How to safely use bleach to kill mold on stone surfaces
Complete guide with dilution ratios and safety information
Sodium hypochlorite (bleach) is a corrosive chemical that can cause severe burns and toxic fumes.
When in doubt about your stone type or proper dilution, consult a professional stone restoration specialist.
Know what you're working with before you start
Sodium hypochlorite (SH) is the active ingredient in household bleach. It kills mold, algae, mildew, and bacteria by oxidizing organic matter. While effective, it's also a corrosive chemical that requires careful handling.
Household bleach comes in different strengths. Always check the label to determine the sodium hypochlorite percentage:
Look for these terms on the label:
Example: If label says "8.25% Sodium Hypochlorite", you have concentrated bleach that's stronger than regular 6% bleach. Adjust dilution ratios accordingly.
Diluting bleach reduces the sodium hypochlorite concentration to a safe level for your specific stone surface.
Final SH% = (Starting SH%) × (Bleach Volume ÷ Total Volume)
Example: 1 part bleach + 4 parts water = 5 total parts
If using 6% bleach: 6% × (1÷5) = 1.2% final concentration
For most homeowners, it's easier to use part-based ratios:
IMPORTANT MIXING RULE: Always add bleach TO water, never water to bleach. This prevents splashing and reduces fume exposure.
Not all stone surfaces can be safely treated with bleach
Recommended SH: 2-4% (final application strength)
Dilution: 1 part 6% bleach to 2-5 parts water
Dwell Time: 15-20 minutes
Notes: Most forgiving surface for bleach treatment. Good for driveways, walkways, patios.
Recommended SH: 2-4% (final application strength)
Dilution: 1 part 6% bleach to 2-5 parts water
Dwell Time: 15-20 minutes
Notes: Similar to concrete. Watch for mortar damage on older brick.
Recommended SH: 1-3% (start low!)
Dilution: 1 part 6% bleach to 5-10 parts water
Dwell Time: 10-15 minutes
CRITICAL: Colored pavers may fade. ALWAYS test first on hidden area.
Recommended SH: 1-2% (maximum)
Dilution: 1 part 6% bleach to 2-5 parts water
Dwell Time: 10-15 minutes maximum
CRITICAL: Very porous - rinse thoroughly. Can cause discoloration. Always test first.
Recommended SH: <1% (very dilute only)
Dilution: 1 part 6% bleach to 10+ parts water
Dwell Time: 10 minutes maximum
CRITICAL: Can dissolve minerals and discolor. Consider alternative cleaners.
Recommended SH: 0.5-1% (very dilute)
Dilution: 1 part 6% bleach to 10-15 parts water
Dwell Time: 5-10 minutes, rinse promptly
CRITICAL: Only if sealed. Can damage or strip sealant. Test first. Consider pH-neutral cleaner instead.
Recommended SH: <0.5% (extremely dilute)
Dilution: 1 part 6% bleach to 15-20 parts water
Dwell Time: 5 minutes maximum
CRITICAL: Highly porous calcium-based stone. Can etch and discolor. Consider professional cleaning instead.
Why: Bleach causes permanent discoloration, etching, and structural damage to marble.
Alternative: pH-neutral stone cleaner only
Why: Bleach causes severe discoloration and can delaminate layers of slate.
Alternative: Water and mild detergent or specialized slate cleaner
Why: Bleach etches and permanently discolors bluestone surfaces.
Alternative: Low-pressure washing with water only
Why: Causes severe discoloration and weakens stone structure.
Alternative: Oxygen-based cleaners or low-pressure washing
Why: Sodium salt deposits cause irreversible breakdown of stone structure.
Alternative: Professional stone conservator only
Why: Bleach will etch polished surfaces and stain unsealed granite.
Alternative: pH-neutral granite cleaner
Common dilution ratios for 6% household bleach
| Target Final SH% | Ratio (Bleach:Water) | Example Mix | Best For |
|---|---|---|---|
| 0.5% (very weak) | 1:11 | 1 cup bleach + 11 cups water | Sensitive stone, test areas |
| 1% (weak) | 1:5 | 1 cup bleach + 5 cups water | Limestone, delicate surfaces, light cleaning |
| 2% (moderate) | 1:2 | 1 cup bleach + 2 cups water | Concrete, moderate mold, general cleaning |
| 3% (strong) | 1:1 | 1 cup bleach + 1 cup water | Heavy mold on concrete, stubborn stains |
| 4% (very strong) | 2:1 | 2 cups bleach + 1 cup water | Severe mold on concrete only |
If using concentrated 8.25% bleach instead of regular 6% bleach:
Formula: (6% ÷ 8.25%) = 0.73, so use 73% as much bleach or add proportionally more water
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Chemical-resistant pump sprayer perfect for applying bleach solutions to stone surfaces.
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If you're not certain what type of stone you have, professional assessment prevents costly damage.
For valuable, antique, or historic stone surfaces, always consult a professional stone conservator.
Large surface areas or complex landscapes with extensive plantings require professional equipment and expertise.
Professional soft washing ensures proper dilution, application, and protection for your property.
Learn About Our Professional ServiceWe offer professional soft washing with proper equipment, experience, and insurance
