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Pasadena, Altadena, San Marino, Arcadia, La Canada Flintridge, Glendora, Claremont and the foothill communities strung along the base of the San Gabriel Mountains. This is the oldest and most architecturally significant residential stock in the region, sitting on some of the hardest municipal water in it, directly beneath the range that generates the county's worst wind events.
The San Gabriel Valley runs on basin groundwater, and that is the first thing that separates it from the Westside. Water pumped from the Raymond and Main San Gabriel basins has spent a long time moving through limestone-rich alluvium washed down out of the mountains, and it carries the calcium to prove it: twelve to eighteen grains per gallon is normal across this valley. Combine that with irrigation running through a seven-month rainless season and you get the defining foothill glass problem, which is not dirt at all but bonded carbonate that washing does not touch and that etches into the silica if it is left long enough.
The second variable is the mountains themselves. The San Gabriels rise abruptly out of the valley floor, which produces two effects on glass. Santa Ana events funnel down the canyons at their most violent anywhere in the county, arriving from inland and loading the north-facing elevation with decomposed granite. And the entire foothill strip from La Canada through Altadena, Sierra Madre, Monrovia and Glendora sits in the wildland urban interface, which means in some years the deposit is smoke residue rather than dust: alkaline ash, fine carbon and condensed tars that behave nothing like road film.
Third is the building stock, which is the best argument for hiring carefully in this region. Pasadena, Altadena and San Marino hold an extraordinary concentration of Craftsman, Greene and Greene influenced, and period revival houses with original wood sash, art glass, and steel casements with divided lights. Those assemblies are held together by putty that failed decades ago and paint that has been holding it since. A pole system across a divided light pops putty and loosens glass. This is a hand-cleaning region, and where the glazing has already failed we photograph it and tell you, because a casement caught early can be reglazed and one left another decade usually cannot.
These are the variables that make this part of Greater Los Angeles different from the region next door. Every one of them is false somewhere else in this metro, which is the point.
Southern California does not have four seasons in the sense window cleaning advice assumes. It has a rain window, a long dry stretch with no rinse at all, and a wind season that runs offshore. Here is the actual year in the San Gabriel Valley, in the order it happens.
What it does to glass
Almost all of the year's rain lands here and the foothills take more of it than the basin does. The first storm mobilises a dry season of dust and mineral into streaks, and on the foothill slopes it drives runoff and sediment against ground-floor glass.
What we do about it
Clean and clear drainage in late October or November, before the first storm. On historic buildings we check and photograph glazing condition on the way through, because a failed putty joint found in November is cheaper than the same joint found in February.
What it does to glass
Seven months with no rinse, and here the irrigation runs the entire time on very hard water. Every head that clips a pane leaves calcium behind and the sun evaporates it within minutes. This is the season in which permanent damage is created.
What we do about it
We walk the beds before leaving and point out exactly which heads are hitting glass. Moving one head is a ten-minute fix that saves years of restoration cost, and we would rather tell you that than put you on an annual restoration schedule.
What it does to glass
The worst wind in the region, funneled down the canyons at fifty to seventy miles an hour and carrying decomposed granite. It packs tracks and weep holes, abrades screen mesh, and loads the mountain-facing elevation rather than the one you would expect.
What we do about it
Tracks cleared dry before any water, without exception. Exterior work booked into an active event gets rescheduled at no charge, because cleaning glass in a wind event is paying for a result that lasts an hour.
What it does to glass
In the interface communities the deposit in some years is smoke residue: fine carbon, alkaline ash and condensed organic tars. The ash can etch if left wet on glass, the tar smears rather than lifting, and the carbon is abrasive.
What we do about it
Flooded neutral-pH pass to carry the alkaline fraction off, then a separate solvent step, then the wash. Never a dry wipe. It is a different job and it is quoted as one.
What it does to glass
Jacaranda honeydew across the older Pasadena and Arcadia streets, plus a genuine deciduous leaf and seed load from a mature canopy that most of Southern California does not have.
What we do about it
Early July for jacaranda blocks, once the drop finishes. Screens come off and are washed flat rather than brushed in place, because canopy debris felts into mesh when it is brushed.
This is the region where we most often have to tell a homeowner that restoration will improve the glass but not perfect it. Basin groundwater at fifteen grains, sprinklers running year round, and seven months of sun evaporating it within minutes of contact is a recipe for carbonate that has etched into the silica rather than sitting on it. As a rough rule, deposits under about five years old restore fully. Past a decade in this sun, they usually do not. We test before quoting so nobody buys a result that is not available.
Art glass, original wavy lights, leaded panels and eighty-year-old steel casements are ordinary features of a Pasadena or San Marino house and are not replaceable in any meaningful sense. They need hand cleaning, water kept off the frame, no impact loads and no thermal shock from cold water on sun-heated glass. The damage done by aggressive cleaning on these houses is permanent and it is depressingly common.
The Eaton Fire in January 2025 destroyed 9,414 structures and damaged 1,074 more in this corridor, and by the end of 2025 Los Angeles County had issued permits for 931 residences in Altadena. That means a large and ongoing volume of new glass going into finished homes surrounded by active construction. Cement, stucco and gypsum fines cure onto new glass and are harder than it is. Dry-wiping them scratches permanently. New construction here needs a rough clean and a final clean, in that order, and it needs flooding and dwell time rather than a wipe.
Hard. Raymond Basin groundwater has moved through limestone-rich alluvium off the San Gabriels, and blended with imported supply it leaves mineral on glass every time it dries, which in this sun takes minutes.
Hard. Raymond Basin groundwater carrying calcium from mountain alluvium, which means a sprinkler head that reaches a pane leaves visible mineral within a single dry season.
Hard basin groundwater carrying calcium from mountain alluvium. Combined with heavy estate irrigation and a seven-month rainless season, it is the leading cause of permanent glass etching in the city.
Hard basin groundwater from limestone-rich alluvium. It leaves mineral on glass every time it dries, and with landscape irrigation running through a seven-month rainless season it is the leading cause of permanent local etching.
A hard blend. Local wells carry mountain-alluvium calcium and imported supply adds its own load, so any irrigation head that reaches glass deposits mineral through a rainless season.
Hard groundwater carrying calcium from mountain alluvium. Irrigation reaching glass deposits mineral through the whole dry season with nothing to dilute it.
Hard basin groundwater. Irrigation overspray leaves visible mineral on ground-floor glass within a single dry season, and after several seasons it bonds rather than sitting loose.
Among the harder municipal supplies in Los Angeles County. Hard basin groundwater that leaves calcium every time it dries, which under inland sun takes minutes.
Because it is basin groundwater rather than imported surface supply, and it has spent a long time in limestone-rich alluvium washed out of the San Gabriels. Twelve to eighteen grains per gallon is normal across the Raymond and Main San Gabriel basins. Every drop that dries on a window in this climate dries fast and completely, and what stays behind is calcium carbonate. Washing does not dissolve carbonate, which is why the same spots survive repeated cleanings.
Yes, and carefully, by hand. Original wood sash, art glass and steel casements are usually held in place by putty that failed decades ago and paint that has been holding it since. We keep water off the frame, work pane by pane on divided lights, avoid thermal shock on sun-heated glass, and photograph any failed glazing rather than working past it. If a company wants to run a water-fed pole across a house like that, it is worth asking why.
Yes, and it is a different job than a maintenance clean. Wildfire residue is fine carbon, alkaline ash and condensed tars rather than dirt. The ash etches if left wet, the tar smears rather than lifting, and the carbon is abrasive enough that a dry wipe scratches permanently. The correct sequence is a flooded neutral-pH pass, then a solvent step for the tar, then the wash. We quote it separately and we will tell you honestly where a pane has been etched past what cleaning can recover.
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