Illustration for Stucco Stains After a New Roof or Gutter Overflow

A homeowner on a pressure washing forum described the classic case. The house got a new roof last year. Then a stucco stain got worse every month over the winter. Two replies pointed at a clogged gutter that wasn’t draining toward its downspout. The rest argued about bleach mixes. In a home improvement thread, another homeowner watched a second-story gutter pour out its end cap in a heavy rain. Fresh streaks on the stucco sat right below it.

Read the stain’s shape first

The shape and spot point to the water source.

What you see Likely source First fix
Streak running down from where a lower roof edge meets a wall Missing or undersized kickout flashing (InterNACHI) Add or fix the kickout, then wash
Wide sheet of staining below a gutter run, worse after big storms Gutter overflowing (InterNACHI) Clear the gutter and downspout, check the slope
Stain or bubbling near the wall base under a roof spout (canale) Roof water landing at the wall base (Sto) Divert the spout’s water away from the wall
Black or green, lightens with a drop of bleach Mildew or algae (Sto, Sherwin-Williams) Bleach-based soft wash
Gray film that doesn’t lighten with bleach Dirt or grime (Sherwin-Williams) Detergent cleaner, low pressure
White, chalky deposit Efflorescence: water carrying salts out of the stucco (Sika, Sto) Fix the water; light wash with a cleaner made for it

If the streaks are orange, that’s a different problem. See our guide to rust stains from sprinklers.

The bleach drop test

A paint maker’s mildew guide gives a simple check. Put a few drops of household bleach on the stain. If it bleaches away, it’s probably mildew. If it doesn’t, it’s probably dirt.

One stucco finish maker notes that mildew often looks like black dirt. So stucco stains get misread.

Fix the water first

A home inspectors’ association explains one likely culprit after a roof job. A kickout flashing sits at the edge of a roof where the eaves meet a wall. It should stick out from the wall and steer runoff into the gutter. If the flashing along that wall is missing or poor, runoff can get inside the wall and cause serious damage. The same group says rainwater can overflow the gutter in big storms and stream down the walls.

After a new roof, check every place a roof edge ends against stucco. If there’s no kickout, or it looks too small, ask your roofer.

One stucco finish maker lists poorly installed roof flashing among the causes of water getting into stucco walls. The results range from efflorescence and peeling to, in serious cases, water damage to the structure. The inspectors’ group warns that stucco can hide that damage for years. A stain that keeps growing is a reason to have a stucco-savvy inspector check for hidden moisture.

Why it shows up in monsoon season

In Albuquerque, the National Weather Service counts monsoon season as June 15 to September 30. Normal rain at the airport for that stretch is 4.48 inches. That’s about half of the 8.84 inches it gets in a normal year. A streak that darkens in August is often your gutters showing their limits.

How to wash stucco gently

Two makers of stucco finish coats publish cleaning guides that line up:

  • Pressure: no more than 500 psi. One says keep the nozzle several feet away. The other says never closer than 2 feet, with a fan tip of 30 to 45 degrees or wider.
  • Mildew: 1 part bleach to 20 parts water, or a cleaner with bleach. One maker says the mildew comes back without bleach and about 15 minutes of dwell time.
  • Dirt: a mild detergent, sprayed at low pressure so you don’t drive dirt deeper into the texture.
  • Wet the wall below and keep it wet, so runoff from above doesn’t make new streaks.
  • No steam, hot water or solvents.

One maker adds that frequent power washing can wear the finish out early. Our guide to soft washing your own house covers the setup. Not sure your “stucco” is really stucco? Read how to tell stucco from EIFS first. EIFS is foam underneath and needs the same gentle care.

Why stucco rarely looks perfect

Stucco is textured and porous, and old stains can sit deep in it. A new washer on a pressure washing forum posted their first stucco house. A strong mix and extra pressure still left shadows. Replies agreed that outdoor surfaces seldom look brand new from washing alone. A national standards guide covers cleaning masonry, concrete and stucco. It says that in some situations it may be wiser to spot clean, or not clean at all.

Set your expectations before you hire. Ask:

  • Will you test a hidden spot first and let it dry before judging?
  • What pressure and tip will you use, and how will you keep the wall below wet?
  • What happens if a stain lightens but doesn’t disappear?

Our questions to ask a house washing company cover the rest of the call. Take photos first so you can tell wash damage from wear that was already there. If your house is brick, the same runoff story plays out differently. See black streaks under brick windows. Here’s how we check every claim.

Sources

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