Mildew vs. Mold
Mildew and mold are both fungi, but they behave differently. Mildew is a flat, surface-level growth — usually white, gray, or yellowish — that stays on top of a substrate and can typically be cleaned away with soft-wash chemistry. Mold is three-dimensional and penetrates into porous materials (wood, drywall, paper), spreading its hyphae deep into the structure. On exterior surfaces, what most homeowners call "mold" is often mildew or algae; true structural mold on the outside of a house is uncommon without an underlying moisture problem.
In depth
For exterior cleaning, the distinction matters because it dictates treatment. Surface mildew and algae respond beautifully to a properly mixed sodium hypochlorite soft wash with surfactant — dwell, rinse, done. True mold penetrating into wood siding, soffit, or fascia may require longer dwell, multiple applications, a wood brightener, and sometimes follow-up sealing or replacement of the damaged material. If you see mold growing out of cracks or seams, you have a water-intrusion problem that cleaning alone won't fix.
How this shows up on our jobs
We identify what is actually growing on a home's exterior before we quote — 95% of the time it's algae and mildew, which a single soft wash eradicates. True mold is rarer and signals a deeper issue.
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Related terms
Algae vs. Lichen
Algae (most commonly Gloeocapsa magma on roofs) is the photosynthetic organism responsible for the black streaks running down asphalt shingles, the green haze on north-facing siding, and the slippery film on concrete walkways. Lichen is a symbiotic partnership between a fungus and an alga — a completely different, far more persistent organism that looks like a flat crusty patch with a raised perimeter. Lichens physically anchor into the substrate with root-like structures, which is why they do not wash off like algae does.
Soft Wash
Soft washing is a low-pressure exterior cleaning method that uses biodegradable detergents and a controlled bleach solution to kill the algae, mold, mildew, and bacteria that cause staining — rather than scouring them off with brute force. A soft wash rig delivers cleaning fluid at roughly 60-200 PSI, comparable to a strong garden hose, which is safe on siding, shingles, painted surfaces, screens, and caulking. The chemistry does the work: the solution dwells on the surface, breaks down the organic growth at the cell level, and is then rinsed clean with fresh water.
Biocide
A biocide is any chemical agent that kills living organisms — bacteria, fungi, algae, mold, mildew, lichen. In exterior cleaning, the most common biocides are sodium hypochlorite (bleach), quaternary ammonium compounds ("quats"), and hydrogen peroxide. Each has a place: sodium hypochlorite is the fastest and most cost-effective broad-spectrum kill; quats provide residual antimicrobial action that slows regrowth on treated surfaces; peroxide is the mildest and is used where chlorine would damage the substrate.
Sodium Hypochlorite
Sodium hypochlorite (NaOCl) is the active ingredient in household bleach and the primary disinfectant used in professional soft washing. It is sold to exterior cleaners in 12.5% concentration (commonly called "SH" or "pool shock"), roughly twice the strength of consumer bleach. When applied in a properly mixed soft-wash solution, it kills algae, mold, mildew, lichen, and bacteria at the cellular level within minutes and breaks down into salt water and oxygen.
Organic vs. Inorganic Stain
Organic stains are caused by living or once-living things: algae, mold, mildew, lichen, tannins from leaves, berry drip, insect excretion, pet urine, and so on. They contain carbon and respond to oxidizing chemistry — sodium hypochlorite or hydrogen peroxide (percarbonate) — which breaks the carbon bonds and destroys the stain at the molecular level. Inorganic stains are mineral-based: rust, hard-water deposits, efflorescence, metallic irrigation over-spray, fertilizer streaks, and the like. They contain no carbon and will not respond to bleach at all — you need an acid or a chelating agent to dissolve the metal or mineral and carry it away.