Durham's water is soft at 23 to 24 mg/L, so the work is pine and oak pollen in March and April, and aphid honeydew and leaf tannin under mature canopy.
Durham's water is soft and deposits essentially nothing, so anything on the glass arrived through the air: pine and oak pollen in early spring, then aphid honeydew and leaf tannin under the big trees. Wash in April or May after the pollen, or in September and October. Trinity Park and Forest Hills under mature trees carry a heavier load than Southpoint, and a single-story exterior runs $300 to $510.
At 23 mg/L Durham's water deposits essentially nothing, so the supply is not what decides a result here. Anything left on the glass arrived through the air — pollen, honeydew, clay — which makes residue in Durham a diagnostic about the surroundings rather than about the rinse.
| Hardness | 23–24 mg/L as CaCO₃ Soft — both published values sit well inside the 0 to 60 soft band. |
|---|---|
| pH | 7.5 (range 7.4–7.5) |
| Iron | < 0.053 mg/L |
| Manganese | < 0.013 mg/L |
| Sodium | 33.9 mg/L (range 20.2–42.2) |
| Disinfectant | chloramine |
| Source and treatment | Lake Michie on the Flat River and the Little River Reservoir, with Jordan Lake water bought through the Town of Cary in shortfall years · Optimised conventional treatment at the Williams plant (22 MGD) and the Brown plant (30 MGD), with orthophosphate dosed for corrosion control · No softening step. |
Not in the report: alkalinity, corrosion index, chloride and total dissolved solids.
SOURCE · City of Durham 2025 Annual Drinking Water Quality Report (Water System ID 03-32-010), with the EPA SDWIS federal treatment record · How we source these numbers
The pollen wave is the fixed point of the Durham calendar and everything else arranges around it: no residential work worth doing in late March, a heavy reset in late April, then an interval set by canopy density rather than by any mineral cycle. Drought years add a constraint nothing else does, since restrictions on non-essential outdoor water use reach exterior washing directly.
NORTH CAROLINA SEASON BY SEASON →Durham has published hardness at 23 mg/L, under the label 'Hardness - EDTA' in its Tap Into Quality report rather than as total hardness, so it is easy to miss. It does. The record carried 78. Two reservoirs feed two plants, and in dry years Durham buys Jordan Lake water from Cary, so the supply has a second character. Above the tap it is the Duke and Research Triangle institutional book, tobacco-warehouse conversions, and the Piedmont pollen and red-clay pair.
Durham's hardness figure is published as 'Hardness - EDTA' at 23.0 mg/L, with an earlier edition of the same report giving 24.0. The label is the reason people miss it — a search for total hardness returns nothing and at least one water-industry site concluded the city publishes no figure at all. It does, in its own Tap Into Quality report, and the value is soft and stable across editions, although the city's 2025 annual report carries no mineral table at all.
The city runs two supplies and two plants: Lake Michie on the Flat River and the Little River Reservoir, feeding the Williams plant on Hillandale Road and the Brown plant on Infinity Road, about 52 million gallons a day combined. Water can move between lakes and plants by gravity, hydropower or pumping, so which reservoir is in service varies.
In dry years Durham draws Jordan Lake water purchased through the Town of Cary, and it holds its own Jordan Lake allocation. That is a genuinely different supply arriving in the same pipes, and it is the reason a single annual figure describes Durham less well than it describes a single-source city.
Durham's drought history is not abstract. In 2007 both reservoirs fell far enough that the city cut consumption by roughly a third and brought the Teer Quarry online as an emergency store. Outdoor water restrictions in a drought year are a direct constraint on this trade, because exterior washing is non-essential use.
The pine and oak pollen wave through March and April is the heaviest single loading event of the Durham year, and it overlays a canopy that keeps working the rest of the season — tannin from leaf litter and aphid honeydew under mature trees, the latter a sticky sugar film that a pure-water rinse passes straight over.
Iron-rich Piedmont red clay splatters onto lower panes from beds and unpaved edges. Through repeated wet-dry cycles it stains rather than washes off, and lifting it needs an oxalic or acid step — the one reason to carry acid in a city whose water gives no other.
Downtown is a tobacco-warehouse conversion district, and that shapes the commercial work more than any figure does: very large single lights, brick openings, interior atria and heights that sit awkwardly between ladder and lift. It is specified work with architects involved.
Duke University, the Duke health system and Research Triangle Park give Durham an institutional book far out of proportion to its size. All three procure formally on annual or multi-year cycles, so the market rewards bid documentation and compliance over responsiveness.
Canopy and access drive the residential number — Trinity Park and Forest Hills under mature trees carry a genuinely heavier load than Southpoint, and pricing them alike loses money on the older side. The institutional book is the other half of the market and it prices at bid: Duke, the health system and Research Triangle Park all buy on formal annual cycles where documentation and compliance matter more than the rate.
Soft water means a careful homeowner gets a good result on Southpoint-era glazing without special equipment. What does not transfer is honeydew, which needs a surfactant and a dwell rather than pressure and is the usual reason someone concludes their windows cannot be cleaned; red-clay staining, which needs an acid step; and the warehouse and campus work downtown, which is lift access and formal procurement rather than a larger version of a house.
OPEN COST ESTIMATOR →2024 annual mean at the Durham Armory monitor, 801 Stadium Drive, 6.1 km north of the city (EPA site 37-063-0015).
2025 annual mean at the Triple Oak monitor, 2826 Triple OAK Drive, 14.4 km south-east of the city (EPA site 37-183-0021).
Largest air release: SCM Metal Products Inc, 2601 Weck DR (primary metals), inside the city limits, 20,077 lb in 2024.
Durham bars water users from letting city water run off landscaped areas onto pavement, sidewalks, gutters, storm drains or a neighbor's lot through misaimed sprinklers or overwatering, and from irrigating while rain is falling. Declared water shortages add mandatory restriction stages on top of these year-round rules.
Durham City Code §70-555: prohibition of water waste · Durham City Code §70-566: mandatory water restriction stages · checked September 2026
Pouring, pumping or otherwise discharging anything other than stormwater into Durham's drainage system, directly or indirectly, is an illicit discharge unless the code's short list of allowed discharges covers it. Soapy runoff from a window job should be kept out of the street inlets.
Durham City Code §70-511: illicit discharges; containment of spills and discharges · checked September 2026
Durham draws the line where a glass crew works: on local landmarks and in local historic districts, caulking, reglazing and replacing broken panes count as routine maintenance needing no certificate, provided the window's size and style stay the same. Replacing the window itself needs a certificate of appropriateness from planning staff or the Historic Preservation Commission, even if no building permit is required.
City-County Planning, Form 1347: work requiring a certificate of appropriateness (2024) · City of Durham, certificate of appropriateness · checked September 2026
The Triangle and eastern Piedmont parent rock weathers to a more iron-bearing clay than the Atlanta Piedmont. Lower-sash splatter on intown stock needs oxalic rather than citric to lift cleanly. A citric-only protocol leaves a faint orange ghost stain.
Loblolly and shortleaf pine pollen blankets south-facing exposures for two to three weeks each spring. Same handling as Atlanta: wet only, never scrape, light alkaline wash to lift the film.
At 23 mg/L Durham's water deposits essentially nothing, so the supply is not what decides a result here. Anything left on the glass arrived through the air — pollen, honeydew, clay — which makes residue in Durham a diagnostic about the surroundings rather than about the rinse.
Residential window cleaning in Durham typically runs $10–15 per pane or $300–510 for a standard single-story exterior. Canopy and access drive the residential number — Trinity Park and Forest Hills under mature trees carry a genuinely heavier load than Southpoint, and pricing them alike loses money on the older side.
Durham's drier, frost-free months are April and May and September and October, by NOAA's 1991–2020 normals for the Raleigh Durham International Airport station, where July brings the most rain. The calendar on this page marks April (after the March 29 bloom), September (after smoke season) and October (before frost). Frost nights usually run from November 4 to March 31.
Usually yes: Durham Water Management publishes hardness of 23–24 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: sodium up to 33.9 mg/L). It draws on Lake Michie on the Flat River and the Little River Reservoir, with Jordan Lake water bought through the Town of Cary in shortfall years.
In Durham the dominant residue patterns include pine and oak pollen wave (march–april) and aphid honeydew and leaf tannin under mature canopy.
Soft water means a careful homeowner gets a good result on Southpoint-era glazing without special equipment. What does not transfer is honeydew, which needs a surfactant and a dwell rather than pressure and is the usual reason someone concludes their windows cannot be cleaned; red-clay staining, which needs an acid step; and the warehouse and campus work downtown, which is…
Revitalised pre-1920 tobacco-warehouse district with very large fixed glazing in brick openings. Lift access, interior atria, and architect-specified work. Trinity Park and Forest Hills have their own housing stock and glazing as well.
Just over the North Carolina line