A homeowner on a pressure washing forum described it well: black streaks under every window of a white brick house, plain pressure did nothing, and no clear idea what was safe to try next. In a trade-forum thread, a washer quoting a brick job worried about the same thing, runoff from oxidized window-frame paint soaked into the brick. Here’s how to tell what you have and what actually removes it.

What the streaks usually are

The brick industry’s trade association notes that stains from outside sources are usually easy to place by their look and location. Use the table as a starting point.

What you see Likely cause What tends to work
Dark or greenish, worst on shady or north walls Mildew, algae or other growth where water keeps the brick wet (BIA TN 23) Bleach-based or biocide masonry cleaner, low-pressure rinse
Gray-black film that follows the drip path Dirt and carbon from air pollution washed off the sill Masonry “restoration” cleaner (e.g., PROSOCO Restoration Cleaner), pro-applied
Dull streaks under painted frames or trim Runoff from oxidized paint or metal above the wall (BIA TN 23 names paint, aluminum, copper and bronze) Restoration cleaner after testing; repaint or replace the source
Orange or rust-brown Rust or iron; brown can also be a manganese stain (BIA TN 20) A different problem; see the link under the table

If the color is orange, our guide to rust stains from sprinklers on brick and stucco covers that.

Try the bleach test first

The trade association’s cleaning guide (Technical Note 20) says household bleach can remove organic growth from masonry in some cases. Test a hidden streak with household bleach, following the label directions, and rinse. If it fades, it’s likely growth, and a normal soft wash or pro house wash should handle it.

If nothing changes, you’re looking at grime, carbon or paint residue. Bleach won’t dissolve it, and blasting harder won’t either.

How much pressure brick can take

Technical Note 20 sets out pressure bands, measured at the nozzle tip:

  • Very low: under 100 psi, for wetting the wall.
  • Low: 100 to 400 psi.
  • Medium: 400 to 800 psi, only if the brick maker permits it.
  • High: over 800 psi, which it calls not appropriate for brick.

It also calls for a 25 to 50 degree fan tip held at least 12 inches from the wall, and no more than 50 psi when spraying on a cleaning solution. The note warns that high-pressure water can etch mortar joints and strip the brick’s outer surface, causing permanent damage.

What removes the stubborn streaks

Masonry cleaner makers sell “restoration” cleaners for exactly this. One maker’s product page says its version loosens and dissolves dirt, paint oxidation, carbon buildup and other atmospheric pollutants. The same maker sells a thick alkaline prewash for spot-treating spots under window sills and eaves, where it says carbon and mildew stains are most severe.

These are not driveway soaps. That restoration cleaner’s safety data sheet lists 3 to 7% hydrogen fluoride, and its data sheet says it etches glass and architectural aluminum and harms wood, paint and plants. The maker sells it only through approved distributor accounts. That’s why this is usually a job to hire out.

Protect the mortar, the glass and the wall below

Whoever does the work should follow the cleaning guide’s basics:

  • Wet the wall below and keep it wet until the final rinse, so dirty runoff doesn’t make new streaks.
  • Keep acids weak and tested. Strong acid can etch brick and dissolve mortar, and on light brick it can cause “acid burn.”
  • Cover windows, metal and plants.
  • Rinse to neutral, about pH 6.5 to 7.5, checked with pH paper.

The cleaner maker also says to let a test area dry 3 to 7 days before judging it.

Older brick: go gentlest first

Chicago’s Bungalow Belt is a good example of where this matters. A Chicago architecture group says some 80,000 brick bungalows ringed the city by the Great Depression. On older brick you often can’t ask the brick maker. The National Park Service’s masonry cleaning guidance says to start at 100 psi or less, even a garden hose, and generally stay under 300 to 400 psi. It also warns that some stains may never fully come out.

Stop the streaks coming back

The streaks follow the water. Brick design guidance says every sill should have a drip that sends water off the wall, with its inner lip at least 1 inch out from the face. The same guidance calls the caulk joints around window and door frames the most frequent places water gets into a brick wall, and caulk doesn’t last as long as the building. If the paint on the frames is oxidizing, the residue will keep washing down until the frames are repainted.

What to ask before you hire

  • Will you test a hidden streak first and let it dry before judging?
  • What pressure and tip will you use on the brick?
  • Which cleaner will you use, and how will you protect the glass, frames and plants?

Our questions to ask a house washing company cover the rest. Take photos first, too, so you can tell wash damage from wear that was already there. For how we source and check every claim, see our methodology.

Sources