Window Washing in Seattle
Seattle's water is very soft at 24 to 32 mg/L across four sample points, soft enough to be corrosive, so both plants dose lime to raise its pH.
Seattle's water is very soft, so nothing in it marks a pane as it dries; squeegee technique, not rinse purity, decides the finish. The long wet season is the real schedule-setter, and May to September is the time to wash. Slope matters more than glass count here: a single-story exterior runs $380 to $620, more where the back of the house sits on a steep drop.
What the water means for the glass
At roughly 24 to 32 mg/L the supply is very soft, and the practical reading is liberating: nothing in Seattle tap water will mark a pane as it dries. Squeegee technique, not rinse purity, sets the quality of the finish here. The one consequence worth knowing runs the other way — water this low in mineral content is mildly aggressive, which is why the utility doses lime for corrosion control, and it is also why soap and detergent go much further here than a newcomer from a hard-water city will expect.
Seattle tap water, by the numbers
Figures by source, as reported.
| Hardness | 24–32 mg/L as CaCO₃ Roughly 1.4 to 1.9 grains per gallon, within the soft band, as published by Seattle Public Utilities. |
|---|---|
| pH | 7.89–8.52 |
| Alkalinity | 20.1–29.9 mg/L as CaCO₃ |
| Iron | 0.0101–0.0285 mg/L |
| Manganese | 0.00092–0.0031 mg/L |
| Sodium | 0.935–2.58 mg/L |
| Chloride | 2.8–3.5 mg/L |
| Total dissolved solids | 57.7–65.2 mg/L |
| Disinfectant | free chlorine |
| Source and treatment | Two protected mountain watersheds: the Cedar River and the South Fork Tolt River. The Cedar watershed is owned and managed by the City of Seattle and its water is clean enough that it does not require filtration. The system serves about 1.6 million people across the greater Seattle area. · Water goes first to the Cedar River or Tolt water treatment facilities, where the operator describes screening, disinfection, pH adjustment and other treatments, plus fluoridation. As of 2026 the Tolt Water Treatment Facility transitioned from private to public operation under SPU. · No softening step. |
Not in the report: corrosion index.
SOURCE · Seattle Public Utilities 2025 Drinking Water Quality Report, with SPU's Quarterly Surface Water Analysis of 20 May 2026 for the mineral rows (seattle.gov) · How we source these numbers
When a wash holds
What sets the cleaning clock
Growth rather than deposit governs the calendar. Moss and algae advance through the wet months from autumn to late spring and are heaviest on north and east elevations that never fully dry. Pollen washes down in spring and is carried by rain rather than settling as dust. The genuinely good working window is the dry stretch from July into September, and a wash taken then will hold far longer than the same wash taken in November.
WASHINGTON SEASON BY SEASON →What's specific to Seattle
Seattle is one of the few cities on this site where the water can be taken off the list of things to worry about. Seattle Public Utilities measures hardness at four named points across the Cedar and Tolt supplies and publishes readings around 24 to 32 mg/L as calcium carbonate, which is the low end of soft. Tap water is an adequate final rinse anywhere in the city. The utility's corrosion-control record shows how little mineral is present: untreated pH of 6.8 to 7.4 and alkalinity under 9 mg/L, corrected with lime toward targets of pH 8.2 and alkalinity 19. What replaces mineral deposit is biology. Moss, algae and lichen grow on shaded frames and glass through most of a long wet season, and they must be lifted rather than dissolved. Combine that with glacial ridges that turn a back elevation into a four-storey drop and the Seattle job is defined by access and by aspect, not by water chemistry.
Seattle Public Utilities measures hardness itself and publishes the numbers at four named sample points: the Cedar treatment plant outlet, Cedar distribution at South Stacy Street and Utah Avenue South, the Tolt filtration plant outlet, and Tolt distribution at Northwest 122nd Street and 1st Avenue Northwest. Across quarterly analyses the total hardness readings sit around 24 to 32 mg/L as calcium carbonate.
Those readings put the supply near the bottom of the soft band, roughly 1.4 to 1.9 grains per gallon. For glass, that means the rinse water is simply not the problem. Tap water here is clean enough to be the last thing that touches a pane on any job in the city, and a deionised final pass is money spent solving something Seattle does not have.
The water is soft enough that the utility has to correct for it. SPU's own corrosion-control material records untreated pH at 6.8 to 7.4 with alkalinity of 6.4 to 8.9 mg/L, and optimised targets of pH 8.2 and alkalinity 19 mg/L, achieved by adding lime at Cedar and lime plus carbon dioxide at Tolt. Water this low in mineral content is aggressive to plumbing.
One quarterly analysis is worth knowing about because it shows the limit of any single figure. On one sampling date the Cedar plant outlet read 30.7 mg/L while the Cedar distribution point read 77.1, with alkalinity, conductivity and dissolved solids all elevated in step. What leaves a plant and what sits in the mains at a monitoring station are not always the same water.
With mineral deposit off the table, the dominant residue here is biological. Moss, algae and lichen colonise frames, sills and the glass itself on shaded northern elevations, and in a maritime climate with a long wet season they are growing for most of the year rather than appearing seasonally.
That changes what the job actually is. Biological growth has to be lifted rather than dissolved, and it returns on a schedule set by shade and moisture rather than by how dirty the air is. A south-facing window and a north-facing window on the same house genuinely need different intervals.
The city's topography is an operational fact in its own right. Seattle is built on steep glacial ridges, and on the hillside neighbourhoods a two-storey elevation at the back of a house can be a three or four-storey drop. Access, not glass area, is what makes a Seattle quote.
How old the glass is
- Before 1940 · 23%wood sash and older glass
- 1940–1959 · 16%steel casements, early aluminium
- 1960–1979 · 14%aluminium sliders, often single-pane
- 1980–1999 · 14%double-pane units become standard
- 2000–2009 · 12%vinyl frames, low-E spreading
- 2010 or later · 21%low-E common: gentle cleaners only
The city, by neighborhood
What window cleaning costs in Seattle
Slope is the dominant cost here and it is invisible in a pane count. Two houses with identical glazing can differ substantially in price because one sits on the flat and the other has a back elevation dropping down a ridge. Beyond that, aspect drives frequency — north-facing glass needs more visits than south-facing glass on the same property — and downtown curtain wall is a separate contracted market with its own access rules.
Doing it yourself
On flat ground with accessible glass this is easy work, because the water does not fight back and technique alone decides the result. The real dividing line is terrain: on the hillside neighbourhoods the downhill elevation of an ordinary house can be a three or four-storey drop with no level ladder footing, and that is a hire-out situation regardless of confidence. Removing established moss from old wood sash also rewards a light hand, since aggressive scraping damages putty and paint faster than it clears growth.
OPEN COST ESTIMATOR →2025 annual mean at the Seattle - Beacon Hill monitor, 4103 Beacon Hill S, 6.5 km south-east of the city (EPA site 53-033-0080).
2025 annual mean at the Seattle-10th & Weller monitor, 10th & Weller, 3.4 km south-east of the city (EPA site 53-033-0030).
Largest air release: Boeing Commercial Airplane Group - Renton, 737 Logan AVE N (transportation equipment), 1.6 km outside the city limits, 37,404 lb in 2024.
Seattle's code sets no everyday limits on hose use, but lets the Seattle Public Utilities director declare a water supply emergency. Once one is declared, no water may be used outdoors except at the times and under the conditions the director sets, and water used against mandatory restrictions carries a surcharge.
Seattle Municipal Code §21.04.500: emergency water use restrictions · Seattle Municipal Code §21.04.505: emergency rate surcharge · checked September 2026
Seattle's stormwater code bars detergent, commercial and household cleaning materials, dirt and similar substances from reaching any public or private drainage system or waterway, whether directly or by running across the ground. Soapy window-washing water therefore has to be kept out of storm drains and gutters.
Seattle Municipal Code §22.802.020: prohibited discharges · checked September 2026
Seattle's Landmarks Preservation Board must issue a certificate of approval before any alteration or significant change to the controlled features of a designated landmark. In historic districts such as Ballard Avenue, any change to a building's external appearance likewise needs a certificate of approval before a permit can be issued.
Seattle Municipal Code §25.12.080: certificate of approval · Seattle Municipal Code §25.16.070: Ballard Avenue, certificate of approval required · checked September 2026
What actually shows up on Seattle glass
The defining substrate problem of western Washington. Moss and algae establish on north-facing glazing, on stone and brick masonry trim, on aluminum and vinyl frames, and on cedar-shingle exterior cladding within a few years of installation. The growth feeds on the maritime humidity and the deciduous-conifer organic load. Removal requires a sodium-percarbonate or quaternary-ammonium pre-treatment, careful rinsing, and an awareness that aggressive scraping will damage anodized aluminum and vinyl. This is the technical specialty of the Washington trade.
Seattle, answered
How hard is the water in Seattle, Washington?＋
At roughly 24 to 32 mg/L the supply is very soft, and the practical reading is liberating: nothing in Seattle tap water will mark a pane as it dries. Squeegee technique, not rinse purity, sets the quality of the finish here.
How much does window cleaning cost in Seattle?＋
Residential window cleaning in Seattle typically runs $13–19 per pane or $380–620 for a standard single-story exterior. Slope is the dominant cost here and it is invisible in a pane count.
When is the best time of year to clean windows in Seattle?＋
Seattle's drier, frost-free months are May to September, by NOAA's 1991–2020 normals for the Seattle Sand Point WSFO station, where November brings the most rain. The calendar on this page marks May (after the April 7 bloom and after smoke season) and June (before frost). Frost nights usually run from November 23 to March 8.
Can I rinse windows with Seattle tap water?＋
Usually yes: Seattle Public Utilities publishes dissolved solids of 57.7–65.2 mg/L, low enough that tap-rinsed glass tends to dry clean. Iron, manganese, sodium and chloride all sit under this site's flags (highest: iron up to 0.0285 mg/L). It draws on two protected mountain watersheds: the Cedar River and the South Fork Tolt River.
Why do my windows look dirty so quickly in Seattle?＋
In Seattle the dominant residue patterns include very soft supply at roughly 24–32 mg/l as caco3, measured by the utility at four named sample points and water soft enough to be corrosive, with lime dosed at both plants to lift ph toward a target of 8.2. Growth rather than deposit governs the calendar.
Do I need a professional to clean my windows in Seattle?＋
On flat ground with accessible glass this is easy work, because the water does not fight back and technique alone decides the result. The real dividing line is terrain: on the hillside neighbourhoods the downhill elevation of an ordinary house can be a three or four-storey drop with no level ladder footing, and that is a hire-out situation regardless of confidence.
What sets Downtown and the waterfront apart for window cleaning in Seattle?＋
High-rise curtain wall on contracted access, with salt air reaching the western elevations directly off Elliott Bay. The only part of the city where marine deposit competes seriously with organic growth. Capitol Hill and Queen Anne have their own housing stock and glazing as well.
Other cities we cover in Washington
Nearby cities in neighboring states
Just over the Washington line