Post-Tension Slab Foundations: What They Are, Why You Never Drill Them, and How Repair Differs

Quick Facts

How Do I Know If My House Has a Post-Tension Slab?

Look at the garage floor first. Many post-tension slabs are stamped in the concrete with a warning, usually near the garage door or the step into the house, that reads along the lines of "Post-Tension Slab — Do Not Cut or Drill" (Concrete Network, accessed Oct 2026). If you find that stamp, the whole foundation is post-tensioned, not just the garage.

Next, walk the outside of the house and look at the vertical face of the slab just above the soil. You may see a row of small, round grout patches at regular intervals. Those are stressing pockets: the recesses at each tendon's anchor that were filled with grout after the cables were tensioned (PTI, Jul 2010). In residential slabs, tendons spaced 48 inches on center are common (Concrete Network, accessed Oct 2026), so the patches repeat every few feet.

No stamp and no patches proves nothing; the builder's foundation plan or the city permit file will name the type. Houston's Foundation Performance Association lists slab-on-grade — "both post-tensioned and conventionally reinforced" — as the most commonly used foundation type in Southeast Texas (Foundation Performance Association, Mar 2003).

Why You Should Never Drill or Cut a Post-Tension Slab

Each tendon is a steel strand inside a greased plastic sheath, anchored at both slab edges and pulled to high tension. Concrete Network describes residential tendons pulled to 33,000 pounds (Concrete Network, accessed Oct 2026). Cut one and two things happen: the slab loses that line of compression, and the stored force can be released at once. STRUCTURE magazine warns that "the force released by a cut tendon may cause the tendon to break through" its anchor and "pose a threat to nearby people or equipment" (STRUCTURE magazine).

Homeowners hit tendons on ordinary jobs: bolting a safe or car lift to the garage floor, trenching for a new bathroom drain, or coring for interior piers. Before any of them, have the slab scanned. A ground-penetrating radar scan "provides a wealth of information concerning the location, size, and material of all slab-encased elements" (STRUCTURE magazine), so the hole can go between tendons instead of through one.

If a tendon has already been cut, don't patch over it; PTI runs a separate certification for unbonded post-tensioning repair personnel (PTI, accessed Oct 2026).

Signs of a Problem in a Post-Tension Slab

Not every crack means anything. PTI's technical note on cracking in these slabs separates two kinds (PTI, Aug 1995):

• Shrinkage (restraint-to-shortening) cracks: vertical cracks through the full slab depth, usually near the middle and quarter-points of the slab, often formed before the tendons were even stressed. PTI concludes they are "of no structural consequence."
• Load (flexural) cracks: cracks near the slab edge zone — typically 6 to 10 feet in from the perimeter — that run parallel to the edge. PTI says these don't reduce strength but can increase deformation, and that slabs showing them "must be evaluated on an individual basis by a structural engineer."

The signs worth acting on:

• A measurable tilt: more than 1.25 inches of difference between two points 10 feet apart on the slab surface, which is the ACI construction tolerance cited by PTI (PTI, Jul 2000).
• Distress in the house that matches the tilt: diagonal drywall cracks, doors racking, separations — all on the side of the house that the elevation survey shows has moved.
• Rust staining, spalling or missing patches at stressing pockets: PTI calls the pocket patch the "first line of defense for the corrosion protection of the stressing anchorage" (PTI, Jul 2010). A failed patch lets water reach the anchor.
• A tendon end sticking out of the slab edge: a sign of a broken or released tendon; keep people away from it and call an engineer.

How Post-Tension Slab Repair Differs

Piering works the same way as on a conventionally reinforced slab; the difference is that any interior pier or plumbing access hole has to be placed after a tendon scan. In Houston, slab-on-grade repairs already require plans from a Texas-licensed engineer showing the location and type of supports (see Houston costs).

Cost by severity

• Cosmetic: shrinkage cracks, slab within tolerance — $0, or $250–$800 per crack to seal (Angi, Dec 2025).
• Evaluation: an engineer's report with an elevation survey, $340–$780 (HomeAdvisor, Jun 2026).
• Moderate: a corner or one side settled, a few piers at $1,000–$3,000 each (HomeAdvisor, Jun 2026).
• Major: a settling slab needing a full pier plan, $5,000–$25,000 (HomeAdvisor, Jun 2026), plus any tendon repair. Size it with the cost calculator and check Texas prices.

Call a structural engineer if: the slab is out of tolerance, there are edge-parallel cracks with distress above, a tendon is exposed or broken, or you need to cut the slab. Call a plumber first if: the movement is local and appeared suddenly; a leak under the slab can mimic soil movement. Call no one if: the cracks are shrinkage-type, the slab is within the 1.25-inch tolerance and nothing is changing.

Why This Happens

PTI describes post-tensioned slabs-on-ground as a solution "for problems associated with ground-supported residential foundations on shrink-swell soils" (PTI, accessed Oct 2026). Since the 2024 edition, IRC R506.2 requires post-tensioned slab-on-ground floors on expansive or stable soils to be designed to PTI DC10.5. The slab is built to bend a little and stay in one piece; it moves when the soil under its edge changes faster than it was designed for.

1. Soil moisture changes at the edge. Most soil volume change happens within 2 to 6 feet of the slab edge, while the soil under the middle stays relatively constant (PTI, Aug 1995).
2. The edge lifts or drops. Wetter edge soil swells and lifts the perimeter (edge lift); drier edge soil shrinks and lets it drop (PTI calls this center lift, "more appropriately called edge drop").
3. The slab bends in its edge zone. The highest bending happens about 6–10 feet in from the edge; if it exceeds what the slab was designed for, cracks form parallel to the edge and the house above shows distress.

What makes it worse:

• Drainage toward the slab: PTI asks owners for 3–5% slope away from the foundation within the first 5 feet and no roof drains discharging at the perimeter (PTI, Apr 2015).
• Trees near the edge: roots dry the edge soil unevenly and drive edge drop on one side.
• Leaks under the slab: add water to one area and cause local edge lift or heave.
• Damaged tendons: a cut tendon removes compression along its line and weakens that strip of slab.
• Built-in unevenness: PTI notes typical single-strikeoff residential slabs start 0.8 to 1.25 inches out of level between points 10 feet apart (PTI, Jul 2000), so a floor that was never flat can look like movement.

What To Do Next

1. Free: confirm the slab type, photograph cracks next to a ruler with the date, and note whether they run parallel to the outside wall near the edge.
2. Free: fix drainage — downspouts away from the slab, soil sloping away within the first 5 feet (PTI, Apr 2015).
3. Free: stop any drilling, anchoring or remodel work until the tendons are located.
4. $340–$780: a structural engineer's evaluation with a floor-elevation survey (HomeAdvisor, Jun 2026). Ask for the maximum difference between points 10 feet apart, and whether cracks are shrinkage or edge-zone cracks.
5. $1,000–$3,000 per pier: only if the engineer finds active movement, with a pier plan that accounts for tendon locations (HomeAdvisor, Jun 2026).

When You Don't Need Repair

If your cracks sit away from the slab edge or cross it at right angles, and the floor stays within 1.25 inches over any 10 feet, you don't need repair. PTI concludes that most cracks in post-tensioned slabs on ground "are not detrimental to their structural function," noting thousands of slabs built since the late 1960s with visible cracks that function satisfactorily (PTI, Aug 1995). Save your money: a garage-floor crack is not a reason to buy piers. Seal it if it lets in water, and re-check the elevation if new distress shows up inside.

Related Issues to Check

• Gaps where interior walls meet the ceiling. When a post-tension slab's edge lifts, interior partitions are pushed up relative to the trusses, and the gap at the ceiling shows the direction of movement.
• A house that was leveled before. Earlier piers that were set without a tendon scan may have damaged a tendon near the pier holes, so ask for the old repair plan and look for exposed strand.
• Pool decks, patios and driveways poured against the slab. Flatwork that settles toward the house sends surface water to the slab edge, which is exactly where post-tension slabs are most sensitive to moisture.

Frequently Asked Questions

What is a post-tension slab? A concrete slab foundation with steel tendons that are tensioned after the concrete cures, compressing the slab so it resists cracking and bending on shrink-swell soil. PTI describes it as a solution for residential foundations on expansive soils, and the 2024 IRC (R506.2) requires them to be designed to PTI DC10.5.

How do I know if I have a post-tension slab? Look for a "do not cut or drill" stamp in the garage floor and a row of small round grout patches along the slab edge outside. If neither is visible, the builder's foundation plan or the city permit file will say.

Can you drill into a post-tension slab? Only after the tendons have been located with a ground-penetrating radar scan and the hole is placed between them. Cutting a tendon weakens the slab and can release the cable's force suddenly (STRUCTURE magazine).

What happens if you cut a post-tension cable? The slab loses compression along that tendon, and the stored force can push the strand out through its anchor. The tendon is then spliced and re-tensioned or replaced by a PT repair contractor under an engineer.

Are cracks in a post-tension slab normal? Yes, most are. PTI says shrinkage cracks are "of no structural consequence"; only cracks near the slab edge running parallel to it need an engineer's evaluation (PTI, Aug 1995).

Can a post-tension slab be repaired with piers? Yes. Settled sections are underpinned with piers the same way as other slabs, at $1,000–$3,000 per pier (HomeAdvisor, Jun 2026). The difference is that interior piers and access holes must be placed after a tendon scan.

Who should I call for post-tension slab repair? A structural engineer first, for an elevation survey and diagnosis ($340–$780, HomeAdvisor, Jun 2026); then a PT repair contractor for tendons or a foundation contractor for piers, working from the engineer's plan.

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The question for a post-tension slab is whether it is out of tolerance and still moving. A free inspection with floor-elevation readings answers that before anyone drills.

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Last reviewed: October 2026 · Sources verified against current industry data