If your windows are looking dirty or streaky within days of a clean, there are usually a small number of identifiable causes and most of them are fixable.
This guide covers the most common causes and what you can do to keep windows looking cleaner for longer.
Soap or Detergent Residue
The most common cause of windows looking dirty shortly after cleaning is residual soap or detergent left on the surface.
Even a thin film of cleaning solution, which is invisible when wet, dries and leaves a sticky layer on the glass that actively attracts dust, pollution, and airborne particles.
The professional solution is pure water cleaning, using deionised or filtered water that contains no minerals or chemicals. Pure water leaves no residue when it dries, producing a streak-free finish that stays cleaner for longer.
Hard Water & Mineral Deposits
London has some of the hardest water in the UK, often exceeding 280–300 ppm of calcium carbonate in many parts of the city.
When tap water is used to clean windows and allowed to dry on the glass, it leaves behind white, powdery mineral deposits, commonly known as hard water stains or limescale, that create a hazy film on the surface.
Source: Thames Water, 2026
This film also provides a rough surface that traps further dirt and pollution more easily than clean glass. Over time, repeated cleaning with tap water can cause mineral deposits to build up and become increasingly difficult to remove.
Pure water cleaning eliminates this issue entirely. By removing the minerals from the water before it touches the glass, there is nothing left behind when it dries.
If your building already has significant hard water staining, our guide on how to prevent limescale buildup on glass buildings covers how to treat existing deposits and prevent them from returning.
City Air Pollution
Buildings on or near busy roads, in high-traffic areas, or close to active construction sites are exposed to a constant stream of fine particles that settle on glass surfaces.
If your building is in a high-pollution location and windows are getting dirty within a week or two of cleaning, it’s advised to organise a more frequent cleaning schedule. Adjusting your maintenance contract to reflect the building’s location and exposure is the most effective way to manage this.
Seasonal Factors
Pollen settles on glass surfaces and creates a yellow or greenish film that can make recently cleaned windows look neglected within days. Buildings near parks, squares, or tree-lined streets, are particularly susceptible.
Algae, moss and lichen is a slower process but equally relevant for buildings that are not cleaned regularly. Once this clings itself on glass, it accelerates the rate at which windows get dirty as spores spread across the surface.
Scheduling professional cleans in spring and autumn, before peak pollen season and after the main growing season, is the most effective way to manage seasonal grime on windows.
Broken Window Seals
If the dirt on your windows appears to be inside the glass, between the panes rather than on the surface, this is a sign that the sealed unit has failed, allowing moisture, condensation, and eventually dirt to enter between the panes.
Failed seals can be recognised by persistent fogging or condensation between the panes that does not clear regardless of temperature or weather conditions. In some cases, dirt or discolouration between the panes will be visible even on a dry day.
This is a glazing issue rather than a cleaning issue, and the only solution is window unit replacement or glass restoration. If you notice this on your building, our glass restoration service can assess and replace affected units as part of a wider building maintenance programme.
The Cleaning Method Used
Common issues that lead to faster dirt and grime on windows include:
- Tap water instead of pure water leaves mineral deposits that attract further dirt.
- Dirty cloths or equipment spreads grime rather than removing it, leaving a residue that builds up over time.
- Cleaning in direct sunlight causes the cleaning solution to evaporate before it can be properly rinsed, leaving streaks and chemical residue on the glass.
- Wrong cleaning products, as certain chemicals leave a residue on glass that attracts dust, particularly products containing ammonia or alcohol.
At All Clean London, we use pure water systems as standard across all commercial window cleaning work. Our equipment is maintained and cleaned between jobs, and our operatives are trained to clean in appropriate conditions and use the correct products for every surface type.
How Often Should Windows Be Cleaned?
The right cleaning frequency depends on your building’s location, type, and level of exposure to pollution, pollen, and environmental factors. As a general guide for commercial buildings:
- Office buildings in high-traffic areas: Every 4 to 6 weeks
- Apartment blocks and residential buildings: Every 4–8 weeks.
- Hotels and hospitality venues: Every 2–4 weeks, depending on guest-facing standards.
- Buildings near parks, construction sites, or busy roads: More frequent cleans may be required, it’s best to assess based on how quickly dirt occurs.
A regular maintenance contract is more cost-effective than reactive cleans and produces consistently better results, as windows maintained on a regular schedule never reach the level of grime that requires intensive treatment, which keeps both cleaning time and cost lower over the long term.
For more information on how often windows should be cleaned, check out our blog.
Frequently Asked Questions
The most common causes are soap or detergent residue left on the glass, hard water mineral deposits, and environmental factors such as urban pollution, pollen, and dust. In busy cities, high levels of airborne particulates mean windows get dirtier faster.
Streaks are usually caused by soap residue, tap water mineral deposits, or cleaning in direct sunlight. Professional contractors using pure water systems, deionised or filtered water with minerals removed, produce a streak-free finish because there is nothing left on the glass when the water dries.
Yes. London has some of the highest levels of airborne particulates and nitrogen dioxide in the UK. Buildings in central London, near busy roads, or close to construction sites will re-soil significantly faster than buildings in quieter locations, this is an environmental factor, not a reflection of cleaning quality.
The most effective solution is pure water cleaning, which uses deionised or filtered water that leaves no mineral deposits when it dries. For existing hard water staining, specialist glass restoration treatments can remove built-up limescale before a pure water cleaning programme is established.
Pure water window cleaning uses water that has been filtered to remove all minerals and impurities. Because there is nothing in the water to leave a residue when it dries, the glass stays cleaner for longer and there are no mineral deposits to attract further dirt.
Most commercial buildings in London benefit from cleaning every 4–8 weeks, depending on location and building type.
Buildings in high-traffic or high-pollution areas may require more frequent cleans. A regular maintenance contract ensures windows are cleaned consistently and never reach a level of soiling that requires intensive treatment.
Keeping Your Windows Clean
Windows that get dirty quickly after cleaning are usually the result of soap or detergent residue, hard water mineral deposits, air pollution, or a cleaning method that isn’t suited to the building.
At All Clean London, we use pure water systems as standard, maintain our equipment to the highest standard, and work with property managers across London to build cleaning programmes that reflect the specific needs of each building.
Request a free quote today and find out how a regular cleaning programme can keep your building looking its best all year round.
