Cracks telegraphing: warning signs

Cracks telegraphing through epoxy show as a fine line or a raised ridge that follows a crack in the slab underneath. The line is usually straighter than a coating defect would be, it reappears in the same place after a recoat, and a coin tap alongside it often sounds hollow. Those three signs together are enough to identify it.

The line

The first sign is a thin line in the coating, often darker than the surrounding floor because dirt collects in the depression. It follows the crack in the concrete, which may be visible at the edge of the slab or under a lifted patch.

A coating defect such as a scratch is usually in the traffic direction, has a start and an end, and does not continue under a wall or across the whole floor. A telegraphing line runs with the slab.

Shine a light along the floor at a low angle. A crack line casts a shadow that a scratch does not, and the shadow reveals the depth.

The ridge

When the slab shifts, the coating can be pushed up into a low ridge along the crack. Run a straight edge across the line: a ridge shows as a gap under the middle of the straight edge.

A ridge means the movement is not purely horizontal. The two sides of the slab have moved vertically relative to each other, which is a more serious sign than a hairline with no displacement.

A ridge also catches tires and brooms, which is how many owners first notice it.

The sound test

Tap along both sides of the line with a coin. A bonded film gives a sharp click. A hollow, drummy sound means the film has separated from the concrete alongside the crack.

That separation is common because the crack is a path for moisture and because the slab movement breaks the bond at the edges. A hollow strip 1 to 2 inches wide along the crack is typical.

Telling it apart from other defects

The decisive test is to expose the concrete. A small grind or a lifted edge that shows a crack directly under the line confirms telegraphing. If the concrete underneath is intact, the line is in the coating only, and a recoat can fix it without any slab work.

A second test is to watch the line across a season. A crack line that widens in winter and narrows in summer is following the slab’s thermal movement. A scratch does not change width with the weather, and that difference alone is often enough to settle the question.

Why it appears after a season

A crack that was tight at installation can open when the slab cools in winter or dries in summer. The coating was applied over a closed crack and looked perfect; the movement came later.

That timing is diagnostic. A line that appears in the first heating season is slab movement. A line that appears in the first week is more likely a coating application defect, such as a lap line or a missed spot.

The first is a slab issue; the second is an installation issue, and the remedies differ.

A line that appears after several years is usually a crack that has always been there but has widened enough to show through. The coating did not fail; the slab changed. Matching the timing to the cause tells you whether to call the installer, the engineer, or a coating contractor.

When the sign is urgent

Most telegraphing is cosmetic. It is a line in a floor, not a structural emergency.

The exceptions are a crack wider than 1/8 inch, visible vertical displacement, cracks that are spreading or multiplying, and a slab that sounds hollow across a large area. Those point to a base or structural problem that a coating cannot fix.

In those cases, an engineer’s assessment comes before any repair. Coating over a moving slab is a temporary cosmetic measure at best.

The inspection routine

1. Walk the floor with a light at a low angle and mark every line.
2. Run a straight edge across each line to check for a ridge.
3. Tap both sides with a coin and note hollow strips.
4. Compare the line with the slab edges and any visible cracks.
5. Photograph with a tape measure for scale.
6. Note the date the line first appeared.

That record tells a contractor whether the crack is stable or active, and it tells you whether the floor is getting worse between inspections.

What the signs mean for repair

A flush line with a sound tap is cosmetic and can be left or recoated. A line with a hollow strip needs the film cut back and the crack filled. A ridge needs the slab assessed before coating again.

A line that returns after a recoat is the clearest signal that the crack is active and needs a membrane or a different strategy rather than another coat.

For the repair routes, see cracks telegraphing repair options. For prevention, read cracks telegraphing prevention, and for the cost of each path, check the cracks telegraphing cost guide.