Quick Summary
- Most cracks in concrete floors can be repaired before epoxy installation — but the repair method must match the type of crack.
- Cosmetic/surface cracks (hairline, dormant) are straightforward to fill and coat over.
- Active or structural cracks require specialized repair and may require evaluation before coating.
- Epoxy installation over unrepaired cracks will result in the cracks telegraphing (showing through) the coating or causing failure at those points.
One of the most common questions homeowners ask before an epoxy floor installation is: "What about all these cracks?" The answer is nuanced. Some cracks are purely cosmetic and easy to fill. Others signal structural issues that no coating can fix — and trying to hide them under epoxy will make the eventual reckoning more expensive. Here's how to tell the difference.
Types of Cracks and What They Mean
Hairline cracks (under 1/16" wide): The most common type. Usually caused by normal concrete shrinkage during curing or minor thermal movement. Typically cosmetic and dormant. Easy to address with epoxy injection or polyurea crack filler before coating.
Shrinkage cracks: Similar to hairline cracks but sometimes slightly wider. Appear during the first year of a concrete slab's life as it cures and dries. Dormant and cosmetic in most cases.
Control joint cracks: Many slabs have saw-cut or tooled control joints designed to guide where cracking occurs. These are intentional and should be filled with a flexible joint filler (not rigid epoxy), then incorporated into the floor design.
Settlement cracks: Wider cracks (1/8" or more) where one side has dropped relative to the other — a step or displacement is visible. This indicates soil movement or subsidence beneath the slab. A structural concern that should be evaluated before coating.
Active cracks: Cracks that continue to move (opening and closing seasonally, or still growing). Applying a rigid coating over an active crack will result in the coating cracking at the same location, typically within 12–24 months.
💡 Tip
To test if a crack is active, apply a small dab of plaster of Paris across the crack. Mark the date. Check it in 2–4 weeks. If the plaster has cracked, the underlying crack is still moving. This is important information for your contractor and the repair method chosen.
Repair Methods for Different Crack Types
Epoxy injection: For narrow dormant cracks (under 1/4"), low-viscosity epoxy can be injected to fill the crack completely and restore structural integrity. The repaired area becomes stronger than the surrounding concrete.
Polyurea or polyurethane crack filler: Flexible fillers that accommodate minor movement. Ideal for control joints and cracks with slight seasonal movement. Applied as a two-part liquid that cures in minutes.
V-groove and fill: For wider cracks, contractors often use an angle grinder to V-groove the crack (open it up into a V shape), clean it out, and fill it with a high-strength filler. This provides a better mechanical bond for the repair material.
Rigid epoxy patching compounds: For spalling, pitting, and surface deterioration. Troweled smooth and feathered to blend with the surrounding surface before coating.
💡 Tip
Tell your contractor about all cracks you know of — including any history of water coming through cracks, any movement you've noticed, and any cracks that appeared suddenly after a weather event. This context helps determine the right repair approach.
What Epoxy Can and Can't Hide
Epoxy CAN cover and integrate: Hairline and dormant cracks that have been properly filled, minor surface pitting and spalling (repaired and feathered), surface staining (though deep stains may require extra prep), control joints (filled and coated).
Epoxy CANNOT fix: Active structural cracks (it will re-crack), hydrostatic water intrusion through cracks (water will push under the coating), significant slab settlement (the floor displacement remains after coating), deep contamination from oil or chemicals not removed during prep.
💡 Tip
Be skeptical of any installer who says they'll "just coat over" cracks without any filling or preparation. Even hairline cracks should be addressed — at minimum, they should be chased out, cleaned, and filled before the first coat goes down.
Bottom Line
Most concrete cracks are repairable and no barrier to a successful epoxy floor — provided they're handled correctly. The key is honest assessment: cosmetic cracks get filled, control joints get flexible treatment, and structural concerns get evaluated before any coating goes down. A good contractor will identify each crack type and explain the repair approach. If they're not doing that, ask.