# Signs a Slab Foundation Needs Repair | San Antonio Guide > How slab-home owners recognize repair-worthy observations, how to document them, and when to request an evaluation. URL: https://sanantoniofoundationrepair.net/guide/signs-a-slab-foundation-needs-repair/ Last-Modified: 2026-09-16 Homeowner guide # Signs a Slab Foundation Needs Repair How slab-home owners recognize repair-worthy observations, how to document them, and when to request an evaluation. Published September 16, 2026 · 5 min read ![Diagonal crack running from a door corner on a slab-home interior wall](/images/featured/diagonal-crack-running-from-a-door-corner-on-an-in.webp) ## What Slab Movement Looks Like From Inside Slab-on-grade homes move differently from raised ones, and the difference shows in what you notice. A slab is a large rigid element. When the ground under part of it changes, the slab bends across a broad area rather than dipping in one small spot. That means the symptoms tend to appear as a pattern across a section of the house rather than as an isolated soft place in one room. If you are noticing something and want to know whether it matters, this guide describes what is worth documenting. Our slab foundation repair [/slab-foundation-repair/ →](/slab-foundation-repair/) page covers what the repair itself involves, and our guide on what slab foundation repair involves [/guide/what-slab-foundation-repair-involves/ →](/guide/what-slab-foundation-repair-involves/) walks through the work stage by stage. ## The Observations That Matter Most ### Diagonal cracks from opening corners The most characteristic slab symptom. A crack running diagonally up from the corner of a door or window frame, inside or outside, occurs because openings interrupt the structure and concentrate stress at their corners. What to note: the length, whether it is widening along its run, whether one side sits proud of the other, and whether it is one crack or several at different openings across the same elevation. ### Separation at baseboards and trim A gap opening between the baseboard and the floor, or between the trim and the wall, particularly along an exterior wall. On a slab home this often indicates the slab edge has moved relative to the interior. ![Gap between baseboard and floor along an exterior wall](/images/content/gap-opening-between-a-baseboard-and-the-floor-alon.webp) ### Cracking at the exposed slab edge Outside, the slab edge is often visible below the brick ledge. Cracking there, particularly where it lines up with cracking above, is worth photographing. ### Doors that bind at one corner Not doors that stick evenly, which is usually humidity, but doors where the gap around the edge tapers: wide at one corner and closing at the diagonally opposite one. That is the opening having racked. ### Floors that feel uneven across a room A broad slope rather than a soft spot. Slabs do not develop localised soft areas the way a wood subfloor does, so a sensation of the floor falling steadily in one direction is more consistent with slab movement. ### Exterior masonry cracking Stair-step cracking through brick mortar joints, most commonly at opening corners. Worth photographing with a scale, and worth noting whether it appears on one elevation or several. ## What These Signs Do Not Establish Any of the above can occur without significant slab movement. Drywall cracks seasonally. Brick cracks thermally. Doors respond to humidity. Trim gaps open as framing dries. What none of them can tell you is how much differential exists, where the low point sits, or whether the movement is active. Those come from an elevation survey, which is the substance of a foundation evaluation [/foundation-evaluation/ →](/foundation-evaluation/) . > **Watch for the shape, not the item** > > One crack at one window is an observation. Cracking at three openings along the same elevation, increasing toward one end, alongside a door binding at that end and a floor that falls that way, is a pattern with a direction. The pattern is what suggests a foundation question. Individual items rarely do. ## Signs That Change the Urgency Some observations move a situation from “document it” to “measure it soon”: - A crack that has visibly widened between dated photographs. - Displacement appearing, with one side of a crack proud of the other. - A crack reopening after having been repaired. - Doors that have worsened progressively across seasons rather than cycling. - Several of these appearing in the same part of the house. - Any of this alongside damp areas in the floor, unexplained water usage, or sewer odours, which raise the plumbing question. ## Documenting Properly 1. Photograph every crack with a ruler held flat against the surface, plus a wider shot for context. 2. Mark the ends with a light pencil line and write the date. 3. List which doors and windows bind, and at which corner. 4. Note where floors feel uneven and which direction they fall. 5. Photograph the exterior slab edge and any masonry cracking. 6. Note where water stands outside after rain. 7. Repeat after a wet season and a dry one. The comparison between two dated sets is worth more than any single set, including one taken by a contractor. ## When to Request an Evaluation Request one when your dated comparison shows change, when several observations cluster in one part of the house, when displacement is present, or when you want a measured baseline before buying, selling, or simply to know where you stand. If your observations are isolated and static, documenting through a season is a reasonable answer and we will say so rather than booking a visit you do not need. Quick answers ## Questions About This Topic ### What signs are specific to slab foundations? expand\_more Diagonal cracks running from door and window openings, separation where the baseboard meets the floor along an exterior wall, visible cracking at the exposed slab edge outside, and localised drops in the floor that you can feel walking across a room. Slabs move as large sections rather than in individual bays, so the pattern tends to be broad rather than confined to one spot. ### Should I act on one small crack? expand\_more Document it rather than act on it. Mark the ends, write the date, photograph with a scale, and check again after a wet season and a dry one. A crack that has not moved across that cycle is dormant. A crack that has run past your marks is active, and that is the point at which measurement becomes worthwhile. ### Can I check my slab myself? expand\_more You can gather useful observations: door operation room by room, where floors feel uneven, crack locations photographed with dates, and where water sits outside after rain. What you cannot do is measure relative floor elevation across a whole plan, which is what distinguishes an active problem from a static one. ## Related Guides ### How Slab Foundations Are Underpinned with Piers The mechanics of stabilizing a slab with piers: placement, hydraulic lifting, load transfer, progressive recovery, and restoration. Read guide arrow\_forward [How Slab Foundations Are Underpinned with Piers →](/guide/how-slab-foundations-are-underpinned-with-piers/) ### Hydrostatic Plumbing Testing Before and After Slab Repair What a hydrostatic plumbing test is, why lifting against broken plumbing causes problems, and when pre/post-lift testing is recommended. Read guide arrow\_forward [Hydrostatic Plumbing Testing Before and After Slab Repair →](/guide/hydrostatic-plumbing-testing-before-and-after-slab-repair/) ### Slab Repair vs. Slab Replacement Weigh repairing versus replacing a slab foundation: criteria, scope and cost differences, when replacement is warranted, and why it's property-specific. Read guide arrow\_forward [Slab Repair vs. Slab Replacement →](/guide/slab-repair-vs-slab-replacement/) ### Under-Slab Tunneling vs. Interior Access Compare exterior under-slab tunneling with interior access: flooring impact, disruption, cost and timeline factors, and when each is chosen. Read guide arrow\_forward [Under-Slab Tunneling vs. Interior Access →](/guide/under-slab-tunneling-vs-interior-access/) Browse all guides [/guide/ →](/guide/)