A crack in an Allen garage slab is not one thing. Before garage floor epoxy goes on that concrete, each line has to be sorted into a control joint cut on purpose, a dormant shrinkage crack that can be filled, or a crack that is still moving and may point to soil or slab movement. Almost every concrete slab cracks. The useful question is what kind of crack it is, and whether it is still changing.
Three lines that are not the same repair
From the open garage door, a saw cut, a hairline, and a crack with a small lip can all look like a broken floor. They are not the same repair. Control joints are straight, even, and usually run across the bay or in a simple grid. Shrinkage cracks wander, branch, or die out in the middle of a panel. Movement cracks often show one side of the concrete sitting higher than the other, or they grow after the ends are marked.
A coating does not stop a slab from cracking, and it does not turn a moving crack into a stable one. A cracked slab can still receive garage floor epoxy once active problems are separated from lines that have stopped. Coating over a crack that is still opening only hides the evidence until the line comes back through the finish.
Control joints are planned cracks
Control joints are the straight saw-cut or tooled lines in the slab. They are planned places for cracking to happen, so the concrete can shrink along a straight line instead of tearing a random path across the bay. In an Allen garage they often run from the overhead door toward the back wall, or they divide a two-car or three-car slab into panels.
Many installers leave control joints open, or fill them with a flexible material rather than a rigid one, so the coating does not crack along them. A rigid fill locks the joint. When the slab keeps shrinking, or the joint opens a little with the season, the coating has nowhere to flex and a straight crack shows up on the joint line. That is a finish failure from treating a joint like a random crack. It is not proof that the house has a foundation problem.
Leaving the joint open means the coating is detailed at the cut so movement stays there. A flexible fill lets the joint move a little without tearing a brittle film. Neither choice erases the joint. A solid color floor will still show the line. Flake broadcast systems hide filled joints better than solid color coats, because the flake pattern breaks up a straight edge. That is concealment, not a repair, and it does not mean the joint has stopped moving.
Filling every straight line solid is the wrong default. A control joint packed with rigid filler can look smooth the day the floor is done and still telegraph a crack after a long dry stretch, when moisture leaves the clay beside the foundation.
Hairline cracks that stay the same width
Hairline shrinkage cracks that wander across the slab and do not change width are usually stable, and they can be filled before coating. Concrete shrinks as it cures and as it dries. A garage slab poured as one piece has to relieve that shrinkage somewhere. If the control joints did not catch all of it, a thin crack finds its own path. These lines are common on slab-on-grade garages in Allen. They are not, by themselves, a verdict on the foundation.
A dormant crack can take a rigid epoxy crack filler. The filler bonds the edges and gives the coating a continuous surface to sit on. Wider cracks are often chased out with a grinder or a saw blade before filling, so the filler has a shoulder to grip instead of sitting in a tight V that it cannot wet. After filling, the repair is ground flush. The coating needs a flat surface. A ridge of filler left proud will print through the finish as a shiny streak or a raised line that catches a broom and a shoe.
A hairline that shows through later is often cosmetic. Garage floor epoxy is a film on top of the concrete, and a filled line can still read as a faint shadow, especially under a solid color when the door is open and the light rakes across the bay. That shadow is not the same event as a crack that is opening again. Watch the width. A line that stays the same width is a surface note. A widening line is the one to treat as active.
Ask what happens if a filled crack reappears, and whether a spot repair is possible. A patch only holds if the crack underneath has stopped.
When one side of the crack sits higher
Cracks with one side higher than the other point to movement of the soil or the slab, and they need a different conversation than a fill-and-coat visit. A lip means the two edges are no longer in the same plane. Grinding the high side down and filling the gap does not put the slab back where it was. If the soil keeps moving, the lip returns and the coating splits along the same line.
An active crack will usually reflect through any coating eventually. No topcoat can stop structural movement. Flexible fillers suit cracks that move a little. They stretch where a rigid epoxy filler would snap. They do not make a moving crack disappear for good. If the movement is large or keeps going, the flexible fill fails too.
Mark the ends of a crack with a pencil and date the mark. If the crack grows past the mark over a few weeks, it is active. In Allen, a hard rain followed by a hot, dry stretch can change soil moisture beside the slab quickly. Take photos of all cracks before coating, with a coin or a tape in the frame so the width is visible later. Those photos are the baseline. Without them, a faint line in the new floor cannot be told from a crack that opened after the coating cured.
Door edge damage and wall cracks
Pitting and spalling near the garage door edge are common where water and road grit collect. That loss of surface is not automatically a structural crack. It still has to be repaired before coating, because a film will not hold on crumbly concrete or hide a pitted band. Loose material comes out, sound depressions get filled, and the repair is ground flush.
Check for cracks running from the slab into exterior walls or brick veneer, and for interior doors that stick or show a new gap. Those signs call for a foundation professional before any coating. A lone hairline that stays the same width, with doors that latch as they always have, is a different case. Not every crack is a foundation problem, and a moving crack should not be coated over.
Water beside the slab and pending repairs
Keep gutters and downspouts directing water away from the garage. Uneven wetting of clay beside the slab encourages movement. A downspout that dumps at a garage corner wets one side of the foundation while the other stays dry. Expansive clay in this area swells when wet and shrinks when dry, so that difference can shift an edge of the slab over a season. Clay is not the cause of every crack. It is a reason to keep moisture even.
Consistent soil moisture around the foundation during long dry spells is a common homeowner step to limit shrinkage. The aim is even moisture, not a soaked strip on one wall and dust on the other.
Do not coat a slab while foundation repairs are pending. Let the repair settle first. Drainage work, pier work, or a plumbing leak repair under the slab can shift the concrete again as the soil finds a new moisture balance. Wait, repeat the pencil marks, and compare them with the photos. A crack that holds its length and width through a wet week and a dry week is a candidate for fill. A crack that grows is not.