How to Clean Stucco Without Damaging It
Hard coat stucco and foam backed EIFS look alike from the street but take very different pressure. What Sto, a major stucco maker, specifies for cleaning each one.
The short answer
Clean stucco with a cleaning solution, a soft to medium brush and controlled water, never steam or solvents. Sto's cleaning spec allows real pressure on hard coat stucco over a solid wall, but limits EIFS (foam backed stucco look systems) and foam trim to 500 psi at most. Algae and mildew need a bleach solution, or they come back.
Stucco or EIFS? Find out first
Traditional stucco is a hard cement plaster over a solid wall. EIFS (exterior insulation and finish systems) has a similar look but is a thin finish over foam board. They can be hard to tell apart by eye, and the difference decides how much pressure the wall can take. Sto also notes that foam trim pieces can be attached to stucco, masonry or concrete walls, and those need the same gentle limit as EIFS.
What Sto specifies
Sto Corp. makes stucco and EIFS systems and publishes a cleaning specification for coated concrete, masonry, stucco and EIFS walls (Sto RC100 cleaning spec, revised December 2023). The key points:
- Test first. Try any cleaning solution and technique on a small, hidden area to make sure it works and causes no damage.
- No solvent cleaners, such as acetone, gasoline, mineral oils or turpentine.
- Algae and mildew: a solution of half a quart to one quart of household bleach per gallon of water, left to soak at least 15 minutes, with a second application for heavy growth. Sto notes that existing algae or mildew will come back if a bleach solution is not used.
- Brushes: soft to medium bristle only, never stiff or wire brushes.
- Water: cool or warm, never steam or high temperature where the coating is staying on the wall. Use at least a 30 degree fan tip.
- EIFS and foam trim: do not use high pressure. Limit pressure to 500 psi, maximum.
Sto lists higher pressures for removing loose coatings from solid walls, which is a different job from cleaning.
How we clean stucco
Our house wash is a soft wash: low pressure and a professional grade sodium hypochlorite (SH) cleaning solution, given time to work, then rinsed. Sodium hypochlorite is the active ingredient in household bleach, so the method lines up with Sto’s own algae and mildew step: the solution does the work, not the pressure. Stucco is on the list of surfaces our house wash covers, along with vinyl, aluminum, brick, wood and fiber cement.
Businesses with stucco fronts
If your business has a stucco or EIFS front, commercial washing is scheduled around your business hours. See commercial pressure washing.
What cleaning will not fix
Cleaning will not repair cracks, soft or hollow sounding spots, or failed sealant around windows. Those let water in and need a stucco contractor. If you do not know whether your walls are hard coat stucco or EIFS, find out before anyone uses pressure on them.
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