos Hills.

Los Altos Hills patios tend to be natural stone — sandstone, slate, or dressed flagstone — set on terraces cut into an oak-covered slope. That combination collects a very specific soil load. Summer moves through the property as marine fog rather than dry heat, and the moisture it leaves on north-facing and tree-shaded stone feeds a slick black-green algae film and, over seasons, moss and crusty lichen that root into the stone’s pores. This is biological growth, not surface dirt, and it does not respond to the same approach as sun-baked pavers on an open valley lot.

The oaks add the second problem. Leaf litter, catkins in spring, and acorn debris in fall sit on porous stone through wet weather and leach tannin, staining flagstone with brown halos and darkening the jointing sand between pieces. Natural stone is more absorbent than concrete, so those stains set deeper and a heavy hand with the wand only drives them further in — or blasts the joint sand out entirely and leaves the field loose.

Then there is the grade itself. These patios sit above graded driveways and downslope planting, so a wash is also a drainage event: whatever is lifted, plus any cleaning agent, wants to run straight downhill toward beds, hardscape, and the storm path. Cleaning stone here means matching pressure and chemistry to an absorbent, biologically loaded surface while controlling exactly where the water goes.

1. The partner identifies the stone first — sandstone, slate, or a mixed flagstone field — and reads its jointing (polymeric sand, mortar, or open sand), then sets a pressure and tip that clean the face without etching soft stone or blowing out the joints.
2. Biological growth is treated, not just rinsed. A dwell-time algaecide and moss-and-lichen application breaks the growth down at the root so the stone stays clean for seasons, rather than a high-pressure pass that only removes the surface film and leaves the anchoring spores behind.
3. Tannin and oak-debris staining are pulled with a stone-safe cleaner suited to an absorbent surface, worked into the halos and the joint sand rather than sandblasted — preserving the field and the color of the stone.
4. Because the terrace sits above the grade, runoff is planned before work starts: flow is contained and directed away from downslope beds, the driveway, and the storm path, and joint sand is topped and re-set where the wash has opened it.

• Late spring, once the winter rains close — the wet season and shade have left algae and moss at their heaviest, and dry weather ahead lets treated stone stay clean.
• Before summer entertaining — clear the fog-fed film and tannin halos while the patio is in daily use and photographs matter.
• Through the summer fog months — north-facing and oak-shaded sections that never fully dry are worth a mid-season pass before growth re-establishes.
• Early fall, ahead of the oak drop and first rains — cleaning before acorns and leaf litter settle keeps tannin from staining the stone and joints over winter.
• Any time joint sand has washed out or moss has crept between pieces — on a graded terrace, open joints let each rain carry more of the field downhill.