You show up to a job and see dark patches on the siding or roof. Is it algae or mold? It matters a lot. Wrong diagnosis means the wrong chemical, a failed clean, and a customer calling you back two weeks later. Here's how to tell them apart and the exact technique to remove each one.

Algae vs Mold: The Quick Answer

Algae is photosynthetic and grows where sunlight and moisture meet. Mold is a fungus that thrives in dark, damp areas. Both require soft washing (low pressure + chemical solution) for a proper kill, but the chemical approach is different -- and so is the price.

Algae: Green, black, or dark streaky patches in sun-exposed areas. Slimy texture when wet. Soft wash or medium-pressure wash removes it.
Mold: Fuzzy, discolored patches in shaded or perpetually damp spots. Musty smell nearby. Requires bleach-based chemical treatment first, then a low-pressure rinse.
Pricing: Algae removal runs $0.15-$0.25/sq ft. Mold removal runs $0.25-$0.40/sq ft due to higher chemical and labor cost.

Using the same technique for both is one of the most common pressure washing mistakes. Keep reading for how to identify each one on-site and the step-by-step removal process.

How to Identify Algae

Algae feeds on moisture, sunlight, and organic material -- including the limestone filler in asphalt shingles and the grime that builds up on siding over time. On-site, look for:

Slimy or slick texture when wet (algae forms a biofilm)
Green, bright green, or dark greenish-black color
Surfaces that get direct or indirect sunlight and stay moist -- north-facing roofs in humid climates, siding near downspouts, concrete in shaded lawns
Gradual spreading across large surface areas rather than concentrated spots

The dark streaks running down asphalt roof shingles that homeowners often blame on dirt or rust are almost always algae -- specifically Gloeocapsa magma, a cyanobacteria that accumulates and darkens over time. Pressure washing alone won't keep it away; chemical treatment is needed to kill it at the root.

How to Identify Mold

Mold is a fungus. It doesn't photosynthesize and doesn't need sunlight. It grows best in shaded, perpetually damp conditions with poor airflow. Signs to look for:

Fuzzy or powdery texture (not the wet sliminess of algae)
White, gray, green, or black coloring -- sometimes multiple colors in one patch
Located under eaves, on north-facing walls, behind shrubs or landscaping that blocks airflow
Concentrated spots that spread outward rather than long streaks
Musty smell in the area, especially after rain

Mold is more aggressive than algae. It can penetrate building materials and cause structural damage over time. It also presents health risks if spores get inside through gaps in siding or roofing. Commercial clients -- especially property managers and HOAs -- are willing to pay a premium for proper mold treatment rather than a surface-only wash.

Removing Algae

Algae responds well to both pressure washing and soft washing depending on the surface. For hard surfaces like concrete driveways and walkways, 1,500-2,000 PSI with a surface cleaner attachment removes most algae growth effectively. Pre-treating with a general-purpose detergent for 5-10 minutes before washing speeds the job up.

For roofs, wood siding, and any surface where high pressure could cause damage, go with soft washing. Here's the basic mix:

Sodium hypochlorite (SH) concentration: 2-3% for most algae on siding and concrete
Mix ratio: 30% SH solution to 70% water, plus 1% surfactant to help the solution cling to vertical surfaces
Dwell time: 15-20 minutes before rinsing
Rinse pressure: 100-300 PSI -- enough to flush the dead algae off the surface

The key advantage of soft washing over pressure washing for algae: it kills the root organism, not just the visible growth. Pressure washing alone blasts off the surface layer but leaves the root cells behind, so algae regrows in a few months. A proper soft wash treatment lasts 1-3 years.

Removing Mold

Mold needs bleach-based chemical treatment to actually die. High pressure washing might scrub the surface clean temporarily, but mold spores survive in the substrate and come back within weeks. The only way to get a lasting result is chemical kill first, then rinse.

Proper soft wash process for mold:

Mix your solution. Use 3-6% sodium hypochlorite concentration for heavy mold growth. For severe cases, go up to a 40:60 ratio of SH to water. Add surfactant for better adhesion on vertical surfaces.
Pre-wet surrounding areas. Protect plants, grass, and anything nearby from chemical runoff before applying the solution.
Apply and dwell. 15-20 minutes minimum. On heavily infested surfaces, a second application may be necessary.
Check your progress. If mold is visibly fading after 5-7 minutes, your concentration is right. If it's barely moving, increase SH concentration on the next pass.
Rinse at low pressure. 100-500 PSI is sufficient to flush dead mold off the surface. Do not blast mold with high pressure -- you'll aerosolize spores and spread contamination to surrounding areas.

Safety is non-negotiable on mold jobs. Wear a respirator, chemical splash goggles, and chemical-resistant gloves. Never mix sodium hypochlorite with ammonia-based cleaners -- the combination produces toxic chloramine gas that can put you in the hospital.

Pricing Algae vs Mold Jobs

Mold removal costs more because it requires stronger chemicals, longer dwell time, more careful application, and PPE that standard algae washes don't need. Here's a working price guide:

Algae removal: $0.15-$0.25/sq ft. Standard house wash or surface clean with algae treatment included.
Mold removal: $0.25-$0.40/sq ft. Chemical treatment, extended dwell time, low-pressure rinse, possible second application.
Minimum charge for mold jobs: $250-$400 regardless of size, due to chemical cost and setup time.

If you quote a house wash and discover mold on-site, stop and adjust the price before you start. Walking in at algae rates and discovering a mold job halfway through is how you lose money fast. Inspect the full property before locking in a number.

A Note on Mildew

Mildew is technically a type of mold -- it's a surface-growing fungus that stays flat rather than penetrating deep into the material. It's less aggressive than true mold and responds to the same bleach-based soft wash approach. You'll commonly see it as a powdery white or light gray growth on wood surfaces, painted walls, and grout lines. Treat it like a mild mold job and you'll get a clean result.

Bottom Line

Diagnose before you price. Algae and mold require different chemicals and different techniques, and mold jobs carry a higher price tag for a reason. Using a one-size-fits-all pressure-wash approach on mold leads to callbacks, unhappy customers, and a reputation for sloppy work.

If you want to quote algae and mold removal jobs accurately, try QuoteSnap for free. You can set up separate pricing tiers for standard cleaning vs mold treatment so customers get an accurate estimate on your site before you ever show up.

Pressure Washing Algae vs Mold: Identification and Removal Techniques (2026)