Mold & Mildew on Terrazzo
Mold & mildew on terrazzo appear as dark gray, black, or greenish discoloration in damp or humid environments. While terrazzo itself does not support mold growth, moisture and organic residue on or within the binder can create conditions where biological growth develops.
What’s Happening
Terrazzo consists of decorative aggregate embedded in either a cement-based or epoxy binder. Mold and mildew grow on organic material, not on the stone chips themselves.
Common contributing factors include:
- Persistent moisture or humidity
- Soap residue in showers or locker rooms
- Organic debris in exterior installations
- Poor ventilation in commercial spaces
- Irrigation overspray or standing water
Cement-based terrazzo is more porous and can retain moisture longer, especially if not properly sealed. Epoxy terrazzo is less porous but can still develop surface growth if residue and moisture remain present.
In some cases, what appears to be mold may actually be moisture darkening rather than active biological growth. Proper drying is critical before determining the true cause.
If discoloration appears orange or brown rather than dark gray or black, review Rust on Terrazzo or Iron Staining on Terrazzo once published to rule out oxidation.
What To Do
If your terrazzo is cement-based…
Improve ventilation and allow the surface to dry completely. Clean using a pH-neutral cleaner formulated for terrazzo. Remove organic residue carefully without aggressive scrubbing. If dark areas lighten as the surface dries, moisture retention may be the primary issue. If growth returns repeatedly, inspect grout joints, control joints, and surrounding areas for ongoing moisture intrusion.
Avoid bleach or acidic cleaners, as these can damage the cement matrix.
If your terrazzo is epoxy based…
Dry the area thoroughly before assessing discoloration. Clean gently with a neutral cleaner and soft pad to remove residue. Because epoxy terrazzo is less absorbent, recurring growth often indicates surface moisture rather than deep penetration. Avoid heavy chemical disinfectants not specifically approved for epoxy surfaces, as these can cause dulling or chemical alteration over time.
If you’re not sure which type of terrazzo you have…
Assume the surface is sensitive. Use only pH-neutral cleaning products and allow complete drying before evaluating. Avoid vinegar, acidic mold removers, or abrasive pads. If discoloration persists or spreads, professional evaluation may be necessary to determine whether the issue is surface growth or moisture intrusion.
If the affected area appears dull rather than darkened, review Etch Marks on Terrazzo to rule out chemical surface damage.
What NOT to Do
- Do not use vinegar or acidic mold removers.
- Do not use high-alkaline industrial disinfectants without confirming compatibility.
- Do not aggressively scrub with abrasive pads.
- Do not seal over active moisture or biological growth.
- Do not ignore recurring moisture problems.
When to Call a Professional
Call a professional if mold or mildew returns repeatedly after proper cleaning and drying, or if discoloration remains even when the surface is fully dry. Persistent issues may indicate underlying moisture intrusion or improper maintenance practices. Professional evaluation ensures the correct cleaning and restoration approach based on terrazzo type.
Safety Equipment
- Protective gloves
- Eye protection
- Adequate ventilation when using cleaning products
- Respiratory protection if using refinishing compounds
Prevention
- Improve airflow in humid or enclosed areas.
- Dry surfaces promptly after water exposure.
- Use only terrazzo-safe, pH-neutral cleaners.
- Address leaks, irrigation overspray, or standing water.
- Maintain appropriate sealing for cement-based terrazzo.
Disclaimer:
The information provided through Surface IQ is for general informational purposes only and is not a substitute for professional evaluation or service. Surface conditions, materials, finishes, and installation methods vary, and results cannot be guaranteed.
NextGen Surface Care assumes no liability for damage, injury, or loss resulting from the use or misuse of this information. Always test any treatment in an inconspicuous area before full application. Follow all product instructions carefully and use appropriate personal protective equipment (PPE) with proper ventilation.
For delicate, valuable, or antique surfaces—or if you are unsure about the material, finish, or condition—consult a qualified stone care professional before attempting any treatment.