Confirming the Slab Type Before You Write a Word
A post-tension slab in expansive clay is not always obvious at first glance. Before putting anything in writing, it pays to confirm exactly what is under your boots. Many newer homes in clay-prone regions use post-tension slabs because they resist the swelling and shrinking that can break a conventional footing. Look for the classic warning stamp at the garage entry or near the electric panel: "Post Tension Slab, Do Not Cut or Core." If it is missing or obscured, check for small, evenly spaced grout patches along the slab perimeter. These are the ends of the tendons. Developers usually leave records with the HOA or city, but on-site evidence trumps paperwork for your report and your risk.
If you still have doubts, tap your contacts at the building department. In many subdivisions, the foundation type is recorded with the permit. Beware of assumptions based on neighborhood or builder, there are exceptions on every block. Once you have confirmed the slab type, note it in the preamble to your report. This frames every observation you make next.
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Perimeter Walk: Grade, Downspout Discharge, and Tree Proximity
Start the fieldwork with a slow lap around the house. The perimeter tells nearly half the story of a post-tension slab in expansive clay. First, look at the grade. Is the soil sloping away from the foundation, or is it flat or even sloping back toward the slab? In expansive clay, even a slight inward slope draws water under the slab, where it can swell the soil and lift the corners or center.
Downspout discharge is next. Extendable splash blocks or corrugated drain lines keep roof water well away from the foundation. If you see missing, broken, or misdirected downspouts, photograph them and note the risk to the slab. For every inch of rainfall, a typical roof can dump hundreds of gallons at the edge. In clay, this can set up months of movement.
Trees are another red flag. Roots from even a medium-sized tree can dry out the clay under one part of the slab while leaving the rest unchanged, creating differential movement. Measure the distance from trunks to the foundation. Many inspectors use the "half the mature height" rule for safe distance, but in heavy clay, even further is better. Document any large trees within thirty feet, and look for root heave or soil separation near the slab. Take wide shots for context and close-ups of roots, cracks, or soil erosion.
Where the Tendon Ends Live and Why the Warning Matters
Post-tension slabs contain steel cables, or tendons, that run through the concrete and are tensioned to hold the slab together. The tendon ends are usually visible as small, circular metal plates along the slab perimeter. These ends matter for safety and for your report.
Never recommend coring, drilling, or cutting the slab unless a structural engineer has mapped the cable layout. Severing a post-tension tendon can release stored energy with violent force. The standard warning stamp is not just for builders; it protects anyone who might later work on the slab, from plumbers to homeowners. When you see a repair involving the slab, or proposals for concrete work, always check for tendon locations and call out the risk in your notes.
Photograph several tendon ends, especially if any show signs of corrosion, spalling, or water intrusion. Note any patching or epoxy repairs, as these may indicate past cable failures or leaks that can compromise slab integrity. These images provide crucial documentation if a second opinion is needed later.
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Crack Width, Displacement, and the Patterns Worth Photographing
Not all cracks are created equal. In post-tension slabs, hairline shrinkage cracks are common and often harmless. What matters is the width, displacement, and pattern. Bring a crack width gauge or a standard coin for scale in photographs. Cracks wider than the edge of a nickel, or those with vertical displacement, deserve extra attention.
Look for patterns. Long, straight cracks parallel to slab edges may follow tendon paths and usually reflect shrinkage or minor movement. Diagonal or stair-step cracks, especially those that run up into brick veneer or stucco, suggest differential settlement or heaving. Map out where cracks start and end. Note if they run through control joints or cross into multiple rooms.
Photograph both close-ups and context shots. Use a ruler or card for scale. If displacement is present, angle the camera to show the difference in height across the crack. In expansive clay, cracks that open and close seasonally are common. Document any caulking, patching, or previous repairs, as they signal ongoing movement.
Interior Clues: Door Reveals, Drywall Cracks, and Floor Slope Readings
Once inside, shift your attention to the subtle signs of slab movement. Start with door and window reveals. Doors that stick, rub, or have uneven gaps at the top and bottom can point to slab movement below. Check both interior and exterior doors, and note which way the gap changes.
Drywall cracks tell another story. Focus on cracks at the corners of windows and doors, and those that run at a forty-five degree angle. Horizontal or vertical cracks in open wall space can also indicate movement, but corners are most sensitive to shifting. Photograph cracks with a tape measure or ruler for scale.
Measuring Floor Slope
A simple ball bearing or marble can give a rough sense of slope but for documentation, use a four or six-foot level. Note any areas with more than a quarter inch of slope per foot, especially if these are near crack patterns found outside. Some inspectors use digital levels or laser devices, recording elevation differences at key points. Create a basic diagram for your report, showing high and low spots. These measurements matter most in expansive clay, where the slab can heave or settle unevenly.
See how SlabToRoof handles this for home inspection
Under Slab Plumbing Leaks and What a Static Test Would Add
Most post-tension slab homes in clay soils have under slab plumbing, usually main drain lines for bathrooms or kitchens. A leak here can saturate the clay, causing heave or softening the soil so the slab settles. Look for signs of moisture at the slab edge, unexplained wet spots in flooring, or mildew odors in kitchens and bathrooms.
Water bills that spike without visible leaks are another clue, if the client provides them. Some inspectors use thermal cameras or moisture meters to scan floors near plumbing runs. While these tools can spot anomalies, they are not conclusive. The gold standard for leak detection is a static plumbing test, which requires plugging the sewer line and filling it with water to check for pressure loss. Most Standards of Practice do not require this, but you should note the option in your report if you suspect a leak.
If your inspection suggests a leak, refer the client to a licensed plumber for a static test. Document all evidence: moisture readings, floor discoloration, and any plumbing repairs visible at the perimeter or in crawl spaces (if any). Remember, under slab leaks in expansive clay can change the slab's elevation in just a few weeks.
Standards of Practice Limits and the Structural Engineer Referral
Every state has its own home inspector Standards of Practice, but most include language that limits structural opinions. You are generally required to describe observed defects and recommend further evaluation when a problem is beyond the general scope. In post-tension slab homes, that line is closer than with pier and beam or conventional slabs.
You are not expected to diagnose the cause of structural cracks or determine the adequacy of the post-tensioning. Your job is to observe, document, and recommend. When you see cracks wider than a nickel, displacement, clear signs of ongoing movement, or major slope changes, the referral is clear: recommend evaluation by a structural engineer. Document the specific observations that drive your recommendation, not just a vague concern.
Many inspectors also call out evidence of under slab plumbing leaks, especially with supporting photos or moisture readings. If the client or agent pushes for a definitive answer, refer to the Standards of Practice and explain the limits of a visual inspection. It is better to over-document and refer than to risk liability for a missed structural defect.
Photographs and Elevation Notes That Survive a Second Opinion
In post-tension slab inspections, your photographs and elevation notes are your best defense if another inspector or engineer is brought in. Use a consistent method for photo annotation. Every image should have context, scale, and a brief note about location. Wide shots set the scene, while close-ups show detail. Annotate images to point out cracks, tendon ends, or signs of moisture. If you use a digital device, mark up the photos before leaving the property so nothing is forgotten.
Elevation readings and slope diagrams should be simple, labeled, and tied to your photos. Draw a rough floor plan and mark measurement points. If you use a digital level, export the raw data and keep it with your report. These notes become crucial if an engineer is hired later or if the client seeks warranty repairs.
Same day delivery of your branded, annotated report to both agents and buyers builds confidence and limits confusion. Tools that combine fast photo annotation with organized report assembly can make the difference between a report that stands up to scrutiny and one that leaves questions unanswered. SlabToRoof is built to handle this task, from field photographs to final delivery, giving inspectors peace of mind when the next post-tension slab lands on the schedule.
