White Streaks Running Down Your Windows From the Roof

Why are there white streaks on my windows under my swamp cooler?

They are mineral deposits left by your evaporative cooler's bleed water. A swamp cooler discharges mineral-saturated water on purpose so salts do not concentrate in the pan, and on a roof-mounted unit that water runs down the roof and across the windows below. It evaporates in the sun and leaves calcium and magnesium carbonate behind. It is not soap residue and it is not a sign your window cleaner did a bad job — washing will not remove it, because detergent does not dissolve carbonate. It needs an acidic restoration pass, and then the bleed line needs rerouting so it does not recoat the same glass next summer.

When it happens: Deposits build April through October and are most visible by September.

Can it be fixed?

Usually removable

Deposits under roughly five years of accumulation almost always come off completely with a controlled acidic restoration, and the glass returns to genuinely clear. Past that, it depends on how hard the local water is and how much sun the elevation takes — restoration will improve the appearance substantially but may not fully clear it, because the carbonate has begun etching into the silica. Once a pane is physically etched, no chemistry restores it; the honest options are mechanical glass polishing or replacement. A reputable cleaner will test an inconspicuous section and tell you which category you are in before quoting.

How to Tell That This Is What You Have

Streaks run vertically and start high — at or near the roofline — then fade as they travel down the glass.
They appear on one elevation of the house only: the one directly beneath the cooler.
They are worst at the end of summer and barely noticeable in March, because the cooler runs April through October.
They came back within weeks of a professional cleaning, which is the detail that convinces most homeowners the cleaner was at fault.
Run a fingernail across one. Surface deposit has a faint texture and catches slightly; etched glass feels smooth but stays visible.

What Is Actually Happening

An evaporative cooler works by pulling outside air through wet pads. Water evaporates into the airstream, which is what produces the cooling — but the dissolved minerals in that water do not evaporate. They stay behind and concentrate in the reservoir.

To keep that concentration from destroying the pads and the pan, every cooler bleeds or dumps a portion of its water continuously. That bleed water is by definition more mineral-laden than the tap water it started as.

On a roof-mounted unit, the bleed line typically discharges onto the roof. The water sheets down the slope, over the fascia, and across whatever windows sit underneath — then flash-evaporates in high-desert sun, depositing its minerals directly onto the glass.

This repeats every day for roughly seven months. Each pass adds a layer, and each layer bakes under UV that at Albuquerque's elevation is meaningfully stronger than at sea level. Over several seasons the carbonate stops sitting on the glass and starts etching into it.

What it costs to fix

Restoration is priced per affected window rather than per house, because typically only one elevation is involved. Light single-season deposits sit at the low end; multi-year buildup on a large west-facing window sits at the high end. Expect it to be quoted separately from the window cleaning itself — if a company folds it into a flat wash price, they are either not doing the restoration or not telling you what they are charging for.

Can you do it yourself?

There is a real DIY path here, with real caveats. A mild acid — white vinegar at full strength, or a dedicated mineral remover — applied with a saturated cloth and given several minutes of dwell time before agitation will lift light deposits. What does not work: any alkaline glass cleaner (it makes carbonate more stubborn), dry scrubbing, or a razor blade on tempered glass. What to avoid entirely: letting acid run down onto stucco, adobe, lime plaster, or anodized frames, where it causes damage that costs far more than the window cleaning. If the deposit does not respond within two attempts, further scrubbing is just adding scratch risk.

How to Stop It Coming Back

Removing it once is half the job. This is the half that determines whether you pay for it again next year.

1Reroute the cooler's bleed line so it discharges into a drain, a downspout, or the ground away from the building instead of onto the roof. This is the single fix that ends the problem, and on most units it is a short length of tubing.
2If rerouting is not practical, extend the discharge so it lands past the fascia line rather than sheeting down the roof plane above windows.
3Service the cooler at startup each spring. A unit with a failing float or a clogged distribution line often bleeds far more than it should.
4Where the bleed cannot be moved, plan an end-of-season restoration each autumn rather than letting deposits accumulate across multiple summers. One restoration a year is far cheaper than a permanently etched window wall.
5Consider a hydrophobic glass sealant on the affected elevation after restoration. It does not stop mineral from landing, but it makes the next season's deposit release far more easily.

Where We See This Most

This problem is not evenly distributed. These are the markets where local conditions make it common, and why.

Rio Rancho

The metro's hardest water — 100% groundwater with no surface blend — plus near-universal roof coolers. The most severe streaking we encounter anywhere.

Albuquerque

Near-universal evaporative cooling across the metro; the single most misdiagnosed window problem in the city.

Bernalillo

Hard municipal wells combined with roof-mounted units on most of the housing stock.

Los Lunas

Hard valley groundwater and a long cooler season at the lowest, warmest end of our New Mexico territory.

Common Questions

Is this the window cleaner's fault?

Almost certainly not. Cooler bleed deposits reappear within weeks regardless of how well the glass was cleaned, because the source is still running every day. The legitimate complaint is if a cleaner charged you for restoration and delivered a wash, or never told you what they were looking at.

Will a water softener fix it?

It helps meaningfully, because softened water carries far less calcium and magnesium into the cooler. But most evaporative coolers are plumbed off a line that bypasses household treatment, so check what actually feeds yours before investing on that basis.

Can I just move the cooler?

You do not need to move the cooler — only its bleed discharge. That is usually a short tubing run to a drain or a downspout, and it is the fix that actually ends the problem rather than managing it.

How do I tell surface deposit from etched glass?

Wet the area thoroughly. Surface mineral largely disappears while wet and reappears as it dries. Etching stays visible even when the glass is soaked, because the damage is in the surface rather than on it.