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.
In depth
In regulated settings (food-service exteriors, healthcare facilities, certain municipal contracts) biocide use is governed by EPA registration, label rate, and state pesticide-applicator rules. A professional tracks kill concentration at the surface (not in the bucket), documents dwell time, and matches biocide choice to the target organism. On a residential roof, that means getting 1-2% sodium hypochlorite at the shingle for at least 10 minutes of dwell — what the industry now considers the ARMA-endorsed baseline for effective algae kill.
How this shows up on our jobs
Every soft wash we perform is a biocide application. We document chlorine concentration, dwell time, and landscape pre-soak on commercial jobs for compliance and warranty purposes.
Services where this matters
Related terms
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.
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.
Dwell Time
Dwell time is the interval between applying a cleaning chemistry to a surface and rinsing it off. It is the single most misunderstood variable in exterior cleaning. Chemistry does not clean instantly; it needs time to break chemical bonds, kill organisms, or dissolve minerals. Too little dwell and the chemistry is wasted. Too much dwell — especially in direct sun — and the chemistry can damage the surface, bleach landscaping, or drive the stain deeper.
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.
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.