Hard water spots on glass: the sprinkler is usually the culprit
Almost always irrigation. Sprinkler overspray and run-off from render or limestone dry on the glass and leave dissolved minerals behind, cycle after cycle, until the deposit bonds. While it is still a deposit, the right treatment takes it off — Schmick window cleans start at $150. Once it has etched the glass, no clean brings it back. Move the sprinkler head first.
The pattern on the glass names the culprit
Hard water spotting almost never covers a house evenly. Walk the perimeter and it shows up in a pattern: ground-floor windows only, the lower two-thirds of the pane, worst on whichever glass sits closest to a garden bed. Sometimes you can read the arc of the sprinkler across the pane in dried rings. Upstairs windows, five metres above the nearest head, stay clean. That uneven distribution is the diagnosis. Weather arrives everywhere at once. Irrigation arrives where you pointed it.
The second tell is direction. Spots that fan upward from the sill were thrown by a head. Chalky streaks running vertically down the pane came from above — rain sheeting off a rendered wall, a limestone sill or a sandstone lintel, picking up minerals on the way. Check in low afternoon sun with the glass dry; wet glass hides everything. Two minutes with your eyes at an angle usually tells you which side of the house owns the problem, and often which head.
Rain is nearly pure water. Your sprinkler is not
What comes out of a mains tap, a bore or a rainwater tank carries dissolved calcium and magnesium. When the droplet evaporates in the sun, the water leaves and the minerals stay. One cycle is invisible. A controller running four mornings a week through a Sydney summer is hundreds of passes over the same pane, each one laying a fractional layer on the last. The crust you can now see took years of small deposits, not one bad week — which is why it seems to appear from nowhere.
Run-off adds to it. Water washing across render, mortar, limestone or sandstone picks up soluble minerals from the masonry and carries them onto whatever sits below, which on most Sydney homes is a window. That is why the worst spotting is often on glass under a rendered upper wall or facing a limestone-paved courtyard, and why it turns up on houses with no irrigation at all. Same chemistry, different delivery. The deposit does not care whether the water came from a sprinkler or a wall.
Three ways glass goes cloudy, and only one is the sprinkler
Hard water spotting is discrete and patterned — rings, arcs, speckles, often with a texture you can feel with a fingernail. Salt haze is the coastal one: an even grey film across the entire pane that you notice at four in the afternoon when the sun goes low, and it covers the whole house rather than a sprinkler's reach. Different causes, different treatments. Telling them apart before anyone starts scrubbing is the difference between a clean that works and one that disappoints.
The third is not dirt at all. Fog sitting between the two panes of a double-glazed unit means the perimeter seal has failed and moisture has got into the cavity. Wipe the inside, wipe the outside, and the cloud stays exactly where it was, because it lives where no cloth reaches. It tends to come and go with the weather, which is the giveaway. That one is a glazier replacing the sealed unit, and we will say so when we find it rather than charge to wash it.
A deposit has a use-by date
While the minerals are sitting on the surface, they come off. It takes the right chemistry, dwell time and a careful hand rather than a stronger arm — ordinary window detergent will not shift a bonded deposit, and a scourer only adds scratches to your problems. Caught within a season or two, the pane usually goes back to fully clear, edge to edge. That is the good news, and it is the reason to look at your glass this month rather than next spring.
Left long enough, the deposit stops sitting on the glass and starts reacting with it. North and west-facing windows get there fastest, because the droplet bakes rather than dries. What you are left with is micro-pitting: the surface itself has changed, and no clean reverses it. Light etching can sometimes be improved by polishing. Heavy etching is a replacement pane and a glazier's invoice. Anyone promising to wash etching out is selling you something, and we would rather tell you early.
Move the head, then clean on a normal cycle
Fix the water before you fix the glass, or you will pay for the same clean twice a year forever. Rotate or move the offending head so the arc stops at the garden, drop the pressure so it stops misting onto the wall, or replace the run along the house with drip line, which puts water in the soil where it belongs. Watering at dawn does not help — this is minerals, not heat. If the source is run-off from render above, a redirected downpipe or a drip edge does the same job.
Then clean once and go back to a normal cadence. Schmick window cleaning starts at $150; most single-storey homes land between $150 and $550 depending on window count, and two storeys adds about 40%. Book it alongside a house wash or gutters on the same visit and you save 5 to 15%. You do not need anyone standing in your driveway to learn the number — the instant quote asks a few things about your home and returns a firm, bookable price in about 60 seconds, and it holds unless the place is materially different from what you told us.